Advances in Psychology Volume 67 Left-Handedness Behavioral Implications and Anomalies

ADVANCES IN PSYCHOLOGY 67 Editors: G. E. STELMACH P. A. VROON NORTH-HOLLAND AMSTEKDAM NEW YOKK OXFORD 'TOKYO LEFT-...

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ADVANCES IN PSYCHOLOGY

67 Editors:

G. E. STELMACH P. A. VROON

NORTH-HOLLAND AMSTEKDAM NEW YOKK OXFORD 'TOKYO

LEFT-HANDEDNESS Behavioral Implications and Anomalies

LEFT-HANDEDNESS Behavioral Implications and Anomalies

NORTI-I-l-1OL.I.AND AMSTERDAM NEW YORK OXFORD 'TOKYO

N OHTH -H( )I, 1, A N I) LLSEVIER SCIENCE PUBLISHERS B V Sara Burgerhartwaat 25 P.0 B o x 21 I 1000 A E Am\terdam. The Netherland\

L)i\trihutor\ for the United States and Canada: ELSEVIER SCIENCE PUBL.ISHING COMPANY, INC. 655 Avenue o f the Americas New York. N.Y. 10010. U.S.A.

L l b r a r y o f Congress C a t a l o g i n g - I n - P u b l i c a t i o n Data

Left-handedness S t a n l e y Coren.

b e h a v i o r a l i n p l ~ c a t i o n sand anomalies

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e d i t e d by

p. cm. -- (Advances i n psychology ; 6 7 ) I n c l u d e s b i b l i o g r a p h i c a l r e f e r e n c e s and indexes. ISBN 0-444-88438-6 1 . L e f t - a n d right-handedness. 2. L e f t and r i g h t ( P s y c h o l o g y ) I. Coren. S t a n l e y . 11. S e r i e s Advances l n psychology (Amsterdam. Netherlands) , 67. PP335.L44 1990 152.3'35--dc20 90-35954

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ISBN: 0 444 XX43X h @'

ELSEVIER SCIENCE PUBLISHERS B.V., I990

A l l rights reserved. N o part of this publication may he reproduced. stored i n :I retrieval system. or Iranamitted. in any forni or by any means. electronic. mechanical. photocopying. recording or otherwise. without tlie prior written permission o f the puhlisher. Elsevier Science Publishers B.V./ Physical Sciences and Engineering Division. P.O. Box 1991, 1000 B% Amsterdam. The Netherlands. Special regulations for readers iii tlie U.S.A. - This publication has heen I-egisteretl with the Copyright Clearance Cenier Inc. (CCC). Salem. M chusetts. Inlormation can he obtained from the CCC about conditions under which photocopies of parts of this publication nay be made i n the U.S.A. A l l other copyright questions. including photocopying ourside o f the U.S.A.. should hc referred to the copyright owner. Elsevier Science Publishers B.V.. unless otherwise specified. N o responsibility i s n\sumetl by the Publisher for any injury and/or damage to persons or property as a matter o f products liability. negligence or otherwise. or from any use or operation of any methods. products. instructions or ideas contained i n the materi;il herein. Printed in The Netherlands

V

Table of Contents Preface

xiii

Contributors

xvii

SECTION I: BIRTH STRESS AND INTRAUTERINE FACTORS

1. Birth Stress and Left-Handedness: The Rare Trait Marker Model Stanley Coreit and AIari Searlenian

3

Rare Traits and Pathological Conditions 7 Left-Handedness and the Rare Trait Marker Model 9 Factors that Influence the Association between Pathology and the Rare Trait 14 Experimental Assessment of Birth Risk Factors and Handedness 17 Application of the Rare Trait Marker Model 18 Is Left-Handedness a Good Marker for Pathology? 25 Conclusions and Future Directions 27 Acknowledgements 30 References 30

2. NonRight-Handedness and the Continuurn of Reproductive Casualty Paul Bakari The Continuum of Reproductive Casualty 33 Implications of the Reproductive Casualty Context for NRH 35 Historical Approaches to Reproductive Casualty 35 Recent Approaches to NRH and Pathology 45 The Comorbidity Factor 52 The Hypoxia Connection 59

33

vi

Contents NRH and Reproductive Casualty: Small Effects and Negative Results 61 Summary 63 References 64

3. Left-Handedness and Prenatal Complications Murray Schwartz Genetic Theories of Handedness 77 Environmental/Cultural Theories of Handedness Prenatal Hormonal Theory 79 Prenatal Pathological Theory 79 Summary 92 References 93

75

78

4. Intrauterine Factors in Sinistrality: A Review Michel Habib. Florence Tome and Albert M. Galaburda

99

Geschwind's Theories of Handedness and Origins of Sinistrality (1982-1985) 100 Anatomical Observations 102 Hormonal Influences on the Development of Asymmetry 106 Pathological Associations of Left-Handedness 115 Fetal Position, Birth Circumstances, and Handedness 118 Acknowledgements 122 References 122

SECTION 11: PHYSIOLOGICAL AND GENETIC FACTORS

5. Laterality in Hemiplegic Children: Implications for the Concept of Pathological Left-Handedness Mem'll Hiscock arid Cheyl K. Hiscock

131

Contents Background 131 Objectives 134 The Study 134 Implications for Pathological Left-Handedness Hypothetical Characteristics of Pathological Left-Handers 148 Conclusions 149 Acknowledgements 150 References 150

144

6. The Neuroanatomy of Atypical Handedness in Schizophrenia Paul Satz, Michael Foster Green, Steven Ganzell, George Bartzokis, Anthony Bledin, and Joseph F. Vaclav Introduction 153 Method 156 Results 159 Discussion 161 Acknowledgements References 163

vii

153

163

7. Phenotype in Normal Left-Handers: An Understanding of Phenotype is the Basis for Understanding Mechanism and Inheritance of Handedness Michael Peters 167 Pathology as Basis for a Phenotypical Distinction? 170 Phenotype as Established by Questionnaire 173 Subclassification of Left-Handers by Preference and Performance A Meaningful Subclassification of Nonpathological Left-Handers? Summary and Conclusions 189 References 190

177 185

viii

Contents

SECTION 111: ENVIRONMENTAL FACTORS

8. Cultural Influences on Handedness: Historical and Contemporary Theory and Evidence Lauren Julius Ham's

195

Historical Evidence 197 Current Evidence 217 Further Questions 231 References 245

9. Switching Hands: A Place for Left Hand Use in a Right Hand World Clare Porac, Laura Rees arid Tern. Buller

Switching Hands: Who, How, When and Why Summary and Conclusions 276 Acknowledgements 285 References 286

259

266

SECTION N: COGNITIVE, SPATIAL AND LANGUAGE ABILITY IMPLICATIONS

10. Mental Retardation and Left-Handedness: Evidence and Theories Margaret-Elleii Pipe

Left-Handedness in Retarded Groups Theoretical Accounts 302 Conclusion 313 Acknowledgements 314 References 314

294

293

Contents

ix

11. Handedness, Sex, and Spatial Ability

Richard S. Lewis arid Lauren Julius Harris Left-Handedness and Spatial Skill Spatial Ability in Left- and Right-Handed "High Reasoners" Conclusions 335 References 336

319

320

328

12. Handedness and Its Relationship to Ability and Talent Michael W. O'Boyle and Camilla Persson Benbow

343

The Left Hand Deficit Hypothesis 343 Factors Moderating Relations of Handedness with Ability 346 Strength of Handedness, Familial Sinistrality and Sex 346 Familial Sinistrality and Brain Lateralization 350 Immune Disorders 353 Age and Hand Consistency 354 Reasoning Ability Level 355 Handedness and Talent 356 Mathematical Precocity 358 A Reanalysis of Benbow (1986) 360 Concluding Comment 364 References 365

13. Familial Sinistrality and Cerebral Organization

Walter F. McKeever Clinical Studies 375 Experimental Studies of Normal Subjects 381 Summary of Demonstrated and Suggested Correlates of FS The Difficulty of Assessing FS Influences: Suggested Strategies 402 References 407

373

401

x

Contents

SECTION V: PSYCHOLOGICAL AND SPATIAL IMPLICATIONS

14. Sinistrality and Psychopathology

Pierre Flor-Henry

415

Sinistrality and Psychosis 417 Sinistrality in Autism and Childhood Schizophrenia Sinistrality in Monozygotic and Dizygotic Twins with Schizophrenia 423 Sinistrality and Mood 426 Conclusion 431 References 435

423

15. Autism and Anomalous Handedness

Susan E. Byson Chapter Overview 441 Current Thought on Autism 442 Handedness and Autism 444 Theoretical Accounts 446 The Geschwind-Galaburda Hypothesis Summary and Conclusions 450 Directions for Future Research 450 Acknowledgemenl 453 References 453

441

448

16. Left-Handedness and Alcoholism

Wayne P. London Increased Frequency of Left-Handedness in Alcoholic Men Left-Handedness and Treatment Outcome 459 Left-Handedness and Having an Alcoholic Father 460 Cerebral Laterality and the Study of Alcoholism 462 Alcoholism and Thyroid Disorders 464 Prenatal Environmental Effects 465

457 458

Contents

Season of Birth 466 Correlations with Latitude 469 Left-Handedness and Seasonal Sensitivity 471 Left-Handedness and Light Pigment 472 Left-Handedness and Life Expectancy 473 Alcoholism and Creativity 474 Alcoholism and Other Neurological Phenomena Issues of Methodology 477 Conclusions 480 Acknowledgement 480 References 480

xi

475

17. Left- and Mixed-Handedness and Criminality: Explanations for a Probable Relationship Lee Ellis

485

A Review of Evidence for an Association between Criminality and Sidedness 486 Possible Explanations for HandednessCriminality Associations 488 A Model of How Sex Hormones Influence Hemispheric Functioning in Ways that May Influence Handedness and Criminality 497 Discussion and Conclusions 498 Acknowledgement 500 References 500

18. Laterality and Longevity: Is Left-Handedness Associated with a Younger Age at Death? Diane F. Halpeni arid Stanley Coren

Right-Handedness as the Human Norm 510 Why Left-Handedness? 511 Is Left-Handedness Genetic? 512 Is Left-Handedness Learned? 513 Is Left-Handedness the Result of Pathology? 517 Prenatal and Perinatal Stressors and Sinistrality 518

509

xii

Contents

Variability in Cognitive Functioning 521 Immune System Disorders 523 Alcoholism and Smoking 527 The Alinormal Syndrome 528 Left-Handedness and Mortality Risk 530 Left-Handedness and Environmental Risk Factors Sinistrality and Longevity: Archival Data 533 Left-Handedness and Mortality: Next of Kin Data Are Left-Handers at Increased Risk of Mortality? Acknowledgements 541 References 541

531

537 540

Name Index

547

Subject Index

563

xiii

Preface There is a long history of fascination with handedness. Early references to it can be found in the Bible and appears in some Egyptian tomb writings. The problem of hand preference has caught the attention of many historical thinkers and scientists including Charles Darwin,Benjamin Franklin and Thomas Carlyle. Several early psychologists, such as G. Stanley Hall, James Mark Baldwin, William James and John Watson, have written on the subject. Many of these writings have included speculation about differences between left-and righthanders that transcend the simple asymmetry in manual skill, and suggest that sinistral individuals have psychological and physiologicalcharacteristicsthat make them quite different from their dextral counterparts. Advocates of this viewpoint have pointed to examples from numerous disparate cultures that have associated left-handednesswith evil, weakness, disease, and treachery. In fact the very word lejl comes from the Celtic Lyft, meaning we& or broken. The French word for left is guuclie, which has been adopted in Engllsh with the meaning of uwkward or guwky, while the Latin word for left, sinister, has come to mean evil or unfomtnate, and these examples could be multiplied many times. These traditions reflect the underlying presumption that right-handedness is associated with nonnaiity as opposed to the abnonalify or pathology of left-handedness. Such traditions and linguistic conventions, of course, do not constitute real evidence that there is something different, or anomalous, about left-handers. They do, however, define the context in which investigatorsbegan to consider the possibility that left-handedness might be due to some form of pathological intervention, or perhaps u markerfor some underlying pathology. Over the past several decades, a vast amount of data has been collected on this issue. Some of this data has suggested that left-handedness may be due to birth traumas or intrauterine hormonal imbalances. Other researchers have found linkages between sinistrality and sleep disturbances, retardation, autism, schizophrenia, dyslexia, criminality, alcoholism, homosexuality, bedwetting, verbal and spatial ability, cerebral organization, creativity, minor brain damage, level of neural maturation, and even reduced longevity. This book gathers together a number of researchers who will present evidence and evaluate whether there are actually differences between left- and right-handers, which extend into the broader psychological and physiological realms. The book is divided into five major sections, each of which deals with a different aspect of the problem. The first section deals with some recent suggestions that, for a large segment of left-handers, their sinistrality is due to some forms of pathological factors

xiv

Preface

associated with pregnancy and birth, with particular emphasis being placed upon the conditions surrounding the actual delivery and the intrauterine environment. The chapter by Coren and Searleman, proposes a general model, which explains why one might expect that a higher proportion of pathological conditions would be found to be associated with left-handedness. The chapter by Bakan explores the specifics of the perinatal and birth circumstances to show how lefthandedness can result from pathological factors, although Schwartz, in his chapter, sounds a warning that the association between pathology and sinistrality may not always be very marked. Finally, Habib, Touze and Galaburda describe the effects of hormonal factors and other factors during gestation, that may serve to shift handedness away from the right-handed norm. The second section presents three chapters that use different techniques to study the brain organization and implications of left-handedness. AU three suggest that there may be several subgroups of left-handers. The specific techniques used to reach this conclusion are quite different, ranging from Hiscock and Hiscock’s study of hemiplegiacs, through Satz, Green., Ganzell, Bartzokis, Bledin, and Vaclav‘s use of magnetic resonance imagery to Peters’ use of some clever behavioural indexes. The third section of this volume provides an exploration of the question of whether cultural, environmental or learning factors can alter manifest handedness. In his chapter, Harris presents an historical and cross-cultural discussion of the various pressures that have been placed on left-handers to become right-handed. Porac, Rees and Buller review some cross-cultural differences in the distribution of handedness and also provide some direct empirical evidence of the effectiveness of trying to change handedness through direct intervention. The fourth section of the book discusses the possibility that left-handedness may be a marker that predicts aspects of an individual‘s cognitive abilities. Pipe opens this discussion with a review of the relationship between retardation and an elevated incidence of left-handedness. In their chapter, Lewis and Harris suggest that the relationship between handedness and spatial ability may be clarified if more is known about cerebral laterality. O’Boyle and Benbow go on to demonstrate that left-handedness may be over-represented in both extremes of cognitive ability, both the cognitively impaired and the precocious. In his chapter, McKeever looks at the cerebral organization of language processing in individuals that have a familial history of left-handedness. The fmal section of the book contains some of the most diverse and controversial material in this volume. It looks at some of the psychological and

Preface

xv

physical factors that have been shown to be associated with left-handedness. In his chapter Flor-Henry explores the possibility that left-handedness may be elevated in groups of patients with clinical depression and other forms of psychopathology.Bryson’s contribution extends this discussion into the realm of autism. In the next chapter, London suggests that left-handers may have an increased susceptibility to alcoholism. Ellis returns us to a more behavioural level when he attempts to tease apart the suggestion that there may be an association between criminality and increased incidence of left-handedness. In the fmal chapter, Halpern and Coren attempt to tie together many of these fmdings. Following a consideration of some of the behavioural and physiological differences between left- and right-handers, and with the introduction of some new data, they reach the startling conclusion that left-handers may actually have a shorter life span than their right-handed contemporaries. My hope in collecting all of this material together in one place, is that we will now be better able to assess whether handedness is an indicator of psychological and physiological predispositions that have consequences for our understanding of broader aspects of behaviour. In so doing, I hope that we are closer to learning whether left-handers are indeed different, pathological, or disadvantaged, or whether they are merely a misunderstood and maligned minority.

Acknowledgements During the preparation of this book, my research has been supported by grants from the Medical Research Council of Canada, the Natural Sciences and Engineering Research Council of Canada, and the British Columbia Health Care Research Fund. Finally, let me acknowledge the assistance of Tania Jackson, without whose efforts this book would not have been completed, and also the assistance of David Wong and Wayne Wong who have run my laboratory in a manner that allowed me the time to finish this task. I also must acknowledge Joan, Flint and Wiz, who keep that other portion of my life in working order. Stanley Coren Vancouver, 1990

This Page Intentionally Left Blank

xvii

Contributors Paul Bakan Department of Psychology Simon Fraser Uniwrsity George Barnoliis Department of Psychiatry and Biobehavioral Sclenccs University of California, Los h g e l e s Camilla P e w n Bcnbow Department of Psychology Iowa State University Anthony Bledin Medical Diagnostic Imaghg Thousand Oaks, C X Susan E. Bryson

Department of Psychology Unive rsity of G uelph Terri Buller Department of Psychology University of Bntsh Columbia Stanley Corm Department of Psychology University of British Columbia Lee Ellis Department of Sociology Minot State Uniwnity Pierre nor-Henry Department of Psychiatry University of Alberta

Michael Foster Green Department of Psychiatry and Biobehavioral Sciences University of California, LQS h g e l e s Michel H. Habib Department of Neuropsychology Centre Hospitalier University d e Marseille Diane F. Halpcrn Department of Psychology California State University, San Bcmardino Lauren Julius Harris Department of Psychology Michigan State Univenity Cheryl K. Hiscock

The Methodist Hospital Houston, TX Menill Hiscock Department of Psychology University of Houston Richard S. Lewis Department of Psychology Pomona College Wayne P. London Dartmouth Medical School Walter F. McKeever Department of Psychology Northern Arizona University

Albert M. Galabunla Hanard Medical School

Michael W. O'Boyle Department of Psychology Iowa State University

Steven Ganzell Department of Psychiatry and Biobehavioral Sctences University of California, Los Angeles

Michael Peters Department of Psychology University of Guelph

xviii Margaret-Ellen Pipe Department of Psychology University of Otago

Alan Searleman Department of Psychology St. Lawrence University

Clare Porac Department of Psychology University of Victoria

Florence Touze Department of Neuropsychology Centre Hospitalier University de Marseille

Laura Rees Department of Psychology Carleton University Paul Satz Department of Psychiatry and Biobehavioral Sciences University of California, Lw Angeles Murray Schwam Department of Psychology Victoria General Hospital, Halifax

Joseph F. Vaclav Department of Psychiatry and Biobehavioral Sciences University of California, Los Angeles

SECTION I: BIRTH STRESS AND INTRAUTERINE FACTORS

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LEFT-HANDEDNESS Behavioral Implications and Anomalies, S. Coren (Editor) 0 Elsevier Science Publishers B.V. (North-Holland), 1990

3

Chapter 1

Birth Stress and Left-Handedness The Rare Trait Marker Model Stanley Coren University of British Columbia and Alan Searleman St. Lawrence University Given the fact that approximately nine out of every ten individuals is right-handed, it is understandable that left-handers have often been viewed as, not just a minority, but as if they were "different" or "strange." Perhaps it is this attitude that explains why there is such a long history of attempts to explain why some individuals deviate from the dextral norm. For instance, one philosophical note, dated 1686, refers to left-handedness as a "digression or aberration from the way which nature generally intendeth (quoted in Wile, 1934, p.92). Around the turn of the century, a series of experimental studies began to show that left-handedness was more prevalent in selected "non-normal" populations such as retardates, criminals, epileptics etc. (Lombroso, 1903; Redlich, 1908; Woodruff, 1909). This rapidly led to the hypothesis that left-handedness might be more than a marker for other deficits, but rather that left-handedness itself may be an aberration. The general tone of this early theorizing about the pathological genesis of left-handedness is well summarized by Brewster (1913) who noted:

4

Coren and Searleman A sound and capable stock, like a right-handed one, breeds true generation after generation. Then something slips a cog, and there appears a left-handed child, a black sheep, or an imbecile. (p. 183)

He makes it clear that slipping a cog refers to some form of damage or insult that results in the left-handed tendencies when he continues: An adult brain, wrecked on the educated side by accident or disease, commonly never learns to do its work on the other; the victim remains crippled for the rest of his days. But a child in whom the thinking area on the other side is still uncultivated, hurt on one side, can usually start over again with the other. A shift of this sort carries the body with it, and the child, instead of being permanently disabled, becomes left handed. (p. 179) Wilhelm Fliess, who, in addition to being a physician and a biologist, was Sigmund Freud’s closest friend, associated left-handedness with several pathological conditions, including homosexuality. In 1906 he wrote: Where lefthandedness is present, the character pertaining to the opposite sex seems more pronounced ... Since degeneracy consists in a displacement of the male and female qualities, we can understand why so many left-handed people are involved in prostitution and criminal activities. (quoted in English by Fritsch, 1968, p. 133) Other deviations in personality, cognitive ability or character were also often associated with left-handedness. In his book, The Backward Child, Cyril Burt (1937) notes:

if it is even safe to treat left-handedness as a sign or symptom, it should be regarded rather as a mark of an ill-organized nervous system. (p. 287) While Blau (1946) also wants to use sinistrality as a pathological marker, but for psychopathology:

Rare Trait Marker Model

5

Sinistrality thus can be used a cue by the psychiatrist in his studies of people with personality difficulties... Sinistrality may then be regarded not only as a neurotic symptom but as one of the signs of an infantile psychoneurosis. (p. 96 and 115) Most of these theorists suggest that there is something congenital about the pathological condition that leads to left-handedness. The congenital aspect refers either to some form of genetic malfunction (e.g., a chromosomal abnormality) or to damage during pregnancy or delivery. Gordon (1921), who looked at handedness in 3,298 normal children, compared to 4,620 children in schools for the retarded, provides a good example of this early theoretical presumption of an association between left-handedness and pathological birth processes. After concluding that there was a higher percentage of left-handed children in "mental defective" as opposed to normal elementary schools, Gordon proposed: It also seems probable that the percentage of naturally left-handed in mental defective schools may be no higher than that in ordinary schools, the increased percentage being due to some cause that has brought about the change, before birth, at birth, or at some subsequent date. (p. 334, emphasis added) Harris and Carlson (1988) make a distinction between a one-type model of left-handedness, that maintains that all left-handedness is pathological, and a two-type model, that maintains that there are two types of left-handedness, one of which is pathological in origin and one that is not. Although most of the researchers discussed above seem to have adopted the one-type (all pathological) model, some other theorists were hesitant to claim that all left-handers were to be regarded as being pathological in some way. Thus, contemporary with some of the earlier researchers mentioned above, was the geneticist H. E. Jordan (1922) who maintained that left-handedness "is not necessarily a stigma of inferiority" (p. 379). He wanted to distinguish between "pure" (presumably genetically based) left-handedness "which constitutes the bulk of the left-handed population" and the subset of anomalous left-handedness that marks some neurological or psychological problem. His viewpoint has generally been accepted by modern theorists, since today it is common to distinguish between presumably genetic left-handers and an additional group commonly designated

6

Coren and Searleman

as pathological left-handers (Satz, 1972, 1973; Satz, Orsini, Saslow, and Henry, 1985; Silva and Satz, 1979). However, this distinction still recognizes the other side of the coin, in other words, that left-handedness may come about through some form of neurological insult or genetic damage. Why is left-handedness often perceived as not merely an indication that the individual displaying the trait is different, but also that the individual is somehow inferior, or damaged? It is easy to demonstrate that the left side in general, and the left hand in particular, has come to be associated with something impure, imperfect, or even evil. This shows itself in many ways. For example, Wile (1934) found some 80 references in the Bible to the right hand, each according to it honours, virtues, and powers. However, Wile reports that "there is not one honourable reference to the left hand." Such negative and disparaging attitudes have found their way into the language as well. For instance, the French word gauche means both left and also clumsy, and has been adopted into English as the word gawky. The German word for left is links, and carries a strong negative connotation that is used pejoratively in forms such as linkisch, meaning clumsy. The word "left" itself comes from a similar negative source, namely the Celtic lyjt meaning weak or broken. In Spanish there is an idiom no ser mrdo that has come to mean "to be very clever," however, the literal translation of the phrase is "not to be left-handed." The Italian word ritancino not only means a left-handed man, but also a dishonest one as well. As a final example, in Australia, a slang term for left-hander is mol$-dooker, where "dook or "duke" is slang for hand, while "molly" is an effeminate man. The reason for all of this bad press for left-handers may not simply be due to the attitude that "different is bad," but may actually have some basis in fact. A number of researchers have reported that increased proportions of left-handers are found in a variety of groups with special problems (Molfese & Segalowitz, 1988; Porac & Coren, 1981; and of course this volume itself, all contain several chapters reviewing these sometimes controversial data). Some of the problems or conditions that have been positively correlated with lefthandedness include: 1) 2) 3) 4) 5)

brain damage epilepsy reading disability neuroticism alcoholism

Rare Trait Marker Model

7

drug abuse homosexuality aggression criminality mental retardation allergies autoimmune disorders migraines emotionality birth stress chromosomal damage poor spatial ability poor verbal ability school failure attempted suicide autism psychosis vegetarianism sleep difficulty slow maturation

Rare Traits and Pathological Conditions There's an interesting observation, that rare traits are often markers for neurological, physical, or genetic deficits. While, at first blush, this seems inexplicable, it is, nonetheless, a valid observation. For instance, rare coloured animals, whether it be a "blue-marl" collie or an albino human, often have major sensory deficits affecting their vision or hearing. Rare physical markers or characteristics, even though their only apparent effects are cosmetic, often show an association with various physical deficits. Consider the following list of relatively rare physical characteristics. 1) 2)

3) 4)

two or more whorls in the hair low seated ears asymmetrical ear heights adherent ear lobes

8

Coren and Searlernan

fifth finger curved third toe as long or longer than the second toe webbing or partial adhesion between the two middle toes overly large gap between first and second toe tongue furrows one lateral flexion crease across the palm instead of two lower lateral flexion crease continues to the edge of the hand fine "electric" uncombable hair many radial loops in the fingerprint patterns The interesting parallel to our discussion of left-handedness is that these so called "minor physical anomalies," which, of themselves, seem to have no particular linkage to psychological or neurological status, have, in fact been associated with a number of problems (see, for example, Bell & Waldrop, 1982; Campbell, Geller, Small, Petti & Ferris, 1978; Krouse & Kauffman, 1982). Some of the problems or conditions that have been associated with these rare features are: attention deficits hyperactivity aggression impulsivity emotionality mental retardation brain damage autism neuroticism low spatial ability low verbal ability psychosis learning disability delinquency

One might notice a degree of overlap between this list and the list previously mentioned as being associated with left-handedness. Is there something in

Rare Trait Marker Model

9

common between left-handedness and a curved fifth finger, an overly long third toe, or the existence of two hair whorls? Yes there is. The common factor, however, is a statistical one. The crucial similarity is that left-handedness and all of these other features that we have mentioned are relatively rare. We believe that such rare features (defined as statistically uncommon or infrequently occurring in the population) will often be associated with "problems" of a psychological or a physiological nature. This association between rare traits and pathological conditions is not capricious, but can be shown to be theoretically predictable, as we shall see below.

Left-Handedness and the Rare Trait Marker Model Since left-handedness is only present in about one out of every ten individuals, it may be classified as a relatively rare behavioural trait. Based simply on statistical considerations, it is possible to predict (even in the absence of any specific physiological mechanism) that left-handedness will be associated with an increased degree of pathology. The theoretical framework that we will use to make this prediction we will call the Rare Trait Marker Model. In describing the assumptions and mechanics of the Rare Trait Marker Model, it should be clear that although we will be concentrating upon left-handedness as the rare trait that we are interested in (cf. Satz, 1972, 1973), we actually could be dealing with any rare trait. We could have chosen any of those minor physical abnormalities that were listed earlier, or many other physiological or behavioural traits that we have not listed. Basically, the model has only two requirements that must be fulfilled. The first is that we have a dominant trait in the population and an alternative trait that may occur in its stead, but is relatively rare. The second requirement is that there should exist some form of pathology that can disrupt the development or emergence of the dominant or common trait. Suppose, for example, that we consider handedness in light of the model that we are going to present. For purposes of analysis, let us suggest some plausible, but hypothetical numbers. First, let us begin with a population in which the natural distribution of handedness is 90% right and 10% left if development proceeds normally. For sake of argument, let's assume that human laterality and lateral preferences are normally under the control of a some form of genetic mechanism or some form of maturational gradient (e.g., Corballis, 1983; Corballis and Morgan, 1978; Morgan and Corballis, 1978). Accordingly, if the genotype expresses itself normally, there may be an asymmetry in growth

10

Coren and Searleman

rate that will result in more rapid development on the left side of the human brain. Because of the contralateral neural link between brain and limb control, this maturational gradient could result in the emergence of a preference for the right hand in 90% of the population. The remaining 10% of the population might have a reversed maturational gradient due to genetic factors and become left-handed. The reader should not get too involved with the specifics of the theory outlined above, since it is merely an example, and we will show that the specific mechanisms involved make little difference for the model we are proposing. The important point to note is that, through some mechanism, if development proceeded normally, the population would contain a majority of right-handers (!lo% in this hypothetical example) and left-handedness would be the relatively rare trait (10%). If we can assume that this is the case, then we have fulfilled our first requirement, namely, the existence of a rare trait in the normal, undisturbed, population. To meet our second requirement we must next suppose that there is some pathological intervention that interrupts this normal maturational process, which, in turn, causes (for argument's sake) 10% of the population to switch their hand preference to the side opposite their natural, physiologically determined side of preference. In this hypothetical example let us assume that the pathology is a chromosomal abnormality of some sort. Again, it is not necessary to be very specific about the mechanism involved, all that is required is that, under appropriate circumstances, this pathology will result in a failure in some individuals to develop the genetically programmed right-handedness, resulting in a "pathological" left-hander, or conversely, that a genetically programmed left-handed individual might have normal development disrupted resulting in a "pathological" right-hander. If some pathology causes a 10% shift away from the genetically programmed handedness, this process would then cause 10% of the natural right-handers (9% of the population) to switch to left hand preference, and 10% of the natural left-handers (1%of the population) to switch to right hand preference. This model is illustrated in Figure 1, where the circles represent individuals with naturally determined, nonpathological preference, and the squares represent the

Rare Trait Marker Model

RIGHT

11

LEFT

Starting Population Prior to Pathological Insult

(A)

RIGHT

LEFT

Resultant Population After Pathological Shift of 10%

(B)

Figure 1:

Diagrammatic representation of the Rare Trait Marker Model applied to Handedness. Circles represent percentages of normal individuals and squares represent percentages of pathologically affected individuals.

12

Coren and Searleman

pathological members of the population. In this arbitrary example, the resultant population contains a distribution of 82% right-handers and 18% left-handers after the effects of the pathological insult are included. Before we go any further in this discussion it may be worthwhile to notice, that in terms of our presuppositions, we are adopting what Harris and Carlson (1988) called a two-type model since we assume that any given handedness pattern may be the result of either natural or pathological factors. In order to describe the population that we have just created we now would benefit from the use of some quantitative indexes. For this purpose we will borrow some measures from the field of epidemiology. In the epidemiological literature the concept of risk is used when describing outcomes in a population. In its simplest form the risk is much the same as rate of an outcome. Thus, the incidence of the outcome in a population is equivalent to an individual member’s risk, or probability of developing the outcome. For any targeted population and any given outcome, this is simply defined as: Percentage o f Population w i t h t h e Outcome

(1) Outcome Risk = T o t a l Target Population

Since this is equivalent to the incidence rate, we would then say that the risk of left-handedness in the population shown in Figure 1 is 0.18. At this level we have not added anything very much to a simple description of incidence, however, the concept of risk becomes more useful when we want to compare the incidence of an outcome in various groups, such as a group with a particular marker and one without. This is usually done by using the ratio of two risks, which is called the relative risk. For any given outcome, then, the relative risk would be defined as: Risk i n p o p u l a t i o n w i t h marker

(2)

Relative r i s k = Risk i n p o p u l a t i o n w i t h o u t marker

Each of the individual risk factors is estimated by Equation 1, and the percentage of individuals in that group actually having the outcome under investigation. Now returning to our hypothetical example, the outcome that we are interested in is for an individual to be affected by some form of pathology (of sufficient severity to affect the natural course of the development of handedness). The marker that we are considering is, of course, left-handedness.

Rare Trait Marker Model

13

When we look at the risk of pathology in each hand preference type, we find that the risk for left-handers is 0.5 or 50% (9/18), but only 0.122 or 1.22% (1/82) for the right-handers. If we now compute the relative risk from Equation 2, in order to compare the two handedness groups, we find that the relative risk is 40.98 (50/1.22). Simply put this means that the probability of finding a pathological individual in the left-handed group is nearly 41 times greater than the probability of finding one in the right-handed group in the example diagrammed in Figure 1. Now the reader might be quite skeptical at this point, since we seem to have made many assumptions about the causes of natural left-handedness (genetic predisposition) and the nature of the pathological intervention (chromosomal abnormality) that we postulated to work out this example. Recall, however, that we indicated we were not too concerned as to the actual mechanism or the nature of the actual pathology involved. In fact, these mechanisms can be considered to be completely hypothetical themselves, with the cause of natural handedness designated as merely Mechanism X , while the nature of the pathology could by Mecliartisni Y. The relative risk of pathology is always higher in the left-handed (rare trait) group regardless of the mechanism(s) causing either the original population distribution differences or the pathological shift to the other side. This occurs because of the statistical considerations from which the Rare Trait Marker Model is derived. All that is required is an asymmetrical population distribution for a given trait and a risk factor that can thwart the development of the targeted trait resulting in a shift in the phenotype. If the normal development of handedness were determined by intrauterine factors, such as the position in which the baby rested (rather than the genetically programmed maturational gradient that we suggested earlier) and the pathological intervention were due to the intervention of some maleficent deity (rather than a neurological insult or chromosomal abnormality) it would make no difference at all. As long as the initial population split was 90% vs 10% and the incidence of pathological shift away from the normal phenotype was lo%, the results would be the same. Any stressor that disrupts the normal development of lateral preference, regardless of mechanism, would be expected to show itself in higher relative proportion in the lower frequency pattern due to the initial skew in the population distribution of the preference pattern and the proportional nature of the pathological change. Although mechanisms and causes are not important in the Rare Trait Model, there are some characteristics of the population and the probability of pathology that do affect the association between pathology and the rare trait, as we will see in the following section.

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Coren and Searleman

Factors that Influence the Association between Pathology and the Rare Trait Although the pattern of results obtained from the rare trait marker model is completely insensitive to the nieciianisnis that cause the original population asymmetry or the pathological shift, the results are very sensitive to the initial population dislribufiort of the trait. The rarer the trait, the higher the likelihood (relative risk) that it will be found in association with pathology. This can be seen in Table 1 for a number of different initial population distributions. Notice that for our original example, the population distribution of 90% for the common trait (right-handedness) and 10% for the rare trait (left-handedness) resulted in a relative risk (likelihood of pathology) that was 40.98 times higher for the left-handers. If the starting population was split 95% for the common trait and 5% for the rare trait, the relative risk of pathology for individuals with the rare trait jumps to 117 times higher than for individuals with the common trait. If the original population was split 80% for the common trait and 20% for the rare trait, the relative risk is still high, but is now considerably less with pathology 11.40 times more likely for the rare trait group. The relative risk continues to drop as the initial difference in the percentage of subjects with the rare and common traits lessens until it disappears completely when the initial population is split evenly between the two traits. Thus we can conclude that the usefulness of a rare trait as a marker for pathology increases when the rare trait becomes rarer. The sensitivity of the Rare Trait Model to the asymmetry of the trait in the population, as shown in Table 1, makes it clear why handedness might prove to be such a good marker for pathology. The usual 90% vs 10% split in favour of dextrality which is normally found in most surveys, provides powerful statistical pressure toward increased likelihood of pathology in the left-handed population. Other aspects of laterality (ix., footedness, eyedness, and earedness) will simply not provide as much resolution since the split between right and left side preference is not as great as it is for handedness. Thus the percentage of the population that is left-footed is 19%, left-eyed 29% and left-eared 40% (Porac & Coren, 1981), which produce considerably lower relative risk values than the 10% associated with left-handedness. Up to now we have only considered the effect of the distribution of the rare and common trait as a factor. The model also incorporates another

Rare Trait Marker Model

15

Table 1: Variations in the distribution of the rare and common traits as a function of the initial population distribution, given a 10% likelihood of a pathological switch.

I n i t i a l Population Distribution ( X )

-r Percent p a t h o l o g i c a l i n F i n a l Population

Relative Risk

Conon

Rare T r a i t

Trait

Trait

Conon T r a i t

5

95

67.86

0.58

117.00

10

90

50.00

1.22

40.98

20

80

30.77

2.70

11.40

30

70

20.59

4.55

4.53

40

60

14.29

6.90

2.07

50

50

10.00

10.00

1.oo

I

quantity, namely the percentage of pathological shift. In our original hypothetical example we selected a value of 10% for this parameter. The assumption that 10% of the population is shifted away from its "natural" handedness by some form of pathological intervention may seem rather arbitrary, and possibly too large of a figure. What would happen if the rate of pathological shift were lower? Changing the rate of pathological shift does have an effect on the rare trait marker model of handedness. The effect, however, is somewhat counterintuitive in terms of likelihood of pathology. If, for example,the rate of pathological shift in the example shown in Figure 1 had been 1% instead of lo%, then out of a hypothetical sample of lo00 subjects, only 1 of the 100 natural left-handers would become a pathological right-hander, whereas 9 of the natural right-handers would become pathological left-handers. This has the effect, of course, of reducing the overall percentage of pathological individuals.

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Coren and Searleman

Table 2: Variations in the distribution of rare and common traits as a function of the percentage of shift due to pathological intervention and the initial population distribution of traits.

Percent Shift

Initial Population D i s t r i b u t i o n (%:

Percent patho log i c a 1 in Final Population

Relative Risk

Oue

To

Rare

Comnon

Rare

Comnon

Rare T r a i t

Patho log)

Trait

Trait

Trait

Trait

Comnon T r a i t

1

95

16.10

0.05

322.00

10

95

67.86

0.58

117.00

20

95

82.61

1.30

63.55

1

10

90

8.33

0.11

75.76

10

10

90

50.00

1.22

40.98

20

10

90

69.23

2.70

25.64

1

20

80

3.88

0.25

15.52

10

20

80

30.77

2.70

11.40

20

20

80

50.00

5.88

8.50

Instead of the pathological left-handers comprising 50% of the full sample of left-handers, they now make up only 8.33%, while the percentage of pathological right-handers falls from 1.22% to 0.11%. Although the incidence of pathological individuals is proportionally less in the left-handed group, the relative risk of pathology is now actually greater. With a 10% shift in handedness a left-hander was 40.98 times more likely to be pathological than was a right-hander, but with a shift of only 1%, the relative risk rises to 75.76 times.

Rare Trait Marker Model

17

Table 2 illustrates the relative risk of pathology for some different starting populations and for different percentages of pathological shift. Notice, that as the rate of pathological shift increases, so does the proportion of pathological individuals in both the rare and common trait groups. However, the risk of pathology in the rare trait group is proportionally less when compared to the common trait group. 7 7 1 ~the s usefulness of a rare trait as a niarker forpossible pathology diminishes if the rate of pathology is high.

Experimental Assessment of Birth Risk Factors and Handedness In our brief historical review, it was noted that some early researchers had suggested that pathology due to birth stress could be a cause of left-handedness. Curiously, despite the early interest in this issue, research on the relationship between hand preference, pathology, and the birth process virtually disappeared during an interval that went from the 1940’s to the early 1970’s. The reemergence of interest was not rekindled until the publication of a one page article by Paul Bakan in 1971. Bakan reported that a sample of male (but not female) left-handed college students were more likely than their right-handed counterparts to have been born in what he claimed was a “high risk birth order. According to Bakan, one would be classified as having a high risk birth order if they were either first born or fourth or later born, on the assumption that individuals born in these birth positions are more likely to suffer from perinatal complications. Largely due to the interest engendered by Bakan’s publication, the last two decades have produced a substantial number of studies that have attempted to find a relationship between left-handedness and indicators of possible birth stress. These studies have not only looked at birth order, but have also monitored several more direct measures of birth stress, in both clinical and nonclinical populations. A recent article by Searleman, Porac, and Coren (1989) has reviewed this literature for the nonclinical samples incorporating meta-analytic techniques. To briefly summarize the results of their review and analysis, it was found that there was no reasonable evidence to indicate that birth order is related to increases in deviations from right-handedness (or from right-sidedness in general) for either males or females. However, when more direct measures of birth stress were used, there was evidence that certain specific birth stressors

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Coren and Searleman

were significantly related to increases in nonright-handedness, particularly for male subjects. Although the size of the effect was quite small for any individual birth stressor, accounting for less than 1% of the variance in handedness, there was evidence of considerable consistency in the overall pattern. Using 10 different indices of birth stress and a composite index (subjects being classified as having had a stressful birth if any of several different birth stressors were present at birth), separate analyses were performed for males only, females only, and collapsed across sex. It was found that 30 of the 33 possible comparisons were in the direction that suggested a positive relationship between increases in the incidence of nonright-handedness and birth stress. This is a much higher proportion than would be expected by chance (p c 0.001), and indicates that when birth stressors are considered collectively, there is some validity to the hypothesis that birth stress is correlated with deviations from right- handedness. In their article, Searleman et al. (1989) drew attention to a number of methodological and theoretical problems that exist in the literature and offered suggestions to help alleviate these problems for future researchers. Specifically, they suggested: examining the variables in question in the context of family studies; decreasing reliance on questionnaire methods and increasing reliance on the use of archival records; conducting longitudinal, prospective studies instead of retrospective studies; using adequately sized samples of birth-stressed individuals; using continuous rather than dichotomous measures of lateral preference; and finally, obtaining a more complete profile of sidedness by examining all four lateral preferences. Until future research adheres to these suggestions, it will be impossible to make final definitive statements concerning the relationships between specific birth stressors and the development of particular patterns of lateral preference among normal individuals. (pp 406-407)

Application of the Rare Trait Marker Model Up to now we have been fairly non-specific in our discussion of the Rare Trait Marker Model. We have only applied it to general cases and hypothetical data. Let us now apply the model to an actual data set. As will be shown below, this will, not only illustrate the model in action, but will also show the kinds of information that the model can extract from empirical data banks. Specifically,

Rare Trait Marker Model

19

application of the model to a particular stressor will allow us to not only calculate the strength of the effect of that particular stressor (in terms of the degree of increased risk of pathology associated with left-handedness) but to also describe the “natural”(genetic?) distribution that would be expected if there were no pathological intervention. For the purposes of demonstrating the application of the rare trait marker model, we will examine the relationship between advanced maternal age at the time of birth and the subsequent incidence of left-handedness in the male and female offspring. Advanced maternal age was selected for several reasons. To begin with, births associated with older mothers seem to be subject to a variety of additional risks across a broad spectrum of birth stressors. For example, if we eliminate the maternal age groups below 16 years, the incidence of congenital malformations, prematurity, miscarriages and still births, increases steadily with increasing maternal age (Montagu, 1962). Lesinski (1975) reviewed 22 studies on birth risk and concluded that all mothers above 30 years of age were above the median risk, and that increasing age beyond this value increased the risk further. Broman, Nichols and Kennedy (1975) conducted a mammoth study that reached the same conclusion. These investigators used data from 169 variables measured on a sample of 26,760 women and found that increasing maternal age produced increasing numbers of sub-optimal births. Maternal age may also operate to produce problems at the genetic level. Many studies have shown that there is a significant correlation between aging and chromosomal changes in normal individuals drawn from populations that are unselected for any disease, disorder, or defect (Court-Brown, Jacobs and Tough, 1967). As a consequence, abnormalities in offspring caused by chromosomal factors increase in incidence as a function of increasing maternal age (Leviton and Montagu, 1971; Matsunaga, 1973; Polednak, 1976; Selvin and Garfinkel, 1972). Another good reason to use advanced maternal age is that it’s typically a much more reliably obtained value than is the case for many other birth stressors. Although it is impossible for a given individual to know, first hand, whether his or her birth was difficult, most individuals can report the current age of their mother (or, if now deceased, the age that their mother would currently be if she were alive today) as well as their own age. This means that the age of the mother at the time of the individual’s birth is usually quite reliable. We can contrast this to asking individuals if they have any knowledge of particular birth stressors associated with their own delivery. This information is, of course, always second hand, and can only be obtained by the individual concerned if it

20

Coren and Searleman

is volunteered by the mother or other members of the family at some time in the past. Over a period of approximately five years, 2,188 freshmen enrolled at the University of British Columbia (1,323 female and 865 male with a mean age of 18.5 years) have been surveyed to obtain medical and demographic information. Patterns of lateral preferences were determined for each individual using the Lateral Preference Inventory. This is a behaviourally validated self-report inventory that is designed to measure hand, eye, foot, and ear preferences. The version used contained four questions to ascertain hand preference based upon items selected from Coren and Porac (1978) and Coren, Porac and Duncan (1979). This self-report battery, which has been extensively used in studies of lateral preference, has been behaviourally validated and the handedness scale has a 98% concordance with individual behavioural testing (see Porac and Coren, 1981 for a full discussion). The questions were presented in a mixed order, and subjects responded "left," "right" or "both to each one. Data on handedness was scored using a dichotomous measure. A continuous measure of handedness was first obtained by transforming the data into an index that incorporates both the direction and consistency of sidedness, using the formula (R-L)/N, where "R"is the number of ''right" responses, " L is the number of "left" responses, and " N is the total number of questions presented. This procedure produces a range of scores from -1 (consistent left-handedness) to t 1 (consistent right-handedness). We could, of course, use this continuous measure to assess whether or not the effects of birth stress is to reduce the strength of dextrality, rather than effecting a shift from right- to left-handedness. However, we need a dichotomous measure of handedness if we are to apply the rare trait marker model that we described in Figure 1 in its simplest and most conceptual form. Therefore, all scores greater than zero (balanced ambidexterity) were classified as right-handed,while scores less than or equal to zero were classified as left-handed, following the format suggested by Porac and Coren (1981). A series of dichotomous classifications were created for advanced maternal age at the time that the offspring was born. In each instance a cut point was used. Thus we can tabulate the percentage of left- and right- handers born to mothers 29 years of age and less and compare that with the handedness distribution for mothers over 29 first, then move the cut point to age 30,and so forth. In this manner we can use advanced maternal age as the risk factor and for each age cutoff we can compute the relative risk of the offspring manifesting left-handedness. These relative risk scores are plotted in Figure 2.

Rare Trait Marker Model

21

Relative Risk of Left-Handedness as a Function of Maternal Age in Years

Lana than

29

30

31

32

33

34

35

30

or Mom

Mother's Age at Time of Subject's Birth

Figure 2:

The likelihood of being phenotypicallyleft-handed as a function of advanced maternal age when the offspring is delivered, based upon data from 2,188 subjects.

First, look at the pattern of results for the total sample. Remember that a relative risk of 1 means that there is no contribution of the particular marker in predicting the specific outcome. In this case, a relative risk of 1would indicate no effect of advanced maternal age on the incidence of left-handedness. Notice that for cut points set at ages 29 and 30 the likelihood of left-handedness in the offspring is not affected by the mother's age. With increasing maternal age, however, there is a gradual increase in the incidence of left-handedness. This is consistent with the supposition that older mothers have more stressful deliveries and gestation periods and that pre- and peri-natal stressors contribute to the appearance of left-handedness. When males and females are considered separately, we find that the pattern of the increase in left-handedness differs as a function of gender. Let us use a relative risk of 1.5 as an arbitrary set point. This is equivalent to a 50% increase

22

Coren and Searleman

in the likelihood of becoming left-handed. This point is reached much earlier for males than for females. For males, a 1.5 relative risk of left-handedness is reached when we consider mothers between the ages of 32 and 33 years of age. For the females, however, this value is not reached until the mother is between 35 and 36 years of age. This suggests that males are more susceptible to the effects of advanced maternal age than are females. This previously unpublished set of data is, of course, another entry into the literature suggesting that birth stress may be associated with an increased incidence of left-handedness. The data presented above are consistent with the hypothesis that we are dealing with the rare trait marker effect, since a presumed set of stressors or pathological interventions (for which advanced maternal age is an indicator) has resulted in the increase in the rare trait (left-handedness). Turning back to the model illustrated in Figure 1,we note two parameters of particular interest. The first is the proportion of individuals whose handedness is pathologically shifted, while the second is the distribution of handedness if no pathology were present in the system. We can use the advanced maternal age data just described to estimate both of these parameters. Before doing this, it might be useful to agree upon some common nomenclature. We will use the term ittherent proportion to represent the proportion of the population who would show a particular handedness if their natural predispositions expressed themselves without disturbance in the targeted phenotype. By pathological proportion we will mean that segment of the population who expresses a particular phenotypic handedness that has been shifted from the naturally encoded side to the other side due to pathological factors. Now, if we represent the inherent proportion of right-handers (i.e., original proportion before it is distorted by pathology) as R, and the proportion of individuals who will be pathologically shifted in their handedness as S, then by working through the model in Figure 1, it’s quite easy to show that the proportion of left-handers with the pathological marker (which we will call PL) is (3)

P L = RS

The value Pu at least for advanced maternal age as the indicator of probable pathology, is known to us from the data set previously presented. It is simply the number of individuals with older mothers who are left-handed. If we want to solve for R and S (to determine the inherent population distribution and the pathological shift proportion) we have two unknowns, and therefore we need another equation. The present data give us a total number of left-handers in the

Rare Trait Marker Model

23

present population (i.e., pathological plus normal left-handers) that we will represent as TL‘This value is the natural proportion of left-handers (I - R) from which we subtract the number of pathological right-handers, (I - R)S, and to which we add the number of pathological left-handers (which is just RS, from equation 3 above). Thus we get T,

(4)

=

1 - R - S + 2RS

With two equations and two unknowns this set of simultaneous equations can be solved. When we do, we get the following solution for the proportion of inherent right-handers: K + (K2 - 4P,)’D

(5) I\

2

where: K = 1 - (T,

-

2PL)

and for the pathological shift factor we get:

There is a long history of empirical work that suggests that males and females differ in susceptibility to birth stress, with males typically faring worse (see Gualtieri and Hicks, 1985 for a review). Combined with the finding that males are more likely to be pathological left-handers (Searleman et al., 1989), the implication is that the pathological shift factor S should differ as a function of gender. There is another reason why we should consider the application of the model separately for males and females. Many investigators have looked at the distribution of handedness as a function of gender. In general, the existing literature suggests that when sex differences for handedness are found they usually indicate a higher percentage of left-handed males than females (e.g., Bryden, 1977; Clark, 1957; Enstrom, 1962; Hardyck, Goldman and Petronovich, 1975; Levy, 1976; Le Roux, 1979; Oldfield, 1971). Estimates of the size of the difference typically vary from 1 to 5 percent. Using the same measure of

24

Coren and Searleman

handedness that was employed in the advanced maternal age study reported above, Porac and Coren (1981) assessed the handedness of 5,147 individuals and discovered that 3.6% more of the females were right-handed than were the males. This suggests that the original proportion of natural right-handers (R in our terminology) may be different for males and females. In the advanced maternal age data it was observed that the percentage of left-handed males was 11.1% as compared to 9.5% for females, giving us 1.6% more dextral females. For all of these reasons, a separate analysis was conducted for each gender to solve for the parameters S and R using the rare trait marker model equations that were derived earlier. We must make a decision as to which set of data to apply the model to, since, as one can see from Figure 2, the risk of pathological shift seems to be changing in a continuous fashion as the mother’s age increases. Perhaps the simplest method of comparing the sexes is to select a maternal age and then calculate the theoretical values. For example, at maternal age of 35 years, 14.6% of the male left-handers had mothers over 35 years of age at the time of their birth as compared to only 8.8% of the male right-handers. For females the difference is smaller, with 11.1% of the left- handers having older mothers and only 8.1% of the right-handers. Applying Equations 5 and 6 to these data, we find that for the males the pathological shift percentage S amounts to 1.8% while for the females it is 1.2%. Note that the values produced by the equations are actually proportions, however for consistency with our previous discussions we will continue to convert the results to percentages when presenting the final product of our calculations. One advantage of presenting these values as percentages is that the value S itself is a risk factor indicating the percentage of the population that should show a pathological shift. Since S is a risk factor, it can be used to estimate the relative risk of a pathological shift in handedness for males and females separately. In this case, the relative risk is 1.5 (1.8%/1.2%) indicating that males are 50% more likely to be pathological left-handers than females. An alternative way to analyze these data would be to decide on a risk value as a threshold, and then compare males and females as soon as they surpass this threshold level. For example, if we select a likelihood value of pathological left-handedness of 1.5, and use it as the threshold level, we find that males first surpass this value when maternal age exceeds 33 years (although there’s a drop below this level at age 36 that we attribute to sampling error), while females don’t surpass this threshold until maternal age exceeds 36 years. At maternal age of 33 years, 24.0% of the left-handed males have the pathological marker (an older mother) while only 15.2% of the right-handed males do. At age 36, 11.1%

Rare Trait Marker Model

25

of the female left-handers have an older mother compared with only 5.9% of the female right-handers. Calculating the pathological shift percentage S produces a value of 3.0% for males and 1.2% for females. Thus the relative risk of pathological shift for males is actually greater, with a value of 2.5 (3.0%/1.2%), than in our previous calculations. Averaging the results of the two calculations of S for each gender reveals that the average shift factor for males is 2.4% and for females 1.2%. For the present data, this indicates that males are approximately twice as likely as females to be pathologically shifted to left-handedness. Until now we have only focused on the likelihood of pathology. Recall, however, that Equations 5 and 6 also permit calculation of the proportion of inherent right-handers (R)if there is no pathological intervention. This is an estimate of the initial population distribution depicted in Figure 1. The original percentage of male right-handers is estimated to be 91.3% and 90.3% respectively, basing our estimates upon data from the maternal ages 33 and 35 that were used in the earlier computations. This provides a mean estimate of 90.8% inherent right-handed males. For females, both computations produce the same estimate, namely 91.4%. This means that females are only 0.6% more right-handed than males. This value is considerably less than one might expect on the basis of simple consideration of the empirical data. For instance, it is only about one third the 1.6% difference found in the raw distributional data from this sample, and one sixth the 3.6% reported by Porac and Coren (1981) using the same measurement scale on a larger, more age heterogeneous sample. The finding of little difference in R as a function of gender has theoretical importance. It suggests that females are just slightly more biased towards dextrality than males, perhaps due to genetics or to the fact that females are somewhat more susceptible to cultural pressures to change handedness (Porac, Coren, and Searleman, 1986). If males and females were to develop without any pathological interventions, the distribution of handedness for the two sexes would be roughly equivalent. We believe that it’s primarily the occurrence of pathological insult that significantly shifts the final phenotype toward increased sinistrality in males.

Is Left-Handedness a Good Marker for Pathology? We have seen that left-handedness does seem to be a reasonable marker for pathology, simply based upon the operation of the statistical mechanisms

26

Coren and Searleman

associated with the rare trait marker model. Are there any other reasons, other than the population split of roughly 90% to lo%, as to why left-handedness might be a useful marker for various psychological and neurological differences? We think the answer is yes. Left-handedness is also particularly useful as a neurological and psychological marker because the control of handedness is neurologically complex, involving a number of brain sites and neural control systems. Discussions of upper limb control by Brodal(1981), Harris and Carlson (1988), and Kupyers (1985) indicate that a variety of different neurological systems are involved. These include three motor systems that originate in the cerebral cortex, several subcortical sites, a sensory system, and several commissural systems. In the cortex, Brodman’s areas 1,2,3,4,5, and 6 have all been implicated in upper limb control. The corticospinal or pyramidal tract, the ventromedial brain stem system (including the reticule-spinal tract) and the lateral brain stem system (including the rubrospinal pathway) all play roles in manual control. Among the subcortical sites mentioned as playing a role in the control of hand use are the basal ganglia structures. The corpus callosum, which transfers information between the two cerebral hemispheres, also seems to play a role in hand control. Sensory systems include, at the minimum, proprioceptive and kinesthetic systems in the postcentral cortex. The interesting issue for left-handedness as a marker for pathology is that pathological insult or malfunctions that affect any of these sites or systems also seem to affect the natural development of handedness. In other words, pathological left-handedness may arise from damage to the cortex, motor systems, sensory systems, or commissural pathways. This suggests that left-handedness is of particular interest because the genesis of pathological shifts in hand dominance may occur due to insult to many different neural centers and systems. This would, of course, increase the sensitivity of left-handedness as a marker for minor neurological damage, that might, in turn, contribute to a number of different behavioural problems. The same minor pathology that causes left-handedness might also interfere with normal learning or language processes, produce emotional, neurotic, or psychotic symptoms, or reduce adaptive and survival ability in a number of different ways. Of course, the large number of possible sites for pathology does have a negative component, in that it also means that there is little specificity (in terms of indicating locus of any lesion) in left-handedness when it is used as a marker for pathology. While the above discussion suggests that left-handedness might be particularly useful as a generalized indication of increased risk for pathology, the

Rare Trait Marker Model

27

actual mechanism that makes it likely that a higher proportion of left-handers will show such effects remains the original asymmetrical distribution of handedness in the population. Thus, the complex neural control associated with handedness makes it highly vulnerable to pathological interventions, however, the fact that it is originally a relatively rare trait is what makes it useful to serve as a marker for increased risk.

Conclusions and Future Directions In this chapter we have presented the Rare Trait Marker Model to illustrate its operation in the context of the relationship between birth stressors and the appearance of left-handedness. The Rare Trait Marker Model is a statistical model that does not rely upon any particular mechanism but simply operates upon a population in which there is an asymmetrical distribution of traits, with one trait or set of traits rarer than the others. The rarer the trait the more strongly it will serve as a potential marker for pathology, where pathology is defined as any situation, condition or intervention that 'will prevent the development of the naturally (genetically) targeted trait, hence resulting in the appearance of its counterpart. To demonstrate the operation of this model, we made a few simplifymg assumptions. To begin with, we presumed that there were only two phenotypically visible traits (in this case, left- and right-handedness). We further simplified by assuming that the traits were dichotomous rather than continuous. In addition, it was presumed that the pathological intervention was either present or absent, and we did not view it as being a continuous variable. All of these assumptions make the computations and the illustration of the operation of the model easier. It should, however, be possible to work out a continuous variable version of the model in which the common trait would be "strong right-handedness" and the stressors would be graded in intensity with stronger stressors producing a larger percentage of shift. If such were the case, the model would have to be changed somewhat to present the pathological shift factor (S) in our model's equations as a function, rather than as a constant. For the purposes of working out our example, we used only one indicator of elevated risk of birth stress, namely advanced maternal age. We are not, of course, arguing that this is the only pathological risk marker that is important in shifting handedness. It is merely a convenient data base that allows us to

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illustrate the operation of the model and also gives us the opportunity to present some hitherto unpublished data that also serve to link birth stress factors with handedness. Presumably, some composite index of birth stressors, incorporating a number of different stressors, would produce a more stable estimate of the total pathological shift factor in the population. This means that our computations of S are most assuredly underestimates of the population value of S, although they would likely be a good indicator of the salience of advanced maternal age as a single risk factor. To that extent, the computational procedures might prove to be a good means for estimating the likelihood that left-handedness would result from other manifestations of birth stress. Each potential risk factor would then produce a particular S value, indicating the influence it would have in shifting the subject’s handedness. The use of the rare trait marker model produced other outcomes of theoretical interest. To begin with, we expected, and the computations confirmed, that males are more susceptible to pathological interventions than are females. In fact, based on the present data set, the relative risk of left-handedness as a function of increasing maternal age is twice as large for males as for females. The data that was presented in Figure 2, however, could indicate that males and females do not differ qualitatively in terms of their susceptibility to stressors, but may differ only in threshold, with females requiring a greater “dose” to trigger the effects. Thus females, just like males, do show elevated risk with increasing maternal age, however the effect requires an older mother (increased stress risk). One unexpected finding of theoretical interest was that the original distribution of right-handedness (R) was very similar for males and females. The usually observed population differences showing significantlymore right- handed females may not be primarily a function of genetic endowment or to greater susceptibility to social/cultural pressures to switch from left-handedness to right-handedness. Instead, much of the difference in the incidence of right-handedness between the sexes could be due to the fact that males have an increased risk for becoming pathological left-handers. Let us conclude by noting that the operation of birth stressors upon the development of handedness does appear to be reasonably established. If one uses the Rare Trait Marker Model, one need not look for specific mechanisms to establish how handedness shifts as a function of the pathological intervention. Simple disruption or retardation of the normal, genotypically programmed, maturational pattern could account for the shift in hand dominance; alternatively, any of a variety of diverse lesions or malfunctions in the complex of systems

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associated with manual control would work just as well. In any event, we have demonstrated that application of this model allows the assessment of the likelihood of developing left-handedness for any specific stressor, and allows us to estimate the initial proportion of left- and right-handedness in a target population prior to the intervention of any pathological factor.

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Acknowledgements This research was supported in part by grants from the British Columbia Health Care Research Foundation and the Natural Sciences and Engineering Research Council of Canada. The authors would like to acknowledge the assistance of Wayne Wong, Joan Donelly, Geof Donelly and Dereck Atha, who assisted in the collection of these data. We would also like to acknowledge the mathematical assistance of Dr. Lawrence M. Ward of the Psychology Department of the University of British Columbia.

References Bakan, P. (1971). Handedness and birth order. Nature, 229, 195. Bell, R.Q, & Waldrop, M.F. (1982). Temperament and minor physical anomalies. In Porter, R. & Collins, G.M. (eds). Temperamental differences in infants and young children (pp. 206-219). London: Pitman. Blau, A. (1946). The master hand. New York: American Orthospychiatric Association. Brewster, E.T. (1913). The ways of the left hand. McClure’s Magazine, 168-183. Brodal, A. (1981). Neurological anatomy in relation to clinical medicine (3rd edition). New York: Oxford University Press. Broman, S.H., Nichols, P.L. & Kennedy, WA. (1975). Pre-school ZQ: Prenatal and early developmental correlates. Hillsdale, NJ: Erlbaum. Bryden, M.P. (1977). Measuring handedness with questionnaires. Neuropsychologia, 15, 617-624. Burt, C. (1937). The backward child. London: London University Press Campbell, M., Geller, B. Small, A.M., Petti, T.A. & Ferris, S.H. (1978). Minor physical anomalies in young psychotic children. American Journal of Psychiatry, 135, 573-575. Clark, M.M. (1957). Left handedness: Laterality characteristics and their education implications. London: University of London Press. Corballis, M. (1983). Human laterality. New York: Academic Press. Corballis, M.C., & Morgan, M.J. (1978). On the biological basis of human laterality, I: Evidence for a maturational left-right gradient. The Behavioral and Brain Sciences, 2, 261-336. Coren, S., Porac, C., & Duncan, P. (1979) A behaviorally validated self- reported inventory to assess four types of lateral preference. Journal of Clinical Neuropsychology, 1, 55-64. Coren, S., & Porac, C. (1978) The validity and reliability of self-report items for the measurement of lateral preference. British Jounial of Psychology, 69, 207-211. Court-Brown, W.M., Jacobs, PA. & Tough, I.M. (1967). Some types of information obtainable from chromosome studies on defined population

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groups. In Human Radiation Cytogenics: Proceedings of an International Sytnposiuni (pp. 115-121). New York: Wiley. Enstrom, EA. (1962). The extent of the use of the left hand in handwriting. Journal of Educational Research, 5.5, 234-235. Fliess, W. (1906). Der Ablauf des Lebens. Vienna: Deuticke. Gordon, H. (1921). Left-handedness and mirror writing, especially among defective children. Brain, 43, 313-368. Gualtieri, T., & Hicks, R.E. (1985). An immunoreactive theory of selective male affliction. The Behavioral and Brain Sciences, 8, 427-441. Hardyck, C., Goldman, R., & Petrinovich, L. (1975). Handedness and sex, race, and age. Human Biology, 47, 369-375. Harris, L.J. & Carlson, D.F. (1988). Pathological left-handedness:An analysis of theories and evidence. In. Molfese, D.L. & Segalowitz, S.J. (eds.) Brain lateralization in children: Developntental iniplications (pp. 289- 372). New York: Guilford Press. Jordan, H.E. (1922). The crime against left-handedness. Good Health, 57, 378383. Krouse, J.P. & Kauffman, J.M. (1982). Minor physical anomalies in exceptional children: A review and critique of research. Journal of Abnormal Child P~ycltology,10,247-264. Kupyers, H.G.J.M. (1985). The anatomical and functional organization of the motor system. In Swash, M. & Kennard, C. (eds.), Scientific basis of clinical neurology (pp 3-18), Edinburgh: Churchill Livingstone. Le Row, A. (1979). Sex differences and the incidence of left-handedness. Journal of Psychology, 102, 261-262. Lesinski, J. (1975). High risk pregnancy: unresolved problems of screening, management and prognosis. Obstetrics and Gynecology, 46, 599-603. Leviton M. & Montagu, A. (1971). Textbook of Human Genetics. New York: Oxford University Press. Levy, J. (1976). A review of evidence for a genetic component in the determination of handedness. Behavior Genetics, 6, 429-453. Lombroso, C. (1903) Left-sidedness. North American Review, 170, 440-444. Matsunaga, E. (1973). Effect of changing parental age patterns on the chromosomal aberrations and mutations. Social Biology, 20, 82-88. Molfese, D.L. & Segalowitz, S.J. (eds.) (1988). Brain lateralization in children: Developniental implications. New York: Guilford Press. Montagu, A. (1962). Prenatal Influences. Springfield, Ill.: Charles C Thomas. Morgan, M.J., & Corballis, M.C. (1978) On the biological basis of human laterality, 11: The mechanisms of inheritance. The Behavioral and Brain Sciences, 2, 270-277. Oldfield, R.C. (1971). The assessment and analysis of handedness: the Edinburgh inventory. Neuropsychologia, 9, 97-113. Polednak, A.P. (1976). Paternal age in relation to selected birth defects. Human Biology, 48, 727-739. Porac, C., & Coren, S. (1981). Lateral preferences and human behavior. New York: Springer.

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Porac, C., Coren, S., & Searleman, A. (1986). Environmental factors in hand preference formation: Evidence from attempts to switch the preferred hand. Behavior Genetics, 16, 251-261. Redlich, E. (1908). Epilepsie and linkshandigkeit.Archives Psychiatria, 44, 59-83. Satz, P. (1973). Left-handedness and early brain insult: An explanation. Neuropsychologia, 11, 115-117. Satz, P. (1972). Pathological left-handedness: An explanatory model. Cortex, 8, 121-135. Satz, P., Orsini, D.L., Saslow, E., & Henry, R. (1985). The pathological lefthandedness syndrome. Brain and Cogriitiori, 4, 27-46. Searleman, A,, Porac, C., & Coren, S. (1989). The relationship between birth order, birth stress handedness and lateral preference: A critical review. Psychological Bulletin, 105(3), 397-408. Selvin, S. & Garfinkel, J. (1972). The relationship between paternal age and birth order with the percentage of low weight infants. Huniari Biology, 44, 501-510. Silva, DA., & Satz, P. (1979). Pathological left-handedness: Evaluation of a model. Brain arid Language, 7, 8-16. Wile, I.S. (1934). Haridedness: Right arid ref. Boston: Lathrop, Lee & Shepard. Woodruff, C.E. (1909). Expansion ofraces. New York: Rebman & Co.

LEIT-HANDEDNESS Behavioral Implications and Anomalies, S. Coren (Editor) 0 Elsevier Science Publishers B.V. (North-Holland), 1990

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Chapter 2

NonRight-Handedness and the Continuum of Reproductive Casualty Paul Bakan Simon Fraser University “the history of man for the nine months preceding his birth would probably be far more interesting and contain events of greater moment, than all the three score and ten years that follow it.” S. T. Coleridge

The Continuum of Reproductive Casualty In 1973 I reported (Bakan, Dibb and Reed, 1973) an association between pregnancy and birth complications (PBCs) and nonright-handedness (NRH). Left-handed and ambidextrous subjects were twice as likely as right-handers, to report PBCs associated with their birth. On the basis of the excess of PBCs among NRH subjects, we suggested that NRH is a result of prenatal or perinatal brain insult. In an earlier paper (Bakan, 1971) a relationship between NRH and PBCs was proposed to account for an observed excess of NRH among first born, and fourth or later born subjects, i.e. births to older mothers; these birth orders are associated with an excess of PBCs. On the basis of these results we concluded that NRH is one manifestation of a “continuum of reproductive casualty” (Pasamanick and Knobloch, 1966). The continuum of reproductive casualty is the result of untoward prenatal or perinatal events suffered by infants “whodo not die, but depending on the degree and location of trauma, go on to develop a series of disorders, extending from cerebral palsy, epilepsy, and mental deficiency, through all types of behavioural

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and learning disabilities, resulting from lesser degrees of damage sufficient to disorganize behavioral development." The events leading to reproductive casualty result from a variety of factors, including maternal-fetal blood type incompatibilities,maternal endocrine disorders,diabetes, radiation, maternal age, chronic stress, drugs, including nicotine and alcohol, infection, and hypoxia (Pasamanick and Knobloch, 1966). Though I was familiar with the concept of a "continuum of reproductive casualty," I was unaware of the interest of Pasamanick and Knobloch in the problem of NRH (Pasamanick and Knobloch, 1966). They concluded, after reviewing twelve manifestations of the continuum of reproductive casualty, that left-handedness "may turn out to be a thirteenth condition within the continuum." They said: It had been our clinical impression that we encountered significantly more left-handedness in preschool age children on whom a diagnosis of brain injury had been made. We felt that it was possible that if the injury was confined largely to the left motor cortex and its efferent system..., the child would... during the maturation of fine motor behavior tend to prefer the left hand ... In the sample of children...we examined the relationship of ambidexterity (non-established handedness) at three years to that of a diagnosis of brain injury made at 40 weeks. A statisticallysignificant association being found....we would like to confirm our clinical impression by examining the relationship of ambidexterity and left-handedness to diagnosis of brain injury in the large cohort of neurologic cases.. Secondly we intend to follow the pattern of investigation we have used before, i.e. to secure an experimental group of left-handed individuals from a school system with a control group of right-handed children from the same classrooms and compare perinatal events in both. (Pasamanick and Knobloch, 1966).

I have been unable to find a published report of the proposed studies. The design described for the retrospective comparison of right and left-handers with respect to PBCs, is essentially the one used in our study (Bakan, Dibb, and Reed, 1973), and the results are essentially those predicted by Pasamanick and Knobloch (1966).

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Implications of the Reproductive Casualty Context for NRH NRH takes on new meaning and significance, when considered in the context of reproductive casualty. This context provides new questions, hypotheses, and experimental approaches with which to advance the understanding of NRH. The context of reproductive casualty broadens the range of variables relevant to the etiology of NRH. Considering NRH in the context of reproductive casualty, is analogous to what happened in the history of the science of nutrition, when isolated syndromes such as pellagra, or scurvy, were understood to be manifestations of malnutrition or vitamin deficiency. Just as pellagra or scurvy are better understood as manifestations of malnutrition, so NRH may be better understood as a manifestation of reproductive casualty. Variables known to be involved in other forms of reproductive casualty can be examined in terms of NRH. Such variables include obstetric complications, hypoxia, birth weight, maternal smoking, use of alcohol and other drugs, and malnutrition. To the extent that some of these factors are associated with socio-economic variables, the context of reproductive casualty broadens still further the range of variables relevant to NRH. The context of reproductive casualty leads to investigation of relationships between NRH and other pathological conditions. It helps to account for numerous relationships between NRH and other manifestations of reproductive casualty, such as epilepsy, learning disorders, attention deficit disorders, mental retardation, and congenital defects. The context of reproductive casualty implies that NRH is not merely a static aspect of the distribution of handedness, but a behaviour subject to increase or decrease, by the action of variables that influence the rate of reproductive casualty.

Historical Approaches to Reproductive Casualty This section traces the history of interest in the relationship between prenatal/perinatal problems and subsequent morbidity. Some of the historical contributions have emphasized morbidity in general, and others have addressed particular outcomes, such as congenital malformations, epilepsy, mental retardation, cerebral palsy, and NRH.

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The Old Testament One of the earliest theories of NRH is found in the Old Testament. The Book of Judges, best known for the story of Samson and Delilah, is not generally noted for a "theory" of the etiology of NRH. But Judges 20:16 relates, that among the inhabitants of Gibeah, "there were seven hundred chosen men, ...left-handed; every one could sling stones at a hair breadth, and not miss." (Jewish Publication Society, 1955). This is one of only two uses in the Old Testament of a Hebrew phrase, ifteryud yemino, translated "left-handed." The other use of the phrase also appears in the Book of Judges (3:15), when "the Lord raised them up a saviour, Ehud, ...a man leff-handed." The left-handed Ehud girded a sword on his right thigh, so that his enemy, King Eglon of Moab, was caught off guard, and was killed by the sword in Ehud's left hand. Translators disagree about the translation of itteryadyemino as "left-handed." In the Septuagint and Vulgate translations, itter yud yemino is translated as "ambidextrous." The difference in translations reflects the fact that the phrase does not contain the Hebrew word for "left," shntol. The word yad means "hand," yemino means "right," and the phrase itter yad yeniino means "a right hand which is itter;" it says nothing at all about the left hand. The implied biblical theory of pathological left-handedness is based on the word itter (spelled afr in Hebrew). This word means impeded, shut-up, or maimed, and may have its origin in the Egyptian word am, meaning injury. The two passages in the Book of Judges refer to a pathological inadequacy of the right hand, and this has been translated either as "left-handed," or "ambidextrous." Where the Bible uses the word "left" to mean direction, as in left hand or left side, the word used is shniol. But in the two cases translated as "left-handed or "ambidextrous," the word itter is used to describe a pathologically inadequate right hand. Left-handedness or ambidexterity is deemed due to inadequacy of the right hand. The biblical theory of pathological left-handedness is further implied in Leviticus (21:17-21), where the rules for selecting priests to perform sacraments in the Holy Temple are described. The basic principle is that a priest with a blemish or physical defect may not be selected. The operational definition of blemish is then given as "a blind man, or a lame, or he that hath anything maimed, or anything too long, or a man that is broken-footed, or broken-handed, or crook-backed, or a dwarf, or that hath his eye overspread, or is scabbed, or scurvy" (Jewish Publication Society, 1955). The issue of the fitness of a priest to serve in the Temple is again discussed, with reference to handedness, in the Talmud, a set of rabbinical commentaries

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on Biblical matters. In the Talmud the word itter is used as a noun, meaning "a left-handed person." This is based on what Preuss (1978, p. 309) considers the "incorrect translation" of itter yud yentino as "left-handed." According to the Talmud, a priest who is left-handed or left-footed, is not suited to serve in the Temple, because these conditions are deemed to be a blemish. The rabbis argue about the acceptability of an ambidextrous priest. One of the rabbis, considers an ambidextrous priest unfit, because he interprets ambidexterity as the result of an abnormally weak right hand. Other rabbis deemed equality of strength of the two hands to be the result of a strong left, rather than a weak right hand, and thus interpreted ambidexterity as non-pathological (Preuss, 1978, p. 309). But they all agree that if left-handedness or ambidexterity is due to a weakness or inadequacy of the right hand, the priest has a blemish, and thus is not fit to serve in the Temple. Ancient India: Caraka A collection of medical writings by the physician Caraka, who lived several centuries B.C., is central to the Ayurvedic medical tradition of Ancient India. Caraka emphasized the importance of prenatal factors in determining developmental outcome. He distinguished between genetic and congenital determination. Congenital abnormalities, he considered as either genetic, or the result of intrauterine factors. Caraka believed that conception involves the semen, the ovum and "the spirit which takes place in the womb." This "spirit" refers to the quality of the intrauterine environment. Caraka believed that congenital malformations, and sensory abnormalities result from poor intrauterine conditions due to "defects of the mother's diet and behavior during gestation." He urged physicians to understand the factors "which are helpful in the formation and development of the fetus, and those which are inhibitive of such formation and growth (Lele, 1986, pp. 165-167). Plato Plato believed there were two circuits or circles in the head, a circle of the Same, and a circle of the Different. This theory, appears to be a precursor to the doctrine of functional hemispheric asymmetry. Plato thought that rotational disturbances of the circles resulted in irrationality, the inability to make judgments about what is the same and what is different among things. Plato

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believed that the trauma of birth deranges the circles, and that the derangement can be alleviated or repaired by a proper education (Plato, 1971; Taylor, 1962). Another pathological result believed due to derangement of the circles in the head by birth trauma, was epilepsy. Interestingly, epilepsy is one of a number of pathological conditions associated with an excess of NRH. Plato himself did not suggest that NRH was related to the perturbations of the circles in the head. In fact he was an advocate of ambidexterity, and he attributed the weakness of the left hand to "the folly of nurses and mothers" in not encouraging the use of both hands equally (Lloyd, 1973, p.185). Sedgewick

James Sedgewick was an eighteenth century English apothecary who believed that many chronic diseases have origins in the prenatal period. According to Sedgewick, "half the ... chronical diseases with which we see children afflicted are only the secondary sighs and groanings, the evidential marks, and reproaches of parentive ill-spent life." He believed that "these consequences...will be brought on infants, by the debauchery of the mother ...so that ...the regulation of the mother, during her pregnancy, is an affair of the highest moment and consideration" (Plant, 1987).

Blondel James Blondel (1729), an eighteenth century obstetrician, also stressed the relationship between the prenatal environment and the welfare of the fetus. He suggested a continuum in fetal development, from loss of life, through feeble growth and weakness, to the healthy state. Among variables considered detrimental to the fetus he cited "distempers of the parents ..., great falls, bruises, and blows the mother receives,...the irregularity of her diet, ....immoderate dancing,...excess of laughing, frequent and violent sneezing, and all other agitations of the body..."And further: "The child may also suffer by the affections of the mother's mind. For the disappointment of what she desires is sufficient to make her uneasy...deprive her of sleep and quiet, and even of food, and...the child runs the risk for want of ...wholesome nourishment, to grow feeble and weak, and at last to lose its life."

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Little William John Little, in a series of lectures, and in a book entitled On the Nature and Treatment of the Deformities of the Human Frame (Accardo, 1989) considered the relationship between cerebral palsy, and perinatal events, in a series of 24 patients. He found spasticity associated with PBCs such as prematurity, difficult labour, instrument deliveries, asphyxia, and convulsions. Little delivered a paper to the Obstetrical Society of London (Little, 1861), in which he anticipated the idea of a continuum of reproductive casualty. He expressed surprise that previous medical authors seemed "quite unaware that abnormal parturition, besides ending in death or recovery, not unfrequently had ...a third termination in other diseases." He proposed a spectrum of long term deformity and disability, secondary to biochemical insult, acting most especially on the brains of "too early and unripe-born foetuses." Accardo (1989) reports that in the discussion following Little's paper there was a reaction of disbelief and defensiveness about this "utterly novel viewpoint that at a single stroke causally united both mental and motor disability in later childhood with the few minutes surrounding birth." And he quotes Cameron (1958) who reported that "Little's original communication unhappily failed to rouse the medical profession's interest in the question on any great scale." Little's emphasis on "the few minutes surrounding birth as the critical time in the etiology of cerebral palsy was criticized by Sigmund Freud (1968, p.257), who argued that difficulties in the birth process were themselves a manifestation of a fetus, compromised earlier, during intrauterine development. The issue of the relative importance, of earlier intrauterine problems, as opposed to problems during delivery, is still an important one. The balance of current evidence increasingly supports prenatal insult as relatively more important than perinatal insult as a determinant of morbidity (Nelson, 1989). The neurologist W.R.Gowers (1888,1893) strongly supported Little. Gowers accepted Little's theory of perinatal etiology, and proposed its extension to a broader spectrum of brain damage syndromes. After describing the symptoms of major brain damage, he suggested that "for every case in which severe effects result from serious injury, there are many cases in which a slighter lesion has more trifling consequences." This statement anticipates the modern idea of a continuum of reproductive casualty.

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Lombroso and Degeneracy Theory

Lombroso's contribution to the idea of reproductive casualty, was based on his observations of a relationship between criminality, and signs of congenital pathology, which he called "stigmata." Lombroso's concept of the "born criminal" implies that criminality, or the congenital conditions favouring it, is a manifestation of reproductive casualty. His work is still echoed in modern research on the relationship between crime or delinquency and conditions associated with PBCs, including NRH (Andrew, 1978, 1980; Gabrielli and Mednick, 1980). Lombroso studied thousands of criminals, alive, and post-mortem, and noted among them an excess of anatomical anomalies. In view of the congenital nature of these anomalies and their increased frequency among criminals, he suggested a congenital determination of criminal tendency, in a class of criminals he described as "born criminals." Lombroso was impressed by the high frequency of congenital defects among criminals. These defects included retreating forehead, exaggeration of the frontal sinus and the supercilliary arches, oxycephaly, open internasal suture, anomalous teeth, facial asymmetries, fusion of the atlas, and anomalies of the occipital opening (Lombroso, 1911). He considered such anomalies as the "result of error in the development of the foetal skull, or a product of diseases which have slowly evolved in the nervous centers." He also found anomalies in the convolutions of the brain, higher frequencies of abnormalities of the foot, precocious wrinkles, absence of baldness, low and narrow forehead, large jaws, peculiarities of hair, iris, ears, nose, teeth heart, genitalia, stomach, and frequent left-handedness or

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ambidextenty

The handedness relationship was developed in a separate paper (Lombroso, 1903). Lombroso concluded that the excess of anomalies among criminals had a prenatal origin "by arrest of development or by disease acquired from different organs, above all, from the nervous centers.." Lombroso noted similarities between the epileptic and the "born criminal;" in fact he considered criminality as one of the manifestations of epilepsy. The various anomalies associated with criminality he considered as signs of degeneracy. Degeneracy was an important concept in late nineteenth century medicine. The term had negative emotional connotations. Lombroso, himself a Jew, had already defended Jews against their being labelled degenerate, during the first decade of the twentieth century, well before the rise of Hitler who used the term "degenerate" to label those he selected for elimination (Gilman, 1985, p. 156). The theory of degeneracy was formulated in 1857 by Benedict-Augustin Morel,

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one of the forefathers of modern psychiatry. Degeneracy, in its early usage, was associated with parental drunkenness, which according to Morel, produced congenital symptoms of depravity, alcoholic excess, and degradation in the first generation of offspring. This continues into the second and third generation, and by the fourth generation the line becomes extinct as a result of sterility (Plant, 1987). The concept of degeneracy came to be applied to a variety of conditions, including cretinism, masturbation, hysteria, madness, melancholia, epilepsy, and neurasthenia. Sigmund Freud, in his early writing, accepted the idea of degeneracy with respect to neurosis, which he felt had roots in prenatal life. By 1917, however, he was protesting the reliance of psychiatrists on concepts such as degeneracy, hereditary disposition, and constitutional inferiority (Gilman, 1985, pp. 205 ff.). In 1892 Mobius introduced the term "endogenous" to psychiatry. This emotionally neutral word began to replace the term degenerate, making it possible to refer to biologically determined congenital events without the negative connotation of terms like degenerate and degeneracy (Gilman, 1985, p. 277). Among the more recent additions to the continuum of reproductive casualty, is the fetal alcohol syndrome (Jones, Smith, Ulleland, and Streissguth, 1973), with symptoms resembling those noted by Lombroso in his "born criminals". The symptoms of fetal alcohol syndrome include prenatal growth retardation, facial anomalies, and central nervous system dysfunction. The syndrome is also associated with skeletal anomalies, cardiovascular defects, kidney anomalies, neural tube defects, and behavioural anomalies such as mental retardation, hyperactivity, sleep disorders and irritability (Abel, 1985). Though the relationship between fetal alcohol syndrome and handedness has not yet been studied, there is reason to predict an excess of NRH in victims of the syndrome (Zimmerberg and Riley, 1988). 1900-1910

During this decade, the problems of reproductive casualty, and the pathological aspects of NRH were of interest to others beside Lombroso. Charles Woodruff (1909) believed that ambidexterity and left-handedness resulted from problems in prenatal development. He suggested a relationship between sinistrality and nervous instability or neurosis. There was interest in the association between NRH and epilepsy (Redlich, 1908; Wallin, 1910). Ballantyne (1902) emphasized prenatal and perinatal factors in abnormal development. He

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believed that toxemia, bleeding during pregnancy, malnutrition, infection, fetal asphyxia, and trauma, were implicated in fetal disease, morbidity, and mortality. Wilhelm Fliess (1906), a physician, and friend of Sigmund Freud, postulated a relationship between handedness and sexual orientation. According to Fliess left-handed men display marked female secondary sexual characteristics, and left-handed women display marked male secondary characteristics. Effeminate men and masculine women, he believed, were more likely to be left-handed or ambidextrous. Fliess believed that left-handedness is symptomatic of incomplete sexual dominance, which might be expressed in homosexual behaviour (Harrington, 1988, p. 94). Fliess was influenced by the degeneracy theory, and considered the essence of degeneracy to consists in a displacement of the male and female qualities. He offered this as a reason why "so many left-handed people are involved in prostitution and criminal activities" (Fritsch, 1968, p. 133). Handedness was a topic in the correspondence between Fliess and Freud. In a letter to Fliess, Freud wrote "it seemed to me that you regarded me as somewhat left-handed, and if this were the case you would tell me, for such a revelation of myself would not hurt my feelings..Actually I am not aware of any preferences for the left, either now or in childhood. It would be more correct to say that there was a time when I had two left hands." In the same letter Freud also referred to himself as "right-left blind (Fritsch, 1978). The matter of a relationship between NRH and homosexuality has recently been resurrected by evidence of excess NRH among homosexuals (Lindesay, 1987). 1911-1950

During this period, an interest in the prenatal and perinatal determinants of schizophrenia was developing. Mackenzie (1912) considered the possible importance of "intrauterine disease" in the development of schizophrenia. He explored the possibility that some cases of schizophrenia were caused by prenatally induced brain deformity, whose effects did not appear until later in life. Turner (1912) believed that schizophreniawas due to developmental defects of the brain, interacting with the stresses of adult life to produce symptoms. Southard (1915) did post-mortem studies of the brains of schizophrenics, and reported morphological abnormalities in the frontal area. He concluded that schizophrenia results from embryonic maldevelopment of the frontal brain. Rosanoff (Rosanoff, Handy, Plesset and Brush, 1934) proposed interaction between genetic factors and early brain injury in the pathogenesis of

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schizophrenia. He likened schizophrenia to epilepsy, in so far as both might be late sequels of birth trauma. This organic view, which has gained support in recent years, was in the 1930's, at a relative disadvantage because of the popularity of psychodynamic or psychoanalytictheories of mental disease. Dalen (1988) in a paper emphasizing PBCs in psychopathology writes: "The idea that some cases of early brain damage only become manifest as psychological disorders in adult life is not new, but it has been kept out of the mainstream of psychiatric thought for a very long time." Quinan (1930) addressed the relationship between handedness and schizophrenia. Wile (1934) in a book on left-handedness, concluded that handedness may be affected by non-hereditary causes, such as factors in embryonic development, birth injuries to nerves, muscles, or brain, and neonatal difficulties. Wile also reviewed the relationship between NRH and cognitive problems such as writing defects, speech deficiencies, and reading disability. The relationship between atypical brain laterality and cognitive difficulties, especially dyslexia, was the focus of Samuel Orton during this period (Orton, 1937). At about the same time, Cyril Burt (1937, p.287) considered the relationship of left-handedness to learning disabilities. He considered left-handedness as "a mark of an ill-organized nervous system." Schwartz (1961) cites two German language papers bearing on the relationship between PBCs and handedness. In the first of these Brander (194O), argues that left-handedness is a persistent characteristic of prematurity, and in the second, Riemann (1949) considers left-handedness as a permanent "disability" following birth trauma. Gesell and Amatruda (1947) in a more general vein stated that perinatal cerebral injury might account for children with personality deviations, dullness, laterality problems, and various other forms of inadequacies and subclinical deficits. 1951-1970

It was during this period that Pasamanick and his associates developed the idea of a continuum of reproductive casualty. In a series of investigations, they found excessive physical and psychological morbidity in the offspring of compromised pregnancies. The continuum of reproductive casualty ranges from a lethal end, consisting of aborted fetuses, stillbirths, and neonatal death, to a variety of sublethal conditions of varying seriousness, including malformations, cerebral palsy, epilepsy, mental deficiency, autism, behavioural and neuropsychiatric disorders, learning disabilities, language problems, strabismus, accident proneness, increased variability, reduced threshold to stress, and as

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described earlier, NRH (Lilienfeld and Pasamanick, 1954; Pasamanick and Knobloch, 1960; Pasamanick and Knobloch, 1961; Pasamanick and Knobloch, 1966; Pasamanick and Lilienfeld, 1955). This wide range of problems, related to PBCs, was attributed largely to a vulnerability of the developing central nervous system to insults associated with pregnancy. Antecedent factors associated with morbidity include prematurity and low birth weight, maternal-fetal blood incompatibilities, maternal endocrine disorders, maternal age, hypoxia, radiation, infection, maternal smoking, alcohol intake, other drugs, and seasonal factors. These variables can effect pregnancy at any stage, in contrast to a prevailing belief that negative outcomes of pregnancy are due to the events around the delivery period. This issue had earlier been considered in a controversy between Little and Freud cited earlier. It was considered wrong, for example to equate breathing difficulty at birth with damage induced during delivery. Rather, prenatally determined pre-existing problems could interfere with adjustment to extra-uterine life and the initiation of respiration (Knobloch and Pasarnanick, 1962). Pregnancy complications believed to have the closest relationship to morbidity are the "prolonged and probably anoxia-producing" complications (Pasamanick and Knobloch, 196fj). This emphasis on atypical intrauterine factors, becoming manifest in perinatal complications, is supported in modern obstetrical thinking (Naeye and Peters, 1987; Nelson, 1989). Pasamanick and his associates drew, from their findings on reproductive casualty, important implications about prevention, socio-economic variables, and genetics. Manifestations of the continuum of reproductive casualty can, in principle, be prevented by any interventions which serve to reduce PBCs. It follows that prenatal medical care, optimal nutrition, and avoidance of harmful substances and harmful environments, can substantially reduce problems of prenatal development, and related reproductive casualty. In this sense the emphasis on reproductive casualty is basically optimistic, because it assumes that appropriate interventions, either medical or social, will reduce reproductive casualty, and the associated costs to society, families, and individuals. PBCs occur in a socio-economic context (Alvarez, 1982; Birch and GUSSOW, 1970; Oakley, Macfarlane, and Chalrners, 1982; Saugstad, 1989), as well as in a medical context. Many variables associated with less than optimal reproductive conditions, are also associated with poverty. Prevention of PBCs, and hence reproductive casualty, can be effected by interventions that minimize the negative effects of socio-economic disadvantage.

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This approach to reproductive casualty also leads to an emphasis on environmental rather than genetic determinants of prenatal maldevelopment, as reflected in the following passages (Pasamanick and Knobloch, 1966): little is known about the predisposing factors which might lead one mother to produce a damaged child while another exposed to the same complications, and even of the same severity, might have a normal offspring. There are some data to indicate that mothers who are themselves of lower- class origin and were exposed to lifelong deprivation from conception to adulthood, as indicated by their poorer physical growth, are the ones who are predisposed to produce children who will exhibit the ...disorders we are discussing. This predisposition is frequently on a post hoc basis, given a hereditary explanation by some writers. Epilepsy...what light do our results cast on the genetic hypothesis? ...It is reasonable to assume that if prenatal and perinatal factors play a significant role in the causation of some forms of epilepsy and genetic factors in others, our cases in which the pregnancy factors were absent should have had more epileptic parents than those cases in which these factors were present. This was not found to be true, and makes it necessary to re-examine the genetic hypothesis in epilepsy. May not the familial aggregation of epilepsy be a reflection of the occurrence of familial aggregation of the prenatal and perinatal factors under discussion...?

Recent Approaches to NRH and Pathology The relationship between NRH and other pathological conditions has been noted for a long time, especially for epilepsy and mental retardation (Pipe, 1988). Various theoretical positions have been taken in approaching this relationship (Porac and Coren, 1977). Since NRH often appears without other obvious pathology, a distinction between two kinds of NRH has been made, namely, pathological NRH and "normal" NRH. The issue is complicated by some evidence of familial factors in NRH, and this has encouraged genetic explanations. Often "normal" NRH is assumed to be genetic in origin. Genetic approaches construct models to account for the distribution of handedness. These models differ in assumptions about the kinds of genes involved. Thus, for example, a model may assume separate genes for right and left-handedness, or

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only a single right-handedness, or right shift gene, with different degrees of expression. Four theoretical positions which address the relationship between NRH and pathology are described, those of Satz, Bakan, Geschwind, and Annett. Satz: Two Kinds of NRH

Satz distinguishes between two kinds of NRH, pathological and normal (Satz, 1972; Satz, 1973). In the extreme, a person with left hemisphere damage, resulting in right hand or right arm paralysis, is forced to use the left hand for many functions. This is clearly a case of pathological left-handedness. More generally, early damage to one cerebral hemisphere results in impaired function of the contralateral hand which, if it is the right hand, results in pathological left-handedness or ambidexterity. This model also allows for pathological right-handedness, if right hemisphere damage results in left hand impairment (Satz, 1972). Satz and his associates have defined a clinical syndrome of pathological left-handedness which is defined by trophic changes in the extremities, reorganization of speech and visuospatial functions, early trauma and PBCs (Silva and Satz; 1979; Satz, Orsini, Saslow and Henry, 1985). This model considers the vast majority of left-handed people have normal or non-pathological handedness, where the left-handedness is due to genetic or cultural determinants. Pathological NRH, in this theory, requires evidence of actual or presumed pathology. When evidence of pathology is not apparent, NRH is considered "normal." A weakness of this model is the sharp distinction made between normal and pathological NRH. This dichotomy results from an overly restrictive definition of pathology. Pathological NRH, according to this model, has to be associated either with obvious pathology, such as paralysis of the right arm or hand (for pathological left-handedness), or other clear-cut evidence of brain dysfunction, such as epilepsy, mental retardation, or PBCs. There is a continuum of overt expression of pathology, where some pathological signs associated with NRH are obvious, such as right arm paralysis, and other signs, such as EEG or biochemical abnormalities are less obvious. In some cases pathology may be apparent, and in other cases pathology may be detectable only by EEG records (Shaw, Colter, and Resek, 1983) or biochemical assay. Pathology is no less pathological if it is less obvious. The criterion of clear evidence of pathology, when used to classify NRH as either normal or pathological, can be quite misleading. Furthermore, the pathologies associated with NRH, and possibly having a common or related etiology, may not be

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obvious, even when NRH is obvious, because they do not become manifest until later. For example, there is evidence of a relationship between NRH and early onset Alzheimer’s disease (Seltzer, Burres, and Sherwin, 1984). Both NRH and early Alzheimer’s disease may have a common precursor in prenatal or perinatal insult to the brain. Should the NRH be considered normal in early life, and pathological some years later when the symptoms of Alzheimer’s disease become obvious? Another condition related to NRH is schizophrenia (Chapman and Chapman, 1987; Cur, 1977; Katsanis and Iacono, 1989; Manoach, Maher, and Manschreck, 1988). Schizophrenia does not usually become apparent until early adulthood. If both NRH and schizophrenia have their origin in prenatal insult to the left hemisphere, is it necessary to wait until schizophrenic symptoms occur before considering the NRH as pathological? There may be other cases where mild brain abnormalities, induced by prenatal or perinatal insults, result in NRH without entailing other obvious signs of neurological pathology, or where symptoms of neurological pathology are greatly delayed, or where pathology becomes manifest in non-neurologicalsymptoms. To consider such cases of NRH as “normal“ or as genetically determined, is to ignore the subtleties of pathological expression.

Bakan: NRH as Reproductive Casualty Bakan (Bakan, 1971; Bakan, 1975; Bakan, 1978; Bakan, Dibb and Reed, 1973) regards NRH, when occurring in the absence of other signs of pathology, as a relatively prevalent and benign manifestation of prenatal or perinatal stress. He suggests that NRH be added to the list of conditions which define a continuum of reproductive casualty. This view is based on relationships between NRH and other forms of pathology, and on the excess of NRH associated with PBCs, even when other signs of pathology are not apparent. This theory holds that it is possible for PBCs to so affect the central nervous system, as to result in a functional reorganization,that produces NRH, without producing other signs of obvious pathology. What might be interpreted as “normal,” or genetically determined NRH in Satz’s model, would be interpreted as not “normal,”and environmentally produced, by Bakan. Bakan suggests that hypoxic insult to the developing nervous system, is a likely mechanism leading to a reorganization, resulting in reduction of right hand use or efficiency (Bakan, 1978). Pyramidal motor neurons, involved in control of hand use, are especially vulnerable to hypoxic damage (Lassek, 1954).

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Damage to these cells need not be associated with gross evidence of pathology. Pyramidal lesions may initially produce transient contralateral motor weakness, power impairment, spasticity and paralysis, followed by rapid and remarkable recovery, and a high degree of compensation, so that functional defects are difficult to detect (Lassek, 1954). Decreased efficiency of the right hand, leading to a change of hand preference, may be the only apparent residual of prenatal or perinatal hypoxia. The major difference between the Satz and Bakan approaches to pathological NRH Lies in Satz's considering most NRH as normally or genetically determined, while Bakan maintains that the determination of NRH results from the consequences of PBCs. Though Bakan does not completely rule out genetic influences in the determination of NRH, he does reject the simple notion of a gene or genes for NRH. Genetic factors could possibly operate by influencing maternal anatomy, intrauterine variables, or perinatal variables that increase the probability of NRH. Essentially Bakan's position rejects two kinds of NRH, normal and pathological, and more parsimoniously assumes one kind of NRH, associated with a continuum from lesser to greater expression of pathology. The hypothesized relationship between PBCs and NRH has stimulated a considerable amount of research and controversy in the neuropsychological literature. Much of this research has been done with non-clinical groups such a$ "normal" university students, without evidence of paralysis, spasticity, or other pathology. Even within such groups there is evidence for a relationship between NRH and PBCs. The evidence for non-clinical populations has recently been reviewed and subjected to a meta-analysis (Searleman, Coren, and Porac, 1989). Taking the studies together they found statistical evidence in support of a relationship between NRH and PBCs in non-clinical samples. Even where individual studies fail to obtain positive results, they very often show directional effects favouring the hypothesized relationship between PBCs and NRH (McManus, 1981). A recent study (VanStrien, Bouma, and Bakker, 1987), not included in the meta-analysis, provides further support for the relationship between NRH and PBCs. In a large non-clinical sample of students, which included 243 who wrote left-handed, there was a significantly raised incidence of PBCs. Left-handed subjects reported two or more PBCs (out of 23 possible) twice as often as right-handers. In another recent study (Schwartz, 1988) left-handedness was associated with significantly lower Apgar scores at birth; low Apgar scores are associated with hypoxia and increased incidence of neurological abnormality (Nelson, 1989). In a recent follow-up study of prematurely born children (Ross,

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Lipper, and Auld, 1987) evaluated at age four, 36% were NRH as compared to 20% for members a control group born at term. The parents of both groups showed the same incidence of right-handedness. The authors of this study conclude that intrauterine events and perinatal illnesses associated with prematurity probably affect the brain in ways which lead to NRH. The model of NRH etiology proposed by Bakan assumes that NRH is a result of PBCs, which result in reduced effectiveness of the right hand, leading to decreased use of the right hand, or NRH. These PBCs may produce other forms of atypical laterality, such as left footedness, crossed dominance, or other manifestations of reproductive casualty. In a more specific form of the model, the noxious event leading to NRH is held to be a reduction in oxygen supply to the fetal brain, and especially to the motor area of the left hemisphere, which is more vulnerable to the effects of hypoxia (Braun and Myers, 1975; Brann, 1989). Direct examination of the noxious events in the prenatal or perinatal environment is not usually feasible. Support for the model has been sought through indirect indicators of prenatal or perinatal stress, i.e. PBCs. There are statistical relationships between these indicators and hypoxic stress, or other kinds of stress, leading to brain and behavioural reorganization associated with NRH. Among the indicators examined are birth order, birth weight, breech birth, multiple births, pre-delivery bleeding by the mother, prematurity etc. Sometimes combinations of indicators are used, such as number of PBCs, or a measure of neonate viability such as the Apgar score. This approach has significant problems, such as inaccuracy of reports of PBCs by the mother or child, inaccuracy and incompleteness of medical records, and relatively low degrees of relationship between PBCs and any single pathological outcome. The timing of the noxious events leading to NRH also poses a problem. Complications may have different effects, depending on whether they occur early or late in the pregnancy, or whether they occur prenatally or perinatally. In sum, there may be a low level of relationship between general indications of stress, and the occurrence of the specific stress or stresses which become manifest as NRH. In a recent study, Bakan (1987) has used reports of maternal cigarette smoking during pregnancy, as an indicator of prenatal chronic hypoxic stress. It is generally agreed that maternal smoking during pregnancy results in a hypoxic state for the fetus (Socol, Manning, Murata, and Druzin, 1982). Maternal smoking also produces low birth weight, higher infant mortality, decrease in fetal breathing movements (Manning and Feyerabend, 1976), and an increase in PBCs. It was predicted that offspring of smoking mothers would be more likely

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to develop NRH because of the chronic hypoxia related to maternal smoking. This hypothesis was supported by a significantly higher frequency of NRH among the offspring of smoking mothers, than among the offspring of non-smoking mothers.

Geschwind The Testosterone Connection Geschwind and his associates (Behan and Geschwind, 1985; Geschwind and Behan, 1982; Geschwind and Behan, 1984; Geschwind and Galaburda, 1985) proposed another theory of NRH, which emphasizes problems in prenatal development. This theory assumes that, in most humans, there is an innate bias to left hemisphere dominance for language and handedness. Certain influences during fetal life can diminish this bias, to produce NRH. The major factor responsible for the events leading to NRH is prenatal testosterone. As a result of either increased concentration of testosterone, or abnormal sensitivity to it, development of the left hemisphere is delayed. This delay leads to altered lateralization and a shift to the left in the distribution of handedness. The atypical development of the left hemisphere also results in cognitive problems such as dyslexia, attention deficit disorders, learning disabilities, and mental retardation, each of which is also characterized by an excess of NRH. Matters are further complicated because of the close relationship between prenatal sex hormones and the developing immunological system. Testosterone not only retards the growth of the left hemisphere, but it also retards the development of the thymus gland, an essential part of the immunological system. Individuals who suffer insult to the left hemisphere are likely to suffer insult to the thymus. Diseases related to immunological dysfunction, are likely to occur more often among those with NRH. The importance of testosterone, a male hormone, also leads to the expectation of sex differences in susceptibility to the effects of testosterone. Geschwind and his collaborators have reported evidence for relationships between gender, NRH, and a variety of medical and psychological problems associated either with malfunctioning of the left hemisphere, or the immunological system. The theory has stimulated research, especially on the medical correlates of NRH, such as immune diseases. The theory places NRH in a pathological context. The Geschwind model emphasizes the importance of testosterone on development of the brain and immunological system. If excess testosterone, or excessive sensitivity to testosterone, has a noxious effect on lateral neurodevelopment, does this imply that testosterone is the only stimulant to the

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neurodevelopmental processes, leading to NRH? Might there be other mechanisms leading to NRH, or immunological problems? Furthermore, what is it that leads to excess testosterone production, or increased sensitivity to testosterone? Prenatal stress may occur in a variety of ways, including malnutrition, psychological stress, maternal fever, infection, alcohol, cigarettes, bright lights, noise, polluted air, high altitude, and hypoxia. Different stresses, occurring at critical periods during gestation, may produce atypical development in physiological and anatomical systems, leading to NRH, atypical immunity, or other pathological conditions, by mechanisms unrelated to testosterone. Though testosterone status may be associated with some of the causes of prenatal stress, it may not be the most likely, or the only factor associated with NRH or immunological pathology. It has been shown, for example, that prenatal stress can produce dopamine asymmetries (Fride and Weinstock, 1989) associated with changes in the laterality of tail positioning in rats. The stress of prenatal hypoxia has been shown to produce increases in adrenalin, noradrenalin, ACTH, and vasopressin, as well as reduced EEG voltage and reduction of fetal breathing movements (Dawes, 1976). Perhaps testosterone abnormalities are secondary to other forms of prenatal stress. Testosterone levels in humans are known to fluctuate in response to stress (Rose, 1984), and stress-induced changes in maternal levels of testosterone may influence fetal levels of testosterone. Maternal stress from bright lights and loud noise can alter plasma testosterone in the rat fetus (Ward and Weisz, 1980, 1984), and demasculinize the behaviour of male offspring. Testosterone may have an effect on lateralization in utero, but it may not be the exclusive mediator of these effects. Annett: Genetics or Pathology? The genetic theory of NRH proposed by Annett (Annett, 1985) is not ostensibly a theory of pathological NRH. Unlike some other genetic theories, it does not postulate a gene for left-handedness and another for right-handedness. The model assumes only a right shift gene, leading to preferred use of the right hand, right foot, and so on. When this gene is denied expression, the result is development of random dominance, ambidexterity, or left-handedness, i.e. NRH. A weakness of the theory is its failure to specify conditions which prevent expression of the right-shift gene. The theory assumes that chance factors or "accidental biases to the left hand" are involved.

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If an additional assumption is added to the theory, then it becomes a pathological theory of NRH. The additional assumption is that some noxious prenatal or perinatal event, be it hypoxia, testosterone, or whatever, leads to developmental reorganization, such that the normal expression of the right shift gene is prevented. In a sense Annett (1988) already implies this in her discussion of the excess NRH reported among homosexuals (Lindesay, 1988). She argues that in homosexuals there may be a blocked expression of the right shift gene, but she attributes this to anomalous delay in cerebral maturation, occurring in the third trimester of pregnancy or shortly after birth. This delay is a likely result of a pathological pregnancy complication. If all goes well and normally, the right shift gene is expressed; if not, its expression is prevented and NRH is the result. With the addition of this assumption the theory becomes compatible with that of Bakan and also that of Geschwind.

The Comorbidity Factor Cooccurrence of NRH with other forms of pathology is the key factor in the diagnosis of pathological NRH. Thus, the joint occurrence of hemiplegia in the right arm or hand, and left-handedness, supports an inference of pathological lelt-handedness. NRH often occurs jointly with mental retardation (Pipe, 1988), often the result of PBCs (Gray, Dean, Strom, Wheeler, and Brockley, 1989; Hicks and Barton, 1975; Naeye, 1987). The joint occurrence of NRH and mental retardation suggests pathological NRH. A similar analysis could be made for epilepsy, often characterized by early brain insult, and also associated with excess NRH. Sometimes the classification of NRH is complicated by temporal factors. As mentioned earlier, diseases associated with NRH, may not become obvious until some years after NRH becomes apparent. Excess NRH is found with schizophrenia, and early onset dementia (Alzheimer’s disease). Victims of either of these diseases may manifest NRH years before the symptoms appear. NRH may have its origin in the same or related insult to the brain that leads to schizophrenia or dementia, but the pathological nature of the NRH would not be apparent until the later appearance of symptoms. If NRH is present, without indication of comorbidity, then a history of PBCs may support the inference of pathological NRH. Sometimes NRH appears jointly with other atypical behaviours, where the relationship to brain insult is possible, but not obvious. This might be the case with sleep disorders, developmental disorders, delinquency, learning disabilities, or accident proneness.

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As additional pathological correlates of NRH are discovered, an increased proportion of NRH, will be attributable to pathological origin. The hypothesis that all or most NRH is pathological has heuristic value, since it encourages the search for correlates of NRH. Theories which emphasize pathological NRH, encourage research on the joint occurrence of NRH with other pathological conditions. Investigators look for excessive NRH in groups with pathological conditions, or they compare the prevalence of pathological conditions in groups with and without NRH. This section summarizes conditions for which excess NRH has been found. Global Correlates

Excess left-handedness was found among men rejected for military service by the U.S. selective service system (Karpinos and Grossman, 1953). Rejection from military service is usually for medical or cognitive problems, and constitutes a global indicator of such problems. Another global indicator of medical pathology is life span. In a study of deceased baseball players, it was found that left-handed players had a shorter life span than right-handed players (Halpern and Coren, 1988). This could be due to early failure of physiological systems, higher accident rates, use of substances such as alcohol or tobacco, higher rate of suicide, or poor adjustment to products designed for a right-handed majority (Coren, 1989; London, 1989). It has also been suggested (Brackenridge, 1981; Neale, 1988) that the secular trend toward increased left-handedness during this century may be due, in part, to the reduction in infant mortality resulting from medical advances. Decreased infant mortality may differentially favor survival of medically compromised and more vulnerable left-handers, resulting in a relative increase of left-handers in the population. The Psychopathology Connection

Certain forms of psychopathology have prenatal or perinatal origins. This can be inferred from the greater frequency of PBCs, anatomical peculiarities of the brain, and neurological soft signs, associated with psychopathology. Since excess NRH is also associated with psychopathology, it appears that the joint occurrence of NRH and psychopathology, is evidence for the related origins of both. The relationship between handedness, psychopathology, and PBCs is especially marked among schizophrenics. Schizophrenics often have a history of

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PBCs (Goodman, 1988;Goodman, 1989; Lewis, 1989). Brain imaging techniques often show abnormalities in schizophrenics (Silverton, Mednick, Schulsinger, Parnas, and Harrington, 1988). Ventricular enlargement, an indicator of hypoxia, and reduced brain size,is a frequent finding ( Volpe, 1987; Weinberger, Torrey, Neophytides, Klein, Rosenblatt, and Wyatt, 1979). Ventricular enlargement is also found with other neurodevelopmental disorders (Bergstrom, Bilk, and Rasmussen, 1984). Abnormal thickening of the corpus callosum, callosal agenesis, ischemic encephalopathy, arteriovenous malformations, and unusual gyral patterns are also more frequent in the brains of schizophrenics (Bigelow, Nasrallah, and Rauscher, 1983; Lewis, 1989). Schizophrenics also have an excess of physical anomalies, and neurological soft signs (Green, Satz, Gaier, Ganzell and Kharabi, 1989). They have a reduced life expectancy (Allebeck, 1989), and are at special risk for suicide, cardiovascular disease, and breast cancer (Harris, 1988). Schizophrenia is associated with various forms of motor dysfunction, resulting from prenatal developmental defects (Crayton and Meltzer, 1976, 1979; Manschreck, 1983; Scheibel and Kovelman, 1981). In addition left hemisphere pathology is associated with schizophrenia (Gruzelier and Hammond, 1976; Gur, 1977), and this is consistent with the excess NRH among schizophrenics (Chapman and Chapman, 1987; Chaugule and Master, 1981; Green, Satz, Smith, and Nelson, 1989; Gur, 1977; Katsanis and Iacono, 1989; Manoach, Maher, and Manschreck, 1988; Nasrallah, Keelor, Schroeder and Whitters, 1981; Piran, Bigler and Cohen, 1982; Shaw, Colter and Resek, 1983). Left-handed schizophrenics have significantly greater ventricular enlargement, and poorer neuropsychological test performance, than right-handed schizophrenics, suggesting a greater degree of cerebral dysfunction (Katsanis and Iacono, 1989). Excess NRH is also a feature of infantile autism or childhood schizophrenia (Barry and James, 1978; Colby and Parkison, 1977; Tsai, 1983). Excess NRH is also observed in affective disorders. There is an old case (Bruce, 1895) of double personality, in a patient alternating between a manic and a depressed persona, while switching between English and Welsh speech, and between right and left-handedness. Left-handedness appeared when the patient was in the depressive state. Sackeim and Decina (1983) found excess NRH in cases of bipolar depression. In a recent study (Bruder, Quitkin, Stewart, Marin, Voglmaier, and Harrison, 1989) a high incidence of left-handedness was found in patients with "non-melancholic atypical depression," i s . a depression where pleasure capacity is preserved, while at the same time there are symptoms of

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depression such as sleepiness, extreme bodily inertia (leaden paralysis), and rejection sensitivity. There is evidence for an excess of both PBCs and NRH among suicides, who are often depressed. Adolescent suicides were found to have a higher than normal occurrence of respiratory distress at birth, absence of early prenatal care, and increased maternal disease during pregnancy (Salk, Lipsitt, Sturner, Reilly, and Levat, 1985). Other variables more often found in the suicide group, included a history of premature births to the mother, abnormal bleeding during pregnancy, infection, problems with labour, and placental disorders. The authors of this study believe that infants who survive adverse perinatal conditions, are more vulnerable to the stressful conditions eliciting suicide. A similar result was found in another group of suicides where PBCs occurred more often than in a control group (Jacobson, Eklund, Hamberger, Linnarsson, Sedvall, and Valverius, 1987). The authors suggest that the effects of hypoxia and obstetric injuries lead to brain damage, which increases the likelihood of self-destructive behaviour. From these results it appears that suicidal behaviour is part of a continuum of reproductive casualty. In yet another study, excess NRH has been found among the victims of suicide (Chyatte and Smith, 1981). Once again there is a pattern of excess NRH, related to a form of psychopathology, which is characterized by an excess PBCs. Excess NRH has been reported for a number of other problems which vary in severity, but have in common a psychological component. Alcoholism is an example. An excess of NRH in a group of hospitalized alcoholics was noted by Bakan (1973), and confirmed by others (Chyatte and Smith, 1981; Harburg, 1981; London, Kibbee, and Holt, 1985; Nasrallah, Keelor, and McCalley-Whitters, 1983). Left-handed alcoholics are more resistant to treatment than right-handed alcoholics (London, 1985; Smith and Chyatte, 1983). It has been found that prenatal exposure of rats to alcohol reduces lateral asymmetry (Zimmerberg and Riley, 1988). Prenatal alcohol also produces a hypoxic effect on the fetus (Abel, 1985), and prenatal hypoxia might account for the joint occurrence of alcoholism and NRH in affected individuals. Since there is a relationship between alcohol consumption and cigarette smoking (Harburg, 1981), it is interesting to note that left-handers are more likely than right-handers to smoke and to smoke more heavily (Harburg, Feldstein, and Papsdorf, 1978). Certain personality problems are also related to excess NRH. Studies have shown increased NRH related to anxiety (Hicks and Pellegrini, 1978), anti-social behaviour (Standage, 1983), field dependence (Silverman, Adevai, McGough, 1966), alexithymia (Rodenhauser, Khamis, and Faryna, 1986), and emotionality

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(Harburg, Roeper, Ozgoren and Feldstein, 1981). There are also relationships between NRH and sleep disorder (Coren and Searleman, 1987), hyperactivity (Bakan, 1988), delinquency (Gabrielli and Mednick, 1980; Grace, 1987), psychopathy (Cuff, 1930; Fedora and Fedora, 1983), accident proneness (Coren, 1989), and homosexuality (Lindesay, 1987). Left-handers are less likely to marry, and more likely to divorce than right-handers (Lansky, Feinstein, and Peterson, 1988), suggesting a possibility of personality problems in social or heterosexual relationships. The Immunological Connection

The Geschwind model of NRH has stimulated interest in the relationship between NRH and diseases implicating the immunological system. An excess of, or sensitivity to, prenatal testosterone, suggested as the cause of delayed left hemisphere development, also impairs development of the thymus gland, and the immunological system (Geschwind and Galaburda, 1985). Infants who suffer prenatal growth retardation are born with reduced peripheral T-lymphocytes, indicating dysfunction of the thymus gland (Ferguson, Lawlor, Neumann, Oh, and Stiehm, 1974). The immunological system is asymmetrically represented in the hemispheres. Left hemisphere brain lesions result in reduction of T-cell mediated activity, whereas similar lesions in the right hemisphere do not have this effect (Barneoud, Neveu, Vitiello, and Le Moal, 1987). Left-handers show less peripheral lymphocyte activity than right-handers (Yokoyama, Hara, and Shoitsuki, 1987). In their research and review of this area, Geschwind and his collaborators have implicated various immunological diseases, including allergies, thyroid disease, rheumatoid arthritis, migraine, myasthenia gravis, and gastrointestinal diseases, such as celiac disease, ulcerative colitis, and ileitis. They provide evidence of relationships between immunological diseases and both NRH and certain developmental disorders as dyslexia, stuttering, delayed speech, childhood autism, hyperactivity, and other learning disabilities (Geschwind and Behan, 1982; Geschwind and Galaburda, 1984). The predictions concerning these relationships have not always been confirmed, but there is a body of evidence showing relationships between developmental, immunological, and laterality variables. Smith (1987) found an excess of left-handers among patients attending an allergy clinic. This was particularly marked for patients with urticaria and eczema. In another study (Weinstein and Pieper, 1988) an excess of NRH was again found in a group of

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allergic patients. There is also convergent evidence of relationships between allergies, PBCs (Bakan, 1977; Salk, Grellong, Straus, and Dietrich, 1974) and EEG abnormalities, suggestive of atypical brain function (Czubakski, Massakowski, Zawisza, and Makowska, 1979). Among other diseases with an immunological component, there is also evidence of excess NRH. Searlman and Fugagli (1987) report excess NRH in patients with Crohn’s disease, ulcerative colitis, and insulin dependent type 1 diabetes. There are more immune thyroid disorders among left-handers (Schachter and Galaburda, 1986; London and Glick, 1988). NRH may be related to migraine disease (Guidetti, 1987), and there is an insignificant trend to more NRH in patients with systemic lupus erythmatosis (Salcedo, Spiegler, and Magilavy, 1985). Cancer is a disease characterized by immunological dysfunction. It was found that the onset of breast cancer occurs about three years earlier in left-handed than in right-handed women (Kramer, Albrecht and Miller, 1985). There is as yet no study implicating handedness in AIDS disease, but there is some indication of a laterality/AIDS relationship (Bear, Agostini, and Saporta, 1988). AIDS patients exhibit a significant reversal of the typically greater right frontal lobe width (CT scan measure), found in a control group. These authors also cite unpublished findings of increased prevalence of childhood learning disabilities in homosexual men undergoing evaluation for AIDS. Excess NRH has been found among homosexuals (Lindesay, 1987). Reversals of normal asymmetry have been reported for nonright-handed subjects (Bear, Schiff, Saver, Greenberg, and Freeman, 1986). Other Correlates of NRH

Other correlates of NRH which have been observed include medical conditions, anatomic, physiological, and behavioural anomalies. Some diseases characterized by chromosomal pathology show excess NRH. This has been found in Down’s syndrome (Giencke and Lewandowski, 1989; Pipe, 1987), Turner syndrome, and Klinefelter’s syndrome, or XXY disease (Netley and Rover, 1982). Various anatomical anomalies are associated with NRH. The coexistence of congenital anatomical anomalies and NRH is further evidence for the pathology/NRH association. Anomalies include increased thickness of corpus callosum (Witelson, 1985;Witelson, 1989), cerebral asymmetry differences (Bear, Schiff, Saver, Greenberg, and Freeman, 1986), dermatoglyphic anomalies (Cummins, 1940; Jantz, Fohl, and Zahler, 1979), arteriographic anomalies

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(Hochberg, and LeMay, 1975), hare lip and cleft palate (Geschwind and Galaburda, 1985; Rintala, 1985), adrenal hyperplasia (Nass, Baker, Speiser, Virdis, Balsamo, Cacciari, Loche, Dumic, and New, 1987), and fusion malformations (Boklage, 1987). Twinning is related to NRH. There is excess NRH among twins, which has aroused considerable interest among geneticists and developmental psychologists (Boklage, 1984; Neale, 1988). Excess NRH appears for both identical and fraternal twins (Springer and Searleman, 1980). Twinning is associated with PBCs and low birth weight, which may predispose toward NRH (Segal, 1989). The very occurrence of twinning may be a pregnancy complication (James, 1977; James, 1983), in that intrauterine hypoxia has been shown to be a determinant of twinning. A number of studies have found differences in physiological variables, between NRH and right-handedness. Left-handers have lower monoamine oxidase levels (MAO) than right-handers (Coursey, Buchsbaum and Murphy, 1979). In the same study it was found that low MA0 levels are also associated with more psychiatric problems, more psychiatric problems in relatives, more criminal convictions, more experimentation with illegal drugs, and elevated scores on the MMPI. Alcoholics, a group with excess NRH also tend to have low MA0 activity. There is evidence that left-handers are more reactive to drugs which influence the brain (Irwin, 1985). Differences between right and left-handers have also been found in EEG measures (Chyatte, Abern, Reddy and Botticelli, 1979), which tend to be more abnormal for left-handers. Left-handedness is also associated with delayed physical maturation (Coren, Searleman and Porac, 1986). Other correlates of NRH include essential tremor (Biary, 1985 ), strabismus and other visual problems (Lessel, 1986), de la Tourette syndrome (Shapiro, Shapiro, Brunn and Sweet, 1978), sleep disorder (Coren and Searleman, 1987), clumsiness (Bishop, 1980), vegetarianism (Chyatte, Chyatte and Althoff, 1979), and Rorschach figure-ground anomalies (Finn and Neuringer, 1968).

NRH and Beyond NRH is considered as one of many outcomes of a less than optimal prenatal or perinatal environment, or as a manifestation of reproductive casualty. The emphasis in the laterality literature, and in this paper, has been on NRH, but similar arguments apply as well to other anomalies of laterality such as pathological right-handedness and crossed dominance. In pathological

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right-handedness excessive reliance on the right hand results from effects of PBCs, leading to weakness or inefficiency of the left hand. Examples of crossed d o m i n a n c e i n c l u d e right-handedness/left-footedness, o r right-handedness/left-eyednesscombinations. Pathological right-handedness and crossed dominance are possible components of a continuum of reproductive casualty. There is very little research on pathological correlates of pathological right-handedness (Annett and Manning, 1989), but there is more for crossed dominance (Krinicki and Nahos, 1979, Piran, Bigler, and Cohen, 1982; Trembly, 1968; Trembly, 1976; Waddy and Kirkby, 1976). The prevalent view considers NRH as either normal or pathological. It is considered pathological, if associated with certain pathologies, (e.g. hemiplegia, seizures), especially those implicating the left hemisphere. However, the literature shows a much wider range of pathological correlates of NRH than has traditionally been considered in the determination of pathological NRH. At the least, this suggests that NRH is more often pathological than has heretofore been believed. NRH should be considered pathological, not only if associated with things like right hemiplegia, seizures, mental retardation (Pipe, 1988) etc., but with an extensive range of problems including various forms of psychopathology, personality disorders, behaviour problems, learning and cognitive disabilities, alcoholism, immunological disorders, sensory and motor disorders, and physiological and anatomical anomalies.

The Hypoxia Connection Hypoxia is a major factor in the production of a suboptimal fetal environment (Nelson, 1989). Impaired delivery of oxygen and other nutrients through the placental-umbilical circulatory system is a prime factor in developmental alterations leading to reproductive casualty (Towbin, 1978; Volpe, 1987). Hypoxia sufficient to alter brain development, may impair other physiological and anatomical systems (Perlman, 1989). Recent research is uncovering specific mechanisms by which hypoxic insult produces reorganization in the brain and other systems of the body (Goodman, 1989; Herschkowitz, 1988; Witelson, 1989). These mechanisms include selective neuronal pruning, synapse elimination, and neuronal misconnections. Such systemic effects may initiate vulnerability to diseases which develop later in life. Diseases, not usually associated with reproductive casualty, may have precursors in fetal development. The reproductive casualty model may include

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more pathological conditions than has been previously envisaged. The model may be relevant to an understanding of visual dysfunction (Groenendaal, Van Hof-van Duin, and Fetter, 1988), cardiovascular disease (Barker, Osmond, Golding, Kuh, and Wadsworth, 1989; Barker, Osmond, Winter, Margetts and Simmonds, 1989), kidney disease (Perlman, 1989), cancer (Kramer, Albrecht and Miller, 1985) and other diseases. The particular applicability of the model of reproductive casualty to NRH is likely due to the special vulnerability of the left hemisphere to hypoxic stress (Brann, 1989; Braun and Myers, 1975), and the fact that one of the most common effects of hypoxia is a decrease in manual dexterity and fine motor coordination. Manual dexterity and fine motor coordination are normally characteristics of the right hand. Hypoxic risk is present during the entire intrauterine period and continues into the perinatal period. There are physiological mechanisms to protect the healthy infant from normal birth-related hypoxia (Volpe, 1987). However, prenatal hypoxia or malnutrition can so compromise the fetus, as to increase complications at birth and thus potentiate the normal tendency to hypoxia, as the infant switches from an umbilical to an atmospheric supply of oxygen (Naeye, 1987). Problems classified as "birth injury" or "birth stress" may be the result of prenatal pathology already well advanced prior to labour. Congenital effects sometimes considered genetic in origin, may in fact be related to the combined effects of intrauterine insufficiency, perinatal hypoxia, and birth trauma. The inclusion of NRH in the context of a continuum of reproductive casualty, suggests an extended list of variables for study in connection with NRH. The enlarged context provides a working hypothesis, namely, that variables associated with reproductive casualty might sometimes be associated with NRH as well. Variables suggested by the context of reproductive casualty include low socio-economic status (Lansky, Feinstein, and Peterson, 1988; Saugstad, 1989), stress, maternal malnutrition, smoking, alcohol consumption, consumption of other drugs, maternal health problems, high altitude hypoxia, polluted air hypoxia, blood type incompatibilities (Kocel, 1977), infection, fever (Shiota and Kayamura, 1989), prenatal coitus (Grudzinskas, Watson and Chard, 1979; Naeye, 1982), male sex of fetus, temperature and seasonal factors (Naeye, 1982; Shiota and Kayamura, 1989), paternal factors (Little and Sing, 1986), and anaesthetics (Kolata, 1978).

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NRH and Reproductive Casualty: Small Effects and Negative Results In a recent review of the relationship between NRH and birth stress (Searleman, Porac, and Coren, 1989), the authors conclude that there is significant positive evidence for the relationship, but that the relationship is weak for "non-clinical"populations. A meta-analysis of relevant studies revealed that "birth stressors...considered collectively, are likely to be related to increases in nonright-sidedness" and "that all of the relationships... were very weak... and accounted for less than 1%of the variance." If NRH is added to the larger class of pathological outcomes which constitute a continuum of reproductive casualty, then small effects and negative results are to be expected. Consider a 2 x 2 table, where one dichotomy is Right-handed (RH) vs Nonright-handed (NRH), and the other dichotomy is PBCs vs. No PBCs. This table yields four cells, RH/PBCs, RH/No PBCs, NRH/PBCs, and NRH/No PBCs. A statistical excess of cases in the NRH/PBC and RH/No PBC cells would clearly support the hypothesized relationship between NRH and PBC. If NRH is a manifestation of reproductive casualty, the logic of this argument becomes diluted. The RH group includes those with PBCs resulting in pathological outcomes other than NRH. It would also include people with pathological right-handedness,where very strong right-handedness is actually a manifestation of PBCs. Or it may include cross-dominants, those who are right-handed and left-footed, or left-eyed. Since there is an association between crossed dominance and PBCs, right-handed cross-dominants would show up in the RH/PBC cell. The result is that the RH/PBC cell frequency will be inflated, not because of absence of relationship between NRH and PBC, but because PBCs can lead to forms of reproductive casualty which are not associated with NRH. The model maintains that where there is NRH, there are PBCs; but where there are PBCs, there may or may not be NRH. Analogously, if a person is a nurse it is very likely that the person is a woman; but if a person is a woman, she may or not be a nurse. The following are some implications of the reproductive casualty model: a)

PBCs can produce a wide variety of pathological results in physiological or anatomic systems, leading to

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The particular result of PBCs is related to when, where, and how the embryo, fetus, or newborn is affected by the complications.

c)

If the PBCs affect areas unrelated to handedness, then any effect produced, though related to PBC, will not lead to NRH. (A similar statement would be applicable to footedness, eyedness, cross-dominance, pathological right-handedness etc.).

d)

If the PBC affects areas that influence the development of NRH, then NRH will result.

e)

The PBC may result in more than one pathological condition, and if the time, place, and degree of the PBC are appropriate, there will be correlations between the pathologies. An example would be joint occurrence of NRH and left-hemisphere oriented cognitivedifficulties, e.g. mental retardation.

f)

Among the various pathological results of PBC there will be varying degrees of correlation, or joint occurrence.

In sum, NRH is associated with PBC, but the measured relationship can be masked by the relationship between PBC and pathological conditions not associated with NRH. There will be varying degrees of relationship between NRH and other pathological manifestations of PBC. Consideration of NRH in the context of a continuum of reproductive casualty, seems to account for many of the facts about NRH, and offers leads for future research about NRH, and reproductive casualty in general. The model has implications that go beyond the considerations of other theoretical approaches to NRH. Because of the high frequency of occurrence of NRH, as compared to other pathologies associated with PBC, NRH frequency can be used as a comparative measure of adequacy of prenatal and perinatal

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conditions. If NRH has a pathological origin, then it follows that reduction of NRH is a worthwhile goal. As NRH is reduced, so other pathologies associated with PBC, are likely to be reduced, because PBC will be reduced. The model emphasizes variables that have not generally been of interest to students of laterality. These variables apply to maternal reproductive efficiency, and include things as maternal nutrition before and during pregnancy, maternal height and weight, maternal health, presence of toxic factors as alcohol, cigarettes, drugs, air pollution etc.,and psychological stress. The model also implicates the political and socio-economic factors which influence the availability of adequate nutrition, health care, and education. Such variables are influential in the reduction of PBCs, as well as many other pathological conditions on the continuum of reproductive casualty. Inclusion of NRH in the continuum of reproductive casualty suggests that a reduction of NRH would accompany a reduction of PBCs. Savings in costs of health care, education, and criminal justice, would more than compensate for the costs of preventing reproductive casualty.

Summary The occurrence of nonright-handedness(NRH) is considered in the context of reproductive casualty. NRH is viewed as one of the many possible results of pregnancy and birth complications (PBCs), which constitute the continuum of reproductive casualty. This continuum ranges from lethal outcomes, such as spontaneous abortion and stillborn births, to relatively minor outcomes. NRH, when unaccompanied by more serious problems, is a relatively benign outcome in a continuum of reproductive casualty. The role of prenatal and perinatal factors in the production of morbidity is a medical problem with a long history. This work is reviewed, especially as it relates to NRH. Four recent theories of pathological NRH are critically considered, those of Satz, Bakan, Geschwind, and Annett. The joint occurrence of NRH with other forms of morbidity is discussed as supporting the pathological origin of NRH. Hypoxia is considered an important factor in the development of NRH and other manifestations of the continuum of reproductive casualty. Research results on the relationship between NRH and PBCs, are discussed in the context of the continuum of reproductive casualty.

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Jantz, R.L., Fohl, F.K., and Zahler, J.W. (1979). Finger ridge counts and handedness. Human Biology, 51, 91-99. Jewish Publication Society. (1955). The Hob Scriptures: According to the Masoretic Text. Philadelphia: the Jewish Publication Society of America. Jones, K.L., Smith, D.W., Ulleland, C.N., and Streissguth, A.P. (1973) Patterns of malformation in offspring of chronic alcoholic mothers. Lancet (June 9), 1267-1271. Karpinos, B.D., and Grossman, H A . (1953). Prevalence of left- handedness among selective service registrants. Human Biology, 25, 36-50. Katsanis, J. and Iacono, W.G. (1989). Association of left- handedness with ventricle size and neuropsychological performance in schizophrenia. Aitierican Journal of Psychiatry, 146, 1056-1058. Knobloch, H. and Pasamanick, B. (1962). Mental subnormality. New England Jounial of Medicine., 266, 1045-1051. Kocel, K.M. (1977). Cognitive abilities: Handedness, familial sinistrality and sex. Annals of the New York Academy of Science, 299, 233-243. Kolata, G.B. (1978). Behavioral teratology: Birth defects of the mind. Science, 202, (Nov. 17), 732-734. Kramer, M.A., Albrecht, S. and Miller, RA. (1985). Handedness and the laterality of breast cancer in women. Nursing Research, 34, 333-337. Krinicki, V.E. and Nahos, A.D. (1979). Differing lateralized perceptual-motor patterns in schizophrenic and non-psychotic children. Perceptual and Motor Skills, 49, 603-610. Lansky, L.M., Feinstein, H. and Peterson, J.M. (1988) Demography of handedness in two samples of randomly selected adults (N = 2083). Neuropsychologia, 26, 465-477. Lassek, A.M. (1954). Tlte Pyramidal Tract: Its Status in Medicine Springfield,Ill.: C.C. Thomas. Lele, R.D. (1986). Ayurveda and Modern Medicine. Bombay: Bharatiya Vidya Bhavan. Lessel, S. (1986). Handedness and esotropia. Archives of Ophthalmology, 104, 1492-1494. Lewis, S.W. (1989). Congenital risk factors for schizophrenia. Psychological Medicine, 19, 5-13. Lilienfeld, A.M. and Pasamanick, B. (1954). Association of maternal and fetal factors with the development of epilepsy. I. Abnormalities in the prenatal and perinatal periods. Journal of the American Medical Association, 155, 719-724. Lindahl, E., Michelsson, K, Helenius, M. and Parre, M. (1988). Neonatal risk factors and later neurodevelopmental disturbances. Developmental Medicine and Child Neurology, 30, 571-589. Lindesay, J. (1987). Laterality shift in homosexual men. Neuropsychologia, 25, 965-969. Little, R.E. and Sing, C.F. (1986). Association of father’s drinking and infants birth weight. New England Journal of Medicine, 314, 1644-5.

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Little, W.J. (1861). On the influence of abnormal parturition, difficult labour, premature birth, and asphyxia neonatorum on the mental and physical conditions of the child, especially in relation to deformities. Transactions of the Obstetrical Society of London, 3, 293-344, abstracted in Lancet, 2, 19 October, 378- 379. Lloyd, G. (1973). Right and left in Greek philosophy. Chap 9 in R. Needham. Riglit and Left: Essays on Dual Symbolic Classifcation. Chicago: Univ. of Chicago Press. Lombroso, C. (1903). Left-sidedness. North American Review, 170, 440-444. Lombroso, C. (1911/1968). Crime: Its Causes and Remedies. Montclair, NJ: Patterson Smith. London, W.P. (1985). Treatment outcome of left-handed versus right-handed alcoholic men. Alcohol: Clinical and Experitnental Research, 9, 503-504. London, W.P. (1987). Cerebral laterality and the study of alcoholism. Alcohol, 4, 207-208. London, W.P. (1989). Left-handedness and life expectancy. Perceptual and Motor Skills, 68. 1040-1042. London, W.P. and Glick, J.L. (1988). Alcoholism, thyroid disorders, and left-handedness (letter). American Journal of Psychiatry, 145, 270. London, W.P., Kibbee, P., and Holt, L. (1985). Handedness and alcoholism. Journal of Nervous and Mental Disease, 173, 570- 572. Mackenzie, I. (1912). The physical basis of mental disease. Journal of Mental Science, 58, 405-477. Manoach, D.S., Maher, BA. and Manschreck, T.C. (1988). Left- handedness and thought disorder in the schizophrenias. Journal of Abnormal Psychology, 97, 97-99. Manning, F.A. and Feyerabend, C. (1976). Cigarette smoking and fetal breathing movements. British Journal of Obstetrics and Gynecology, 83, 262-270. Manschreck, T.C. (1983). Psychopathology of motor behavior in schizophrenia. In B.A. Maher and W.B. Maher (eds.) Progress in Personality Research., vol 12, New York: Academic Press, 53-99. McManus, I.C. (1981). Handedness and birth stress. Psychological Medicine, 11, 485-496. Naeye, R.L.(1982) Environmental influences on the embryo and fetus. In R.B. Hill and J A . Terzian. Environmental Pathology: An Evolving Field. New York: A.R. Liss, pp. 111-128. Naeye, R.L., and Peters, E.C. (1987). Antenatal hypoxia and low IQ values. American Joumal of Diseases of Childhood, 141, 50- 54. Nasrallah, H.A., Keelor, K., McCalley-Whitters, M. (1983). Laterality shift in alcoholic males. Biological Psychiatry, 18, 1065-1067. Nasrallah, HA., Keelor, K., Schroeder, C.V. and Whitters, M.M. (1981). Motoric lateralization in schizophrenic males. American Journal of Psychiatry, 138, 1114-1115. Nass, R., Baker, S., Speiser, P., Virdis, R., Balsamo, A., Cacciari, E., Loche, A., Dumic, M. and New, M. (1987). Hormones and handedness: left-handed bias in female congenital adrenal hyperplasia patients. Neurology, 37, 711-715.

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LEFT-HANDEDNESS Behavioral Implications and Anomalies, S . Coren (Editor) 0 Elsevier Science Publishers B .V. (North-Holland), 1990

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Left-Handedness and Prenatal Complications Murray Schwartz Victoria General Hospital, Halifax

“Left-handedness is one of the degeneracy signs of the born criminal.” This quote by the famous 18th century physician, criminologist and reformer C. Lombroso (1903) is intended as a warning to the reader since the author of this chapter is just such a left-hander. In addition to criminals, left-handers are found in disproportionate frequency in problem populations such as autistics, epileptics, dyslexics, learning disabled, stutterers, schizophrenics and mental retardates; individuals suffering from auto-immune disorders, childhood allergies and migraines; and, that most problematic group, graduate students (Geschwind and Galaburda, 1987; Harris, 1980; Habib & Galaburda, this volume; Halpern & Coren, this volume). It should also be pointed out that sinistrals are also found in disproportionate numbers in populations of architects, mathematically gifted and musicians (O’Boyle & Benbow, this volume; Geschwind and Galaburda, 1987). Many cultures have viewed right-handedness as proper, righteous and correct, and have considered left-handedness as evil, clumsy and cursed (Needham, 1973; Harris, 1980 and this volume). While mammals other than humans display lateral preferences, these are on an individual basis, i.e., any single animal is equally likely to prefer either hand (e.g., Corballis, 1983). Humans, on the other hand are much more likely to prefer the right hand (Hamilton, 1976; Webster, 1976, as cited in Hicks and Kinsbourne, 1976). Although always in the minority, left-handers have been present in the world since prehistoric times (Coren and Porac, 1977; Dennis, 1958; Harris, 1980; Hildreth, 1949; Spennman, 1984; Springer and Deutsch, 1981).

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Estimates of the frequency of occurrence of left-handers in the human population have remained surprisingly static from the beginning of recorded history. Although the estimated range of left-handedness in these retrospective "historical reviews" varies from 3% to 20%, the average is about 9%. The exact percentage of left-handedness in the population changes somewhat from study to study depending on the method used for assessing handedness and/or the means of categorizing left-handedness (e.g., Porac and Coren, 1981). Lefthandedness also appears to be relatively constant across racial and ethnic boundaries (Harris, 1980) although some recent variations are suggested by Porac, Rees and Buller (this volume). A number of more recent studies have suggested an increase in the number of left-handers within the past few generations (e.g., Spiegler and YeniKomshian, 1984). These findings, which appear to be independent of the method of classifying handedness, suggest a revision in the current estimate of lefthandedness from between 9-10% to between 13-14% (Annett, 1973; Ashton, 1982; Brackenridge, 1981; Carter-Saltzman, 1980; Spiegler and Yeni-Komshian, 1984). This increase in the incidence of left-handers, which occurs in the younger age groups, is often interpreted as reflecting a more tolerant attitude towards left-handers in recent decades and a coincident relaxation in cultural pressure to convert to dextrality. Prior to this increased acceptance of lateral preference, lefthacders were either subtly cajoled, or more overtly forced, into converting to right-handedness. The penalties for remaining true to one's natural sinistrality could range from enduring the wrath of a teacher or parent who ridiculed handwriting posture to getting hit across the knuckles to coerce a change in behaviour. Coren and Halpern interpret the decreasing number of left-handers with older generations as a reflection of the greater morbidity and mortality rate for left-handers (Coren, 1989; Halpern & Coren, 1988 and this volume). Numerous explanations have been proposed to account for the presence of left-handers (e.g., Hardwyck and Petrinovich, 1977; Harris, 1980; Hildreth, 1949). There does not appear to be a generally acceptable all-inclusive theory for the development of handedness. Current theories concerning the etiology of handedness can be grouped into three categories: genetic; environmental-cultural; and prenatal/environmental. The latter category includes hormonal influences and the pathological models of handedness. The three categories are not necessarily mutually exclusive, and possibly parts of all three may contribute to the determination of lateral preference.

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Genetic Theories of Handedness Many explanations have been predicated on the presence of a relationship between genetics and handedness. Carter-Saltzman (1980) tested children adopted before they were one year old along with their adoptive and biological parents. He found a significant correlation between the biological parents and their offspring but not between the adoptive parents and their children. Although these findings lend themselves to a genetic explanation, there are problems with the study, not the least of which is the difficulty in obtaining reliable handedness measures prior to one year of age. Both single (Annett, 1964, 1972, 1975, 1978, 1985) and double allele (Levy, 1976, 1977; Levy and Nagylaki, 1972; Nagylaki and Levy, 1973) genetic models of handedness have been proposed and are supported, at least in part, by other researchers (e.g., Geschwind and Galaburda, 1987; Hardyck and Petrinovich, 1977; Hicks and Kinsbourne, 1976; Liederman and Kinsbourne, 1980). In many cases, the proponents of a genetic basis for handedness believe that environmental prenatal (Corballis, 1980a; Corballis and Beale, 1976 Geschwind and Galaburda, 1987; Rife, 1950) or environmental postnatal (Collins, 1970; 1975; 1977; Hecaen and Ajuriaguerra, 1964; Hecaen and Sauguet, 1971; Hudson, 1975; McManus, 1980; Porac and Coren, 1979) contributing factors are also present. The most detailed studies attempting to explain a genetic component for handedness have been done by Annett (e.g., 1985) who advanced the notion of random dominance. She suggests that the majority of persons in the population carry a "right-shift'' gene which increases the probability of left-hemisphere dominance for controlling function (and presumably preference). However, approximately 18% of the population have random dominance for handedness and accidental factors determine the lateralization of handedness for that segment of the population. Annett believes that half of the "random" 18% become right-handers and the remainder become left-handed. The accidental factors can be considered to include a wide variety of prenatal influences. Because the determination of handedness is subject to the whim of so many fetal environmental factors, no purely genetic theory can alone account for the determination of handedness. There are too many prenatal temperature and chemical variants that are known to have an influence on genetic unfolding for any theory to consider the genetic code in a vacuum.

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Environmental/Cultural Theories of Handedness Many theories have been based on cultural folklore (Needham, 1973) and few have survived advances in anatomical knowledge and closer experimental scrutiny. The premise of the cultural/learning position is that handedness is a learned phenomenon that is passed on through generations. Hildreth (1949) states that the proportion of children favouring their left hand decreases considerably between the first and fifth year of life. This can be interpreted as evidence favouring a learned component in handedness (Provins, 1967). Also, Collins (e.g., 1975) has convincinglydemonstrated that paw preference in rats can be altered significantly by environmental experiential factors. Hicks and Kinsbourne (1976) reviewed the human handedness literature and found little support for the hypothesis that handedness is learned. They present the view that any evidence that could be interpreted as favouring a learning/cultural theory of handedness is, at best, equivocal. Frustration over finding a parsimonious genetic explanation is not sufficient justification for defaulting to a learning determination of handedness. Weak and/or equivocal evidence similarly is not an acceptable basis for formulating a solid theoretical stand. Studies demonstrating a decrease in the frequency of occurrence of lefthandedness with increasing age (see Hildreth, 1949; Jones, 1937; Stellingwerf, 1975 as reported in Porac, Coren and Duncan, 1980) are interpreted as reflecting pressure to conform to an essentially right-handed environment, rather than the unfolding of a dextral, biological, maturational process. A few recent surveys report just the opposite trend, namely, a higher incidence of sinistrality with the older age groupings (Brackenridge, 1971; Fleminger, Dalton and Standage, 1977; Levy, 1974). These latter studies, in turn, are interpreted as demonstrating an increasing tolerance or acceptance of left-handedness and a consequent relaxation in cultural pressure to convert to dextrality. Halpern and Coren (this volume), counter this position with a literature survey. Based upon evidence from 34 studies since the turn of the century, they suggest that the percentage of adult left-handers has not changed over the past 80 years.

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Prenatal Hormonal Theory Geschwind and Galaburda (1987) have expanded Annett's idea of random dominance by suggesting that a full one third of the population is subject to random determination of lateral dominance. This theoretical postulation dovetails well with Geschwind's finding that one-third of the population do not manifest a larger left-sided planum temporale. This subgroup of the population, according to Geschwind and Galaburda (1987), have anomalous lateral dominance, is., they are not clear right-handers and randomly develop patterns of hand preference that differ from the norm of strong dextrality. Both Annett (1985) and Geschwind and Galaburda (1987) argue that certain prenatal influences act to diminish the overall innate (genetic predisposition) towards right-handedness to create random dominance. Hormonal influence, particularly the concentration of testosterone, is the most salient hypothesized factor for Geschwind and Galaburda (1987). The higher the testosterone level during certain prenatal periods, the greater the likelihood of random determination of lateral dominance. It is important to note that the higher levels of testosterone along with the "random laterality which results in an increased number of left-handers are not considered pathological events, but aspects of normal prenatal development.

Prenatal Pathological Theory One group of researchers has advanced the position that right-handedness is the norm and that deviation from this manifestation, namely sinistrality, represents a pathological condition (Bakan, 1971, 1975, 1977; Bakan, Dibb and Reed, 1973; Gordon, 1920; Subirama, 1969). The leading proponent of this position, Bakan, claims that pregnancies involving a higher risk of cerebral insult (esp. hypoxia) to the fetus, produce a higher than normal frequency of sinistrality in the resulting children. He argues that prenatal stress causes left cerebral motor damage resulting in a "weakness" of the right hand, thus prompting a sinistral shift in handedness. While some studies support Bakan's position (e.g., Coren and Porac, 1980; Coren, Searleman and Porac, 1982; Hicks and Barton, 1975; Leviton and Kilty, 1976), there is a long list of studies which do not (Annett and Ockwell, 1980; Barnes, 1975; Hicks, Evans and Pellegrini, 1978; Hicks, Pellegrini, and Evans, 1978; Hicks, Pellegrini, Evans and Moore, 1979; Hubbard, 1971; McManus, 1981; Schwartz, 1977, 1988b; Sexton and Schwartz,

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1987; Tan and Nettleton, 1980). With the assistance of an interesting and sophisticated meta-analysis statistical technique, Searleman, Porac and Coren (1989) reviewed and analyzed the findings of over 25 birth stress and laterality studies and found a significant relationship between the presence of birth stressors and a reduced frequency of right-handedness. The effect size, while statistically significant, was small. To further explore the premise that left-handedness may reflect cerebral deficiency, several researchers have compared left-handers and right-handers on a series of cognitive tasks. While some researchers suggest that left-handers show certain relative deficiencies in some cognitive skills (cf. McKeever, this volume; Porac & Coren, 1981), there are many others that find either no difference or a superiority for left-handers on some tasks (Ashton, 1982; Hardyck, Petrinovich and Goldman, 1976; Helm and Watts, 1976; Hicks and Beveridge, 1978; Newcombe, Ratcliff, Carrivick, Hiorns, Harrison and Gibson 1975; Wellman, 1985) while Lewis and Harris (this volume) argue that there may be important mediating factors that are often not considered. In any event, the general pattern of the data in the literature is consistent with Geschwind and Galaburda (1987) who argue that left-handers do not present as an overall disadvantaged group with reference to general disability or morbidity. Because the data has been so mixed in direction, the pathological model of left-handedness has generated much of the recent controversy surrounding the etiology of handedness. It is interesting that it was not until Bakan's (1971) publication of a one page article that the relationship between perinatal stress and left-handedness developed into a controversy (Searleman, Porac and Coren, 1989). The existence of some pathological left-handedness is not denied. On the contrary, the presence of a pathological condition in some left-handers would account for the high proportion of left-handers in problem populations where brain damage is suspected (Hecaen and Ajuriaguerra, 1964; Satz, 1972, 1973; Satz, Baymur and Vlugt, 1979). Brain (1945) introduced the notion of "shifted sinistral" or pathological left-handers, along with naturally occurring left-handers, several decades ago; and Satz and his co-workers have more recently reintroduced and popularized the terms "manifest"or "pathological" left-handers (Orsini and Satz, 1986; Satz, 1972, 1973; Satz, Orsini, Saslow and Henry, 1985). Satz's (1972,1973) succinct explanation is that the high percentage of sinistrals in "problem" populations is due to the expected number of so-called normal genetic left-handers augmented by the presence of pathological left-handers (see also Coren & Searleman, this volume). The pathological left-handers are persons who would have been right-handed were it not for some, presumably prenatal,

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pathology to the left-hemisphere which forced them to use their undamaged, right cerebrally controlled, left hand. The parsimony of Satz's explanation makes it appealing. Annett (1981) and Geschwind and Galaburda (1987) worry that the normal vs. pathological distinction for left-handers has been too widely applied. They argue that the pathological classification should be restricted to those few individuals who have sustained serious prenatal or perinatal damage to the left hemisphere; i.e., those individuals who necessarily end up with a right hemispheric dominance for handedness. In the same vein, Geschwind and Galaburda (1987) do not agree with equating "normal" to "genetic,"or "acquired" to "pathologic," with reference to left-handedness. They strongly feel that the intrauterine influences that produce left-handedness are varied, numerous and only partially genetically determined. Since many normal left-handers became so for normally occurring, non-genetic reasons, they believe the terms "normal" and "genetic" should not be equated. Similarly, left-handers with physical abnormalities of the left hemisphere have had their brain organization altered by a mechanism that, in a less extreme case, likely produces the majority of normal left-handers. Touwen (1972) feels the statement that all left-handers are the products of brain injury is extending the argument beyond reasonable limits and is not logically sound. The salient point is that there is general disagreement with Bakan's view that all sinistrality is pathologic. The stronger prevailing view is that there is a continuum of left-handers with and without accompanying abnormalities. Even if one were more sympathetic to Bakan's views, there are severe methodological considerations which must be taken into account. The studies investigating handedness and birth complications are suspect because of indirect methods of determining perinatal risk or complications and inadequate methods of determining handedness. The presence or absence of birth stress factors usually has been determined by asking college students the nature of their mother's pregnancy (e.g., Bakan, Dibb and Reed, 1973; Schwartz, 1977). While such a procedure is convenient, its validity must be questioned. A few studies have used maternal reports for information about the nature of a pregnancy that occurred from 7 to 15 years earlier (Annett and Ockwell, 1980; Badian, 1983; Coren and Porac, 1980; Coren, Searleman and Porac, 1982; Dusek and Hicks, 1980; Searleman, Tsao and Balzer, 1980; Tan and Nettleton, 1980). Although maternal report is a move in the right direction, it is still dependent on recall of events several years in the past. Chamberlain and Johnstone (1975) questioned mothers hospitalized for the birth of a subsequent child, about their previous pregnancy. They found that 42% of the mothers either did not know the length

82

Schwartz

of their labour or were inaccurate by several hours. Schwartz (1988a) compared hospital pregnancy and birth records and maternal report two years postpartum and found significant discrepancies between the two data sources. In two-thirds of the cases where the mother reported pregnancy complications, there was no corroboration found in the hospital records. When the hospital records indicated the presence of serious stress factors (e.g., meconium staining) or complications (breech birth, toxemia) there was a lack of any similar indication in approximately half of the mothers’ reports. Clearly the findings in the Chamberlain and Johnstone (1975) and the Schwartz (1988a) studies cast doubt on the results of any investigations which rely on retrospective, second-hand source data collection. Consequently, the conclusions and theoretical formulations drawn from those results must be treated as no more than speculation at best. Another methodological problem in most of the birth stress studies is that handedness is determined in the most convenient, rather than the most accurate, manner. For example, Hubbard (1971) allowed for self-classification by his subjects, while Bakan (1971) and Bakan, Dibb and Reid (1973) both used writing hand as the criterion for determining handedness. Both procedures have been shown to be inadequate by themselves in the determination of handedness, particularly left-handedness (Annett, 1985; Benton, Meyers, and Polder, 1962; Bryden, 1977; Coren and Porac, 1978; Coren, Porac and Duncan, 1979; Crovitz and Zener, 1962; Knox and Boone, 1970; Oldfield, 1971; Raczkowski, Kalat and Nebes, 1974; Satz, Achenback and Fennell, 1%7; Steingrubber, 1975). Benton, Meyers and Polder (1962) have indicated that self-assessed left-handers are a highly variable group. Geschwind and Galaburda (1987) have succinctly stated that the high correlation between self-described handedness and test scores is of little use simply because there are many discrepancies in the group that matters most, namely, the subgroup which includes the left-handers. Geschwind and Galaburda (1987) similarly criticize the exclusive reliance on writing hand to categorize handedness. In light of the methodological limitations, it is not surprising that contradictory results abound (see Leiber and Axelrod, 1981). The magnitude of the methodological problem was illustrated by Schwartz (1977) who categorized left-handers according to three different criterion: selfclassification, writing hand or results of a 1Citem laterality questionnaire. He found all three methods yielded the same percentage of left-handers, but the composition in the sample, is., the persons who actually occupied those categories, were different. The personnel differences in the three left-handed categories ranged from 15% to 30%. Given the small percentage of left-handers

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present in the population, the 30% discrepancy in categorization could readily account for many of the discrepancies in results from different studies. It is also important to appreciate that handedness is not a dichotomy but a continuum and that the decision of where on the continuum left, ambidextrous or righthandedness begins and ends is arbitrary and must be operationally defined. Hubbard (1971) and Schwartz (1977) were two of the first persons to report findings inconsistent with those of Bakan (1971). Bakan (1977) attempted to dismiss the negative findings by suggesting that their studies were suffering from sampling error. The argument became somewhat forced when Bakan proposed that the contrary results in the Schwartz (1977) study were due to the fact that subjects used in that study were from a geographic area with a high mortality rate and that the pathological left-handers were dead. Presumably the deceased left-handers, had they lived, would have altered the statistics in a manner more favourable to Bakan’s hypothesis. A decade after the Schwartz (1977) study, Sexton and Schwartz (1987) readministered a laterality questionnaire to over 650 university students. The questionnaire asked specific questions related to birth order, pregnancy complications, birth stress and early developmental problems. The design incorporated all of Bakan’s criticisms of the original Schwartz (1977) study (undifferentiated sample according to sex, lack of detailed questioning about possible pregnancy and birth complications). Nevertheless Sexton and Schwartz (1987) obtained results fully consistent with those of the original Schwartz (1977) study, is., there was no increase in the frequency of sinistrality as a result of high-risk pregnancies (as determined by birth order) or as a result of reported pregnancy or birth complications. The findings were consistent regardless of whether the handedness categorization process was based on self-assessment of handedness or by a 14-item questionnaire. The lack of any increase in sinistrality for high-risk birth orders or for reported complications was consistent whether left-handers were categorized on a strict criterion (performed all tasks always or usually with the left hand) or weakly (performed more tasks with the left hand) or when the sample was divided into right-handed vs. all others (i.e., left-handers t ambidextrous). Some additional measures of perinatal stress were also looked at, namely maternal age at birth, and early developmental problems. Those new measures also failed to provide any evidence favouring Bakan’s hypothesis. A summary of the results of the Sexton and Schwartz (1987) study are shown in Table 1.

84

Schwartz

Table 1: Stress factors and handedness (Sexton and Schwartz, 1987)

Stress Factor

I

I R

Males

Tota 1

Females L

R

L

R

L

Birth Order: 1st or > 3rd 2nd or 3rd

118 85

15 8

207 122

Birth Complications: Yes No Don ' t Know

41 139 26

8 14 1

86 206 36

4 15 2

127 345 62

12 29 3

128 188

7 14

204 312

12 32

Maternal Age: c19 or >30 19-30

Post Neonatal Difficulty: Yes No

NB None

29 15

1 43 291

8 36

of the results are significant.

The previously described study is yet another example of Searleman, Porac and Coren's (1989) complaint! They argue that the standard retrospective paradigm will not put to rest the controversy about whether a relationship exists between birth stress and laterality nor about the nature of the relationship, in non-clinical populations. They point to the methodological problem of surveying offspring or retrospective reports from mothers with no corroborating archival data backup, the lack of a longitudinal, prospective approach, and the use of dichotomous rather than continuous measures of lateral preference. They argue that a definitive statement is not possible until those characteristics are advanced in a more comprehensive investigation. Schwartz (1988a, 1988b) has presented preliminary reports on a longitudinal, prospective study investigating the relationship between perinatal stress and the development of lateral preference. Additional data will be presented here. Although the study does not pretend to be definitive, it is promising in so far as it complies with several prerequisites set forth by Searleman, Porac and Coren

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85

(1989). Namely, the study uses archival as well as laterality questionnaire data, continuous as well as dichotomous values for handedness categorization;stresses longitudinal, prospective rather than retrospective data collection; and compares hospital records to maternally reported information. Over 400 children were tested on a variety of Iaterality measures which taken from included tests of lateral preference (ten-items "show me how you the Harris Tests of Lateral Dominance (Harris, 1947), with the important addition that the children were given concrete objects to manipulate rather than merely pantomiming the activity. The children were also given tests of manual dexterity such as the Purdue Pegboard, the Grooved Pegboard, a telegraph key tapping test and the tapping test from the Harris Tests. Two handedness scores were used as the dependent variables: a dichotomous score which classified the child as left- or right-handed depending on the hand with which the child performed better, or preferred, for the majority of tasks; and a continuous measure of handedness called the RH% score, obtained by taking the percentage of tasks preferred, or performed better with the right hand. The children were tested once yearly for four years, beginning at age 2. The mothers were extensivelyinterviewed when the two year olds were initially tested. The maternal interviews were aimed at obtaining pregnancy and delivery information including the presence or absence of specific stress factors; complications of pregnancy and delivery; post-partum and early developmental problem; developmental landmarks; and familial handedness history. The important third source of data gathered in the study was hospital pregnancy and birth records, The hospital records contained information about the pregnancy in general, including the presence of stress or complication factors and extensive information about the delivery stages and immediate post-partum status of the neonate. The Schwartz study looked at a wide range of stress/risk factors and complications including the following: Birth weight. Weight of neonate, according to the mother and, independently, according to hospital records. This was done since the Chamberlain and Johnstone (1975) study demonstrated that the maternal report of pregnancy birth history may not be accurate. Matenial smokirig. Maternal smoking may be considered a stress/risk factor as there is evidence suggesting that smoking adversely affects the health of the foetus, especially as manifested in lowered birth weight. Pregnancy order. The pregnancy (gravis) order is not necessarily the same as birth order since birth order may not take into account spontaneous abortions, stillbirths and neonatal deaths. ...'I)

86

Schwartz

Maternal age. The child bearing ages associated with the least risk are between 19 and 30 years of age. Apgar Score at 1 min. The Apgar rating (Apgar, 1953) is a neonatal rating out of 10, based on a maximum score of two points for each of the following: respiration, responsiveness, colour, tone and heart rate; and is a measure of the physiological status and well being of the neonate immediately after birth. Scores of 7-10 are considered normal or safe, scores from 4-7 are considered to be a sign of possible distress and should be monitored, and scores of 0-4 are considered to indicate serious distress and active intervention required. Meconium staining. Meconium is a dark secretion from the intestines of the fetus and, when present in different colourations before parturition, is considered a sign of fetal distress. Maternal report of complications. Mothers were interviewed two years postpartum and asked whether there were any complications with their pregnancy, delivery or immediate post-partum period. In addition to the general inquiry, specific stress/risk factors and complicationswere enumerated to the mother and she was asked if these situations were present (e.g., toxemia, blood pressure problems, water retention, circulation problems). Hospital report of complications. The hospital pregnancy/delivery record provided a type of multiple choice check list system for indicating the occurrence of any stress or complication factors at different stages of pregnancy, labour/delivery, and the immediate post-partum period. Not only was there a comprehensive list of possible stress and complication factors compared to previous studies, but two dependent measures of handedness were used: a dichotomous right-left measure (the hand preferred and/or used the majority of times when all the tests were compiled) and a more contiguous RH% measure (the number of times the right hand was used for all the tests, expressed as a percentage of the total number of tasks completed). A summary of the findings for each of the age groups, for all stress/complication measures for the right-left dichotomous measure and the RH% measure are found on Tables 2 and 3 respectively. T-test analyses were conducted for all the measures when the RH% score was the dependent variable with high and low risk defining the groups (Table 2). Chi square analyses were performed for each stress/risk or complication measure when the dependent variable was the dichotomized right-left handedness index (Table 3).

Prenatal Complications

87

It is apparent from the tables that there are a plethora of negative findings between birth stress and handedness in the tables. Does this indicate that the relationship between birth stress and handedness must be abandoned? Well we certainly must call the issue moot. Several of the measures admittedly are what may be called weak stress indicators. These include birth order, age of mother, maternal smoking and birth weight. There are, however, several relatively strong measures of stress or complications (e.g., Apgar scores, meconium staining, maternal or hospital indication of complications). If any relationship between birth stress or complications existed, then it presumably would be manifested when using one or more of these measures. The so-called strong measures indicate the immediate post-partum physiological status of the neonate (Apgar), the amount of fetal distress experienced during birthing (meconium staining) and the complete objective hospital record of events (hospital report of complications). There were only five significant results out of 108 analyses run when the continuous dependent variable (RH%) was used in the analyses. Furthermore, only two of the five were in the direction predicted by the pathological theory of handedness (see Table 2). When the dichotomous (rightleft) dependent variable was used, 8 of 108 Chi square analyses were found to be significant (see Table 3). Certainly, these results cannot be construed as evidence favouring a birth stress and left-handedness relationship in the global sense. The one result from this study that seems consistent with the a relationship between birth stress and sinistrality, comes from the one minute Apgar scores. In the dichotomized data (Table 3) we find three comparisons that are statistically significant, and ten of the twelve comparisons are in the predicted direction, with higher proportions of sinistrality associated with lower Apgar scores. Low Apgar scores have been shown to be associated with hypoxia and increased incidence of neurological abnormality, hence this finding is suggestive. The problem of course is that one cannot prove the null hypothesis. In this study, for example, in Table 3, we are dealing with dichotomized data, with about 300 observations per comparison. There are many fewer stressed births than there are normal births (about 10% for an average comparison of about 270 vs 30 cases). This means that the incidence of left-handedness would have to rise from 10% in the normal group to 22% in the stressed group, to achieve a statistical significance of p<0.05. This is a 120% increase in left-handedness! This is, of course, a massive change in the incidence of the effect and is much larger than we require of most psychological variables, hence assertion of null hypothesis is tenuous. However, the results of the Schwartz study reported herein

%

Table 2: Results for different stress factors using RH% score, (N) Age Risk Factor

High

Pregnancy (gravis) order: (Low = 2nd or 3rd) Males 71.0 (95) 71.2 (113) Females 76.4 (88) 71.9 (74) 73.6 (183) 71.5 (187) Tota 1

Low

5 Years

4 Years

3 Years

2 Years Low

High

Low

High

LOW

High

3

a 72.9 (68) 73.4 (76) 75.5 (61) 74.3 (50) 74.1 (129) 73.8 (126)

79.4 (71) 73.7 (70) 80.0 (62) 76.9 (49) 79.7 (133) 75.0 (119)

72.8 (76) 73.5 (70) 73.2 (55)

69.9 (90) 74.4 (146) 71.6 (145)

70.4 (136) 73.7 (29) 74.5 (110) 70.2 (13) 72.2 (246) 72.6 (42)

72.8 (115) 74.6 (26) 75.8 (97) 67.2 (13) 74.1 (212) 72.1 (39)

75.3 (115) 82.6 (23)' 80.0 (98) 65.0 (12)' 77.5 (213) 76.6 (35)

Birth Weight (according to hospital): (LOW = 2268-40829) Males 71.6 (165) 69.8 (35) 70.4 (130) 73.4 (28) Females 73.3 (130) 74.7 (20) 74.5 (98) 71.4 (16) Tota 1 72.4 (295) 71.6 (55) 72.1 (228) 72.7 (44)

73.1 (111) 74.6 (26) 74.8 (87) 70.8 (15) 73.9 (198) 73.2 (41)

75.3 (112) 82.6 (23)" 68.2 (14) 79.3 (88) 77.0 (200) 77.1 (37)

Maternal Smoking: (Low = c 5 cig./day) Males 71.2 (166) 70.5 (49) Females 74.0 (114) 74.5 (46) 72.4 (280) 72.4 (95) Tota 1

71.0 (125) 70.2 (41) 73.0 (89) 77.1 (33) 71.8 (214) 73.3 (74)

73.5 (113) 70.7 (29) 73.8 (77) 76.7 (32) 73.6 (190) 73.9 (61)

76.4 (107) 77.2 (77) 76.7 (184)

76.4 (31) 81.3 (32) 78.9 (63)

Maternal Age: (Low = 19-30 yrs) Males 71.6 (133) 71.4 (80) Females 74.1 (114) 74.8 (47) Tota 1 72.7 (247) 72.6 (127)

68.9 (104) 74.9 (59) 76.7 (35) 73.1 (88) 70.8 (192) 75.6 (94)'

72.9 (92) 75.2 (47) 75.1 (39) 73.6 (31) 73.9 (171) 74.6 (78)

76.1 (88) 78.0 (76) 77.0 (164)

77.6 (49) 79.3 (34) 78.3 (83)

Birth Weight (according to mother): (LOW = 2268-40829) Males 71.3 (117) 70.3 (37) Females 74.5 (143) 71.9 (18) 72.7 (320) 70.8 (55) Tota 1

n 1

Table 2: Continued Age Risk Factor

3 Years

2 Years Low

High

Low

4 Years High

Low

5 Years High

Low

High

76.8 (101) 77.9 (83) 77.3 (184)

74.6 (36) 80.1 (27) 77.0 (63)

Meconium S t a i n i n g : (Low = not p r e s e n t ) Males 71.8 (153) 68.1 (49) Females 74.0 (121) 73.69 (36) Tota 1 72.8 (274) 70.0 (85)

69.5 (119) 75.8 (43) 73.4 (93) 75.3 (30) 71.2 (212) 75.6 (73)

73.2 (104) 72.0 (36) 73.5 (85) 79.8 (25) 73.3 (189) 75.2 (61)

Apgar at 1 .(High 5 7 ) Males Females Total

72.1 (147) 65.5 (15) 75.6 (108) 63.2 ii6j 73.62 (255) 64.3 (31)

73.1 (126) 71.1 (14) 74.8 (96) 74.7 (14) 73.8 (222) 72.9 (28)

76.6 (123) 78.7 (96) 77.5 (219)

72.3 (14) 77.7 (14) 75.0 (28)

71.4 (101) 70.3 (66) 73.0 (78) 75.9 (46) 72.1 (179) 72.6 (112)

73.1 (87) 72.8 (56) 73.9 (72) 76.4 (39) 73.5 (159) 74.3 (95)

76.7 (86) 78.7 (72) 77.6 (158)

76.1 (54) 78.3 (39) 77.0 (93)

71.9 (67) 73.7 (65) 72.8 (126)

77.4 (68) 78.6 (57) 77.9 (125)

75.8 (73) 78.7 (55) 77.1 (128)

Minute

72.0 (184) 58.9 (18) 75.1 (143) 67.5 ( i 8 j 73.3 (330)* 63.2 (36)*

Complications (according t o m o t h e r ) : (Low = none or minor) 73.5 (126) 68.0 (91) Males Females 73.0 (100) 76.4 (62) Total 73.3 (226) 71.4 (153)

Complications (according t o h o s p i t a l ) : (Low = none or minor) 72.0 (95) 70.5(123) 71.3 (82) 70.9 (86) Males Females 76.1 (81) 72.9 (72) 75.3 (61) 72.8 (65) Tota 1 73.9 (176) 71.5 (206) 73.0 (143) 71.7 (151)

a =

p<.O5

74.3 (77) 76.3 (53) 75.1 (130)

Table 3 Results For Different Stress Factors Using Dichotomous Right-Left Classification

Age Risk Factor

2 Years Low

3 Years

4 Years

Low

High

Pregnancy (gravis) Order: (Low = 2nd or 3rd) Males 22.1 (95) 36.5 (63)* Females 14.8 (88) 20.3 (74) Total 18.6 (183) 27.7 (99)*

14.5 (76) 12.9 (70) 13.7 (146)

B i r t h Weight (according t o mother): (LOW = 2268-4082) Males 19.8 (177) 24.3 (37) Females 17.5 (143) 16.7 (18) Total 18.8 (320) 21.8 (55)

High

i

5 Years

LOW

High

17.8 (90) 9.1 (55) 14.5 (145)

13.2 (68) 14.8 (61) 14.0 (129)

15.8 (76) 12.0 (50) 14.3 (126)

11.3 (71) 16.1 (62) 13.5 (133)

17.1 (70) 14.3 (49) 16.0 (119)

19.9 (136) 10.9 (110) 15.9 (246)

3.5 (29)* 15.4 (13) 7.1 (42)

16.5 (115) 12.4 (97) 14.6 (212)

6.5 (31) 23.0 (13) 11.4 (44)

16.5 (115) 13.3 (98) 15.0 (213)

4.4 (23) 33.3(12)' 14.3 (35)

B i r t h Weight (according t o h o s p i t a l ) : (LOW = 2268-4082) Males 20.0 (165) 25.7 (35) Females 19.2 (130) 15.0 (20) Tota 1 19.7 (295) 21.8 (55)

19.2 (130) 11.2 (98) 18.8 (228)

3.6 (28)* 12.5 (16) 6.8 (44)

16.2 (111) 13.8 (87) 15.2 (198)

7.7 (26) 20.0 (15) 12.2 (41)

16.1 (112) 14.8 (88) 15.5 (200)

4.3 (23) 28.6 (14) 13.5 (37)

Maternal Smoking: (Low = <5 cig./day) 21.1 (166) Males Females 16.7 (114) Total 19.3 (280)

20.4 (49) 19.6 (46) 20.0 (95)

15.2 (125) 13.5 (89) 16.9 (214)

22.0 (41) 6.3 (32) 15.0 (73)

12.4 (113) 15.6 (77) 13.7 (190)

24.1 (29) 9.4 (32) 16.4 (61)

15.0 (107) 19.5 (77) 16.8 (184)

12.9 (31) 6.3(32)' 9.5 (63)

Maternal Age: (Low = 19-30 yrs) 18.0 (133) Males Fmales 16.6 (114) Total 17.4 (247)

22.5 (80) 23.6 (47) 21.3 (127)

21.1 (104) 12.5 (88) 17.1 (192)

16.3 (92) 13.4 (89) 15.8 (171)

9.3 (47) 9.7 (31) 9.0 (78)

14.8 (88) 17.1 (76) 15.9 (164)

12.2 (49) 11.8 (34) 12.0 (83)

8.5 (59)* 8.6 (35) 8.5 (94)

LOW

High

8c N

Table 3:Continued: ~

Age Risk Factor

Low

4 Years

3 Years

2 Years High

Low

High

Low

5 Years High

Low

High

Meconium Staining: (Low = not present) 19.6 (153) Males Females 17.4 (121) Tota 1 18.6 (274)

24.5 (49) 19.4 (36) 22.4 (85)

18.5 (119) 11.6 (43) 10.8 (93) 13.3 (30) 15.1 (212) 12.3 (73)

15.4 (104) 13.9 (36) 16.5 ( 8 5 ) 4.0 (25j 15.9 (189) 9.8 (61)

13.9 (101) 16.7 (36) 15.7 (83). 14.8 i27j 14.7 (184) 15.9 (63)

Apgar a t 1 Minute: (High = 57) Males 20.3 (187) Females 16.1 (143) Tota 1 18.5 (330)'

33.3 (18) 27.8 (18) 30.6 (36)

14.3 (147)' 33.3 (15) 9.0 (108) 25.0 (16) 12.2 (255)" 29.0 (31)

15.1 (126) 14.3 (14) 13.5 (96) 14.3 (14) 15.1 (212) 14.3 (28)

13.5 (126) 21.4 (14) 14.6 (96) 21.4 (14) 14.6 (219) 21.4 (28)

Complications (according t o mother): (High = Both preg. and d e l i v . c a n p l i c . ) Males 20.5 (195) 22.7 (22) Females 17.3 (150) 16.7 (12) Total 19.1 (345) 19.4 (34)

16.7 (150) 17.6 (17) 12.1 (116) 11.1 (9) 14.6 (268) 15.4 (26)

15.1 (126) 11.8 (17) 13.3 (105) 16.7 (6) 14.3 (231) 13.0 (23)

15.7 (121) 5.3 (19) 15.2 (105) 16.7 (6) 18.3 (226) 8.0 (25)

Canplications (according t o hospital): (High = Both preg. and d e l i v . c u n p l i c . ) Males 22.0 (182) 13.9 (36) Females 17.1 (146) 16.7 (18) Tota 1 19.8 (328) 14.8 (54)

16.4 (146) 18.2 (22) 10.8 (111) 13.3 (15) 19.1 (357) 19.4 (37)

14.4 (125) 15.8 (19) 13.9 (101) 9.1 (11) 14.2 (226) 13.3 (30)

14.5 (124) 11.8 (17) 15.8 (101) 9.1 (11) 15.1 (225) 10.7 (28)

a = pc.01 b = pc.05 c = pc.05 (one t a i l )

3er En @

92

Schwartz

should not be viewed alone, but in the context of the two massive, albeit retrospective studies (with more than 17,000subjects in each study), reported by McManus (1981) and Spiegler and Yeni-Komshian (1982). They also report nonsignificance when investigating pregnancy stress/complications and a change in the incidence of left-handedness. I recall when my psychology instructor, Ron Melzack, the noted pain researcher, shook his head in frustration in reference to the specific receptor theory for cutaneous sensation that once a theory has been established, even though almost all evidence is contrary, you can't get rid of it. A similar argument may be raised with reference to Bakan's (1971, 1977) theoretical position that all left-handedness is the result of cerebral insult. The evidence against the strong position that all left-handedness is pathological is compelling. A note of caution must be raised so that the pendulum is not allowed to swing too far and ignore the so-called soft pathological position (Satz, 1973). Some left-handers, and a significant number in some clinical populations, are likely pathological. Even given the difficulty in obtaining statistical significance with population sizes and splits such as we have here, the data from the Apgar scores is still consistent with the hypothesis of some link between certain forms of pathology and sinistrality for a certain subgroup of individuals. To go from that position to the suggestion that all lefthanders are cerebrally pathological is too great a leap and not justified by data or logic.

Summary The hard pathological position says that all left-handedness is the result of cerebral insult. The soft pathological position, and the more tenable, argues that in some clinical or problem populations the number of non-pathological lefthanders is augmented, by perhaps as many as an equal number of pathological ( k . , brain-damaged) left-handers. The accumulated data appear to support the contention that the majority of left-handers in the population are present due to what is considered normal genetic variability, and prenatal environmental influences (e.g., Annett, 1985; Geschwind and Galaburda, 1987). The actual number of left-handers in the general population is higher than the normal category would warrant because "pathological" individuals who, as a result of cerebral insult to certain motor areas, become left-handed.

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References Annett, M. (1964). A model of the inheritance of handedness and cerebral dominance. Nature, 204, 59-60. Annett, M. (1972). The distribution of manual asymmetry. British Journal of psycho lo^, 63, 343-358. Annett, M. (1973). Handedness in families. Annals of Human Genetics, 37, 93105. Annett, M. (1975). Hand preference and the laterality of cerebral speech. Cortex, 11, 305-328. Annett, M. (1978). A Single Gene Explanation Of Right And Lefr Handedness And Brainedness, Coventry, England: Lancaster Polytechnic. Annett, M. (1981). The genetics of handedness. Trends in Neurosciences, 3, 256258.

Annett, M. (1985). Left, Right, HandAnd Brain: The Right Shiji Theory. Hillsdale, N.J.: Erlbaum. Annett, M., & Ockwell, A. (1980). Birth order, birth stress and handedness. Cortex, 16, 181-188. Ashton, G.C. (1982). Handedness: An alternative hypothesis. Behavior Genetics, 12, 125-147. Badian, NA. (1983). Birth order, maternal age, season of birth, and handedness. Cortex, 19,451-463. Bakan, P. (1971). Birth order and handedness. Nature, 229, 195. Bakan, P. (1975). Are left-handers brain damaged? New Scientist, 67, 200-202. Bakan, P. (1977). Left handedness and birth order revisited. Neuropsychologia, 15, 837-839. Bakan, P., Dibb, G., & Reed, P. (1973). Handedness and birth stress. Neuropsychologia, 11, 363-366. Barnes, F. (1975). Temperament, adaptability, and left-handers. New Scientist, 67, 202-203. Benton, A.L., Meyers, R., & Polder, G.J. (1962). Some aspects of handedness. Psychiatric Neurology, 144, 321-337. Brackenridge, C.J. (1981). Secular variation in handedness over ninety years. Neuropsychologia, 19, 459-462. Brain, W.R. (1945). Speech and handedness. Lancet, 249,837-841. Bryden, M.P. (1977). Measuring handedness with questionnaires. Neuropsychologia, 15, 617-624. Carter-Saltzman, L. (1980). Biological and sociocultural effects on handedness: Comparison between biological and adoptive families. Science, 209, 12631265. Chamberlain, G., & Johnstone, F.D. (1975). Reliability of the history. Lancet, I, 103. Collins, R.L. (1970). The sound of one paw clapping: An inquiry into the origins of left handedness. In G. Lindzey, D.D. Thiessen (Eds.), Contribution To Behavior-Genic Analysis - The Mouse A s A Prototype. New York: Meredith Corporation.

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Collins, R. (1975). When left-handed mice live in right-handed worlds. Science, 187, 181-184. Collins, R.L. (1977). Origins of the sense of asymmetry: Mendelian and nonMendelian models of inheritance. Annals of the New York Academy of Sciences, 299, 283-305. Corballis, M.C. (1980). Is left-handedness genetically determined? In J. Herron (Ed.), Neuropsychology Of Left-Handedness. New York: Academic Press. Corballis, M.C. (1983). Human Laterality. New York: Academic Press Corballis, M.C., & Beale, I.L. (1976). The Psychology Of Left And Right. Hillsdale, N.J.:Erlbaum. Coren, S. (1989). Left-handedness and accident-related injury risk. American Journal of Public Health, 79(8), 1040-41. Coren, S., & Porac, C. (1977). Fifty centuries of right-handedness:The historical record. Science, 198, 631-632. Coren, S., & Porac, C. (1978). The validity and reliability of self-report items for the measurement of lateral performance. British Journal of Psychology, 69, 207-11. Coren, S., & Porac, C. (1980). Birth factors and laterality: Effects of birth order, parental age, and birth stress of four indices of lateral preference. Behavior Genetics, 10, 123-138. Coren, S., Porac, C., & Duncan, P. (1979). A behaviourally valid self-report inventory to assess four types of lateral preference. Journal of Clinical Neuropsychology, 1, 55-64. Coren, S., Searleman, A., & Porac, C. (1982). The effects of specific birth stressors on four indexes of lateral preference. Canadian Journal of Psychology, 36, 478-487. Crovitz, H.F., & Zener, K. (1962). A group test for assessing hand and eye dominance. American Journal of Psychology, 75, 271-276. Dennis, W. (1958). Early graphic evidence of dextrality in man. Perceptual and Motor Skills, 8, 147-149. Dusek, C.D., & Hicks, RA. (1980). Multiple birth-risk factors and handedness in elementary school children. Cortex, 16, 471-478. Fleminger, J.J., Dalton, R., & Standage, K.F. (1977). Age as a factor in the handedness of adults. Neuropsychologia, 15, 471-3. Geschwind, N., & Galaburda, A.M. (1987). Cerebral Lateralization: Biological Mechanisms, Associations, And Pathology. Cambridge, MA: MIT Press. Gordon, H. (1920). Left-handedness and mirror writing, especially among defective children. Brain, 43, 313-368. Halpern, D.F. & Coren, S. (1988). Do right-handers live longer? Nature, 333, 213. Hamilton, C.R. (1977). Investigationsof perceptual and mnemonic lateralization in monkeys. In S. Harnad, R.W. Doty, L. Goldstein, J. Jaynes, & G. Krauthamer (Eds.), Lateralization In The Nervous System. New York: Academic Press. Hardyck, C., & Petrinovich, L.F. (1977). Left-handedness.Psychological Bulletin, 84, 385-404.

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Hardyck, C., Petrinovich, L., & Goldman, R. (1976). Left handedness and cognitive deficit. Cortex, 12, 266-278. Harris, A.J. (1947). Test of Laterality, Psychological Corp. Harris, L.J. (1980). Left-Handedness: Early Theories, Facts, And Fancies. In J. Herron (Ed.), Neuropsychology of Lef-Handedness. New York: Academic Press. Hecaen, H. & deAjuriaguerra, J. (1964). Left-Handedness, Manual Superiorify And Cerebral Dominance. N.Y.: Grune & Stratton. Hecaen H., & Sauguet, J. (1971). Cerebral dominance in left-handed subjects. C O H ~7,, 19-48. Heim, A.W., & Watts, K.P. (1976). Handedness and cognitive bias. Quarterly Journal of Experimental Psychology, 28, 355-360. Hicks, RA., & Barton, A.K. (1975). A note of left-handedness and severity of mental retardation. Journal of Genetic Psychology, 127, 323-324. Hicks, RA., & Beveridge, R. (1978). Handedness and intelligence. Cortex, 14, 304-7. Hicks, R.A., Evans, E.A., & Pellegrini, R.J. (1978). Correlation between handedness and birth order: compilation of five studies. Perceptual and Motor Skills, 46, 53-54. Hicks, R.E., & Kinsbourne, M. (1976). Human handedness: A partial crossfostering study. Science, 192, 908-910. Hicks, RA., Pellegrini, R.J., & Evans, EA. (1978). Handedness and birth risk. Neuropsycliologia, 16, 243-245. Hicks, RA., Pellegrini, R.J., Evans, EA., & Moore, J.D. (1979). Birth risk and left handedness reconsidered. Archives of Neurology 36, 119-120. Hildreth, G. (1949). The Development and Training of Hand Dominance. Journal of Genetic Psychology, 75, 197-275. Hubbard, J.I. (1971). Handedness not a function of birth order. Nature, 232,276277. Hudson, P.T.W. (1975). The genetics of handedness-a reply to Levy and Nagylaki. Neuropsychologia, 13, 331-339. Jones, H.E. (1931). Dextrality as a function of age. Journal of Experimental Psy~hology,14, 125-43. Knox, A.W., & Boone, D.R. (1970). Auditory laterality and tested handedness. COH~ 6, 164-173. Leiber, L., & Axelrod, S. (1981). Not all sinistrality is pathological. Cortex, 17, 259-272. Leviton, A., & Kilty, T. (1976). Birth order and left-handedness. Archives of Neurology, 33, 664. Levy, J. (1974). Psychological implications of bilateral asymmetry. In S. Dimond & J.G. Beaumont (Eds.), Heniisphenc Function In The Human Brain. London: Paul Elek. Levy, J. (1976). A review of evidence for a genetic component in the determination of handedness. Bekavioural Genetics, 6, 429-453. Levy, J. (1977). A reply to Hudson regarding the Levy-Nagylaki model for the genetics of handedness, Neuropsychologia, 15, 187-190.

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Levy, J., & Nagylaki, T. (1972). A model of the genetics of handedness. Genetics, 72, 117-128. Liederman, J., & Kinsbourne, M. (1980). Rightward motor bias in newborns depends upon parental right-handedness. Neuropsychologia, 18, 579-584. Lombroso, C. (1903). Left-sidedness. North American Review, 170, 440-444. McManus, I.C. (1980). Handedness in twins: A critical review. Neuropsychologia, 18, 347-355. McManus, I.C. (1981). Handedness and birth stress. Psychological Medicine, 11, 4854%. Nagylaki, T., & Levy, J. (1973). "The sound of one paw clapping" isn't sound. Behavior Genetics, 3, 279-292. Needham, R. (1973). Right and Lefl: Essays On Dual Symbolic Classification. Chicago: University of Chicago Press. Newcombe, F.G., Ratcliff, G.G., Carrivick, P.J., Hiorns, R.W., Harrison, GA., & Gibson, J.B. (1975). Hand preference and I.Q. in a group of Oxfordshire villages. Annals of Human Biology, 2, 235-242. Oldfield, R.C. (1971). The assessment and analysis of handedness: the Edinburgh Inventory. Neuropsychologia, 9, 97-113. Orsini, D.L., & Satz, P. (1986). A Syndrome of pathological left-handedness. Axhives of Neurology, 43, 333-337. Porac, C., & Coren, S. (1979). Individual and familial patterns in four dimensions of lateral preference. Neuropsychologia, 17, 543-548. Porac, C., & Coren, S. (1981). Lateral Preferences And Human Behavior. New York:Springer-Verlag. Porac, C., Coren, S. & Duncan, P. (1980). Life span age trends in laterality. Journal of Gerontology, 35, 715-721. Provins, KA.(1967). Motor skills, handedness and behavior. Australian Journal Of Psychology, 19, 137-147. Raczkowski, D., Kalat, J.W., & Nebes, R. (1974). Reliability and validity of some handedness questionnaire items. Neuropsychologia, 12, 43-47. Rife, D.C. (1950). Application of gene frequency analysis to the interpretation of data from twins. Human Biology, 22, 136-145. Satz, P. (1972). Pathological left-handedness: an explanatory model. Cortex, 8, 121-135. Satz, P. (1973). Left-handedness and early brain insult: an explanation. Neuropsychologia, 11, 115-117. Satz, P., Achenback, K., & Fennell, E. (1967). Correlations between assessed manual laterality and predicted speech laterality in normal population. Neuropsychologia, 5, 295-310. Satz, P., Baymur, L., & Van der Vlugt, H. (1979). Pathological left-handedness: cross cultural tests of a model. Neuropsychologia, 17, 77-81. Satz, P., Orsini, D.L., Saslow, E., & Henry, R. (1985). The pathological lefthandedness syndrome. Brain and Cognition, 4, 27-46. Schwartz, M. (1977). Left-handedness and high risk pregnancy. Neuropsychologia, 15, 341-344.

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Schwartz, M. (1988a). Discrepancy between maternal report and hospital records. Developmen fa1Neuropsychology, 4, 303-304. Schwartz, M. (1988b). Handedness, prenatal stress and pregnancy complications. Neuropsychologia, 26, 925-929. Searleman, A., Porac, C., & Coren, S. (1989). Relationship between birth order, birth stress, and lateral preferences: A critical review. Psychology Bulletin, 105, 397-408. Sexton, J., & Schwartz, M. (1987). Birth Complication and handedness. Unpublished undergraduate honours thesis, St. Francis Xavier University. Searleman, A., Tsao, Y.C., & Balzer, W. (1980). A reexamination of the relationship between birth stress and handedness. Clinical Neuropsychology, 2, 124-128. Spennman, D.R. (1984). Handedness data on the European neolithic. Neuropsychologica, 22, 613-615. Spiegler, B.I., & Yeni-Komshian, G.H. (1982). Birth trauma and left handedness. Paper presented at International Neuropsychological Society Meeting. Spiegler, B.I., & Yeni-Komshian, G.H. (1984). Incidence of left-handed writing in a college population with reference to family patterns of hand preference. Neuropsychologia, 21, 651-659. Springer, S.P., & Deutsch, G. (1981). Left Brain, Right Brain. San Francisco: Freeman. Steingrubber, H.J. (1975). Handedness as a function of test complexity. Perceptual and Motor Skills, 40, 263-266. Subirama, A. in P.J. Vincken and G.W. Bruyn (Eds.), (1969). Handbook Of Clinical Neurology, Vol. 4. Amsterdam: North Holland. Tan, L.E., & Nettleton, N.C. (1980). Left handedness, birth order and birth stress. Cortex, 16, 363-373. Touwen, B.C.L. (1972). Laterality and dominance. Developmental Medicine and Child Neurology, 14, 747-755 Wellman, M.M. (1985). Information-processing ability among left- and righthanded children. Developniental NeuropsycJiology, I , 53-65.

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Chapter 4

Intrauterine Factors in Sinistrality: A Review Michel Habib Florence Touze Centre Hospitalier University de Marseille and Albert M. Galaburda Harvard Medical School

Current biological theories regarding the etiology of handedness are derived from two opposing -- but not necessarily mutually exclusive -- views: one favouring genetic and the other environmental factors. Despite solid theoretical foundations, purely genetic accounts never proved totally satisfactory, being contradicted, for example, by twin studies. On the other hand, purely environmental explanations of handedness invariably come up against repeated statistical evidence of some degree, roughly similar across cultural and geographical variations, of inheritance of handedness. In the present chapter we shall try to extract from the recent literature the most convincing arguments supporting the role of intrauterine influences in the development of handedness, and especially those factors susceptible of being causally linked to the emergence of left-handedness or sinistrality. All along these lines, repeated reference will be made to the work of Norman Geschwind, who devoted the last few years of his life to elaborating a highly stimulating

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model of cerebral dominance, largely inspired form the biological and pathological associations of left-handedness, shrewdly combining the intervention of genetic and epigenetic factors. It thus seemed to us opportune to summarize in an introductory section the broad lines of Geschwind’s theories. Then data will be presented supporting the role of environmental -- especially hormonal - factors on the developing brain and on the development of brain morphologic as well as functional asymmetries in animals and man. Finally, we will review somewhat older data suggesting the role of the fetal intrauterine position as well as more recent evidence of the role of premature or complicated birth, and try to show how these data may also be interpreted in the light of Geschwind’s theory.

Geschwind’s Theories of Handedness and Origins of Sinistrality (1982-1985) Geschwind’s model was based on three assumptions: 1.

Functional aspects of brain lateralization (including handedness) are the direct consequence of the presence of anatomical asymmetries of the cerebral cortex. This postulate was already formulated in 1968 in a short but landmark article in this field (Geschwind and Levitsky, 1968), revealing gross asymmetry in favour of the left planum temporale, a cortical region known to be involved in language processes. This asymmetry was thought to represent the anatomical correlate of the functional specialization of the left hemisphere for language. In this connection, handedness is taken as a reflection, a by-product, of the more general lateralization processes, preferentially considered because it is more directly accessible to experimental evaluations.

2.

Beyond simple handedness data, one may consider that the general population is divided into two groups: one is said to harbour “standard dominance” traits and roughly corresponds to 70 percent of the population. This group mainly includes strong right-handers, whose language lateralization is also supposed to be strongly left-biased. The other 30 percent comprise the

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"anomalous dominance" group, which includes all left-handed and ambidextrous subjects, but also some right-handers who may have some undetectable -- or undetected -- traits of ambilaterality (especially those with family history of lefthandedness). In the "anomalous" group, language is thought to be less strongly located to the left hemisphere, with a greater participation, or even slight predominance, of the right hemisphere. Lateralization characteristics in the standard group would be the consequence of a universal, genetically determined tendency toward a left hemisphere preponderance, while in the anomalous dominance group, this tendency would be hindered by non-genetic factors acting during late pregnancy to produce a random distribution of functional features of lateralization. A minor part of this group, however, could show a reversed pattern of laterality giving rise to a genuine "mirror image" of the standard pattern of left hemisphere predominance. thus, in Geschwind's view, only in such rare cases of so-called "cerebral situs inversus" might left-handedness be strictly genetically determined.

3.

Environmental -- especially hormonal -- factors acting on the fetal brain during the midgestational period would act to slow down the development of specific areas in the left hemisphere', so that homologous areas in the right may undergo compensatory over-development and hence more functional competence. Geschwind suggested that testosterone was the best candidate for subserving this role. This choice is mainly derived from the multiple and complex interactions known to exist between sex, hormones, and brain lateralization (see below), especially the well-documented -- although contested - greater incidence of left-handedness in males than in females.

These hypotheses rely on several observations from various areas of research, which are reviewed in the following sections.

'

As will be developed below, it is now thought that the right hemisphere is more likely the appropriate target of environmentally acting (including intrauterine) factors.

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Anatomical Observations Description of Morphologic Asymmetries

Detailed anatomical accounts of the patterns of asymmetry in the human brain are given elsewhere (Galaburda, 1984; Geschwind and Galaburda, 1985; Witelson and Kigar, 1987), and will not be dealt with here in detail. Only the following points will be pointed out: 1.

Approximately two thirds of human brains harbour a typical pattern of asymmetry involving the posterior end of the Sylvian fissure. The right is shorter and curls upward, whereas the left is longer and straight (Rubens et al., 1975). Likewise, the surrounding cortex is asymmetric, both the planum temporale (Geschwind and Levitsky, 1968) and the parietal operculum (Hochberg and LeMay, 1975) being larger on the left. Other brains (1/3) either are roughly symmetric or, less often, show a reversal of the standard pattern.

2.

These asymmetries correlate fairly well with several markers of functional asymmetry. About 70 percent of right-handers show a typical pattern of asymmetries as evidenced in vivo on carotid angiograms (Hochberg and LeMay, 1975) or CT-scans (LeMay and Kido, 1980), while left-handers and ambidextrous subjects show symmetry or reversed asymmetry more often than righthanders. A larger left parietal operculum on carotid angiogram was found in subjects with left hemisphere dominance for language as assessed by the Amytal test (Strauss et al., 1985). Direct measurements of the planum temporale are now possible with the technique of magnetic resonance imaging. Preliminary results showed a significant correlation between nonrighthandedness and absent or reversed planum asymmetry (Habib, 1989).

3.

Globally, significant results are best obtained when contrasting strong right-handers with nonright-handers, rather than with strong left-handers. This is consistent with Geschwind’s distinction between standard and anomalous dominance groups.

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Development of Asymmetries Macroscopically, two separate observations on the anatomy of the human fetal brain must be pointed out. Firstly, Wada et al. (1975) showed that anatomical asymmetry of the planum temporale is already present on fetal human brains as early as the 30th gestational week. This is of major importance since it definitely rules out an exclusively post-natal influence on cortical asymmetries. Thus, although side differences are present early, Wada and Davies (1977) showed that the degree of asymmetry may decrease between birth and adulthood in males. Secondly, Chi et al. (1977) showed that the development of the convolutions around the Sylvian fissure occurs earlier on the right side, and that structures surrounding the planum temporale may be recognizable on the right side as long as two weeks earlier than on the left. These apparently contradictory findings suggest that the right planum develops earlier but the left grows larger, due to either a faster rate or a longer duration of the maturation process on this side (Rosen and Galaburda, 1985). An alternative explanation, only hinted at here but discussed further below, would be that earlier sulcation on the right reflects faster neuronal loss on that side, reduction in that side, which then leads to anatomical asymmetry. Mechanisms of Asymmetry In the human embryo and then in the fetus, nerve cells undergo a series of steps leading to the formation of the cerebral cortex in the developing forebrain. One of these steps is characterized by neuronal migration, which takes place between approximately the 16th and 25th weeks of gestation, during which primitive cells migrate from germinal zones to attain their final positions in one of the six layers of the neocortical mantle. Each part of the cortex possesses its own timetable such that the perisylvian regions are the last ones to reach their mature appearance (Rosen and Galaburda, 1985). Gyrification, and thus the first gross evidence of asymmetry, appears shortly after this migratory stage is completed (Goldman-Rakic and Rakic, 1984). At the same time, another important process is taking place: the competition among neurons for synaptic targets, a step thought to regulate the rate of cell loss (in that only neurons that reach their cortical targets will survive, the others degenerate). This occurs following rules that are not yet fully understood, but may involve a complex combination of genetic, biochemical epigenetic, and functional epigenetic factors. The process of cell loss may account for the development of cortical

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asymmetries (Geschwind and Galaburda, 1984; 1985). The left planum, being ordinarily smaller than the right, would be at a disadvantage in the competition for synaptic targets, thus attaining a smaller number of ipsi- and contralateral connections. At the same time, if the right side indeed develops more quickly, it may attain a different pattern of connections that are more diffuse, since it meets less competition at the start. According to the Geschwind hypothesis, under the influence of factors modifying the development of the left planum (for example abnormal maternal and/or fetal secretion of hormones), the disadvantage of the right hemisphere may be reduced or reversed, resulting in absence or reversal of asymmetry. More recently, Galaburda et al. (1987) have added some important refinements to Geschwind’s early formulation of his theory. In a reappraisal of the Geschwind and Levitsky (1968) anatomical work, they found that brains the plana of which were most asymmetric, were also those whose total (right plus left) planum area was the smallest. Conversely, the less asymmetric brains displayed the largest right-plus-left planum area. They concluded that the planum asymmetry is only dependent on the size of the right planum, suggesting that any modifying factor acting during brain maturation to produce nonasymmetry (or anomalous dominance) must act on the right -- and not the left - hemisphere. In other words, hormones that would decrease ordinary dorr.inance would do so by enhancing the growth of the right side without slowing down the left. Or, conversely, increased asymmetry would result from (hormonal?) inhibition of the growth of the right side alone. In the latter case, anomalous asymmetry would be accompanied by improved capacity by the right side in the competition for synaptic targets. Galaburda and colleagues also found that the ordinarily smaller right planum was smaller because of diminished numbers of neurons, rather than as a result of side differences in the packing density. This suggests that mechanisms leading to a reduced size of the right planum affected either diminished production or enhanced elimination of neurons (Galaburda et al., 1986). Another issue relevant to the question of development of brain asymmetries is, that of callosal cortico-cortical connections. On the one hand, the abovementioned process of cell loss occurring during the mid-gestational period, which may be part of the mechanism for producing asymmetric cortical areas, is accompanied by some concomitant loss of axons, and as part of that, probably some callosal axons as well. Also, it has been demonstrated that a dramatic loss in callosal projections occurs in the early post-natal period (O’Leary et al., 1981), due not to cell loss but to terminal retraction or axonal pruning (Purves and

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Lichtman, 1985). In all species studied (mainly kittens, rats, and cats), it has been shown that callosal neurons are, at birth, much more widely distributed than in adult animals, and that transitory callosal projections are eliminated during a critical period, which precedes the myelination of callosal fibers and is simultaneous with a quick phase of synaptogenesis (Innocenti, 1986). In the rat, the adult pattern of callosal projections is attained before post-natal day 15, while myelination, starting about day 12-13, continues much later in adult life (Seggie and Berry, 1972). Juraska and Kopcik (1988) have observed variations in myelinated axon numbers with postweaning enrichment of the environment, suggesting long lasting environmental influences on the callosal morphology. In man, the exact timing of these various events is not known, but they are thought to occur in the first weeks or months of extrauterine life. Rosen et al. (1989) described different patterns of callosal connections in rat cortical areas according to whether they are more or less symmetric. Symmetric areas, as compared to asymmetric areas, contained a more diffuse and enhanced pattern of connections, thus paralleling and indeed surpassing the increased number of neurons in the former. Also, abnormal callosal connectivity has been associated with dysgenetic cortical lesions (Rosen et al., 1989), demonstrating that developmental events interfering with the establishment of normal cellular architecture can affect the connectional pattern of the lesioned area. Recently, Oppenheim et al. (1989) reported an interesting comparison of callosal morphology on sagittal resonance magnetic images in five pairs of identical twins. They showed that callosal areas, within each pair, were strikingly similar when compared to unrelated controls. They concluded that genetic influence was of major importance in the determination of callosal shape. Interestingly, in one of the twin pairs, handedness was non-concordant within the pair, since only one of the two subjects was left-handed (LQ=-40). Finally, similarity between callosal shape in twins does not rule out the effect of environmental factors, since twins are subjected to similar prenatal -- and probably also early postnatal -- influences. In this respect, it would be of most theoretical importance to compare dizygotic and monozygotic twins. It is thus probable that factors influencing the pattern of interhemispheric connections -- and hence possibly the pattern of functional asymmetry -- is a complex intermingling of genetic determination, prenatal epigenetic influences, and postnatal activity-dependent pressures.

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Hormonal Influences on the Development of Asymmetry The effect of hormones on the developing brain has been amply documented in animals and man. Sex Differences in Human Lateralization

During the past ten years, numerous studies have been devoted to the issue of sex differences in human brain function, mainly concerning differences in patterns of lateralization (see for instance reviews by McGlone, 1980; Bryden, 1979, Bradshaw and Nettleton, 1983). Although there is still no consensus about this question, it is generally accepted that women exhibit superiority in some aspects of verbal performance, while men are superior in some visuo-spatial tasks. With regard to handedness, most studies suggest that males are more often nonright-handed than females (Porac and Coren, 1981). Despite this latter finding, and mainly because of sex differences in responses to dichotic listening tests, females are thought to have greater bilateral representation of functions than males. Thus, sex differences in functional lateralization remains a poorly characterized and even less well understood subject, whereby much of the methodology used to describe these differences is suspect (see Galaburda, 1989). Sex differences have also been reported in the pattern of anatomical asymmetries, female brains being either less asymmetric (Rubens et al., 1976) or more often asymmetric in the opposite direction (Wada et al., 1975). These data, taken together, suggest that individual female brains could indeed be more asymmetric, in either direction, such that the average is less lateralized. This interpretation is compatible with the Geschwind hypothesis, which proposes that male hormones decrease the degree of anatomic asymmetry, towards more symmetry, whether it starts from right asymmetric or left. Another -- also largely debated -- explanation is that interhemispheric transfer is more efficient in women, a suggestion in accordance with the findings of deLacoste and Holloway (1982) of a larger posterior part of the corpus callosum in females. This observation, however, has not been confirmed in subsequent observations (Oppenheim et al., 1987; Byne et al., 1988), but Witelson (1989) recently found, on post-mortem speciments, a significant interaction between hand, sex, and size of the callosal "isthmus," the part of the callosum containing intertemporal and interparietal fibers - nonright-handed males but not females showing a smaller isthmus than right-handers. This

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observation would be predicted by Rosen et al.'s finding of increased callosal connectivity in symmetric architectonic areas (1989; see above), assuming that, consistent with the Geschwind hypothesis, individual males, particularly lefthanded males, are indeed more symmetric in the temporal and parietal regions2. In any case, findings of gross callosal morphology must be taken with caution at this time since virtually nothing is known about variability of topography of callosal fibers in the human brain. To summarize, some effect of sex on human brain functional lateralization appears quite probable, but further studies are obviously needed before firm conclusions can be drawn. Recent attempts at developing an experimental model in the rat have not yet yielded conclusive results. Berrebi et al. (1988) found a larger callosum in male rats, especially when provided with handling stimulation in the neonatal period, unlike Juraska and Kopcik (1988), who failed to disclose any sex difference in gross callosal size, but found, on ultrastructural examination, a larger number of callosal axons in females, and an increased axonal size in males. Here again, the significance of such findings, as well as their relevance to human lateralization, needs further clarification.

Effects of Sex Hormones on the Brain The best evidence for the existence of sex steroid effects on the brain stems from experimental research on the rat, in which there is a clear-cut sexual dimorphism in the motor behaviours associated with mating. This sexual behaviour can be changed by modifying the hormonal environment during a critical period taking place around birth in this animal (Kelly, 1985). Gorski et al. (1978) demonstrated that a part of the hypothalamus ("the sexually dimorphic nucleus of the preoptic area") is larger and contains more neurons in males than in females and increases in size during the first ten days after birth. If the female neonate is exposed to testosterone during this period, the nucleus will adopt the typical male pattern. Contrariwise, castration at birth of male offspring leads to the female pattern. This is a most demonstrative example of an epigenetic influence on anatomical and thence physiological sex related patterns. Moreover, it provides testosterone with a role most like a trophic

'

Recently, one of us (MH) globally replicated Witelson's findings in a study of 50 normal subjects whose corpus callosum was examined with magnetic resonance imaging. Non-righthanded males had larger total callosal areas, this effect being mainly observable on the anterior half of the callosum, whereas righthanded females tended to show a larger isthmus than righthanded males and females.

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effect, which if true in other regions of the brain, would make predictions in favour of lesser asymmetry and more axonal connections, including callosal axons. The Geschwind hypothesis predicts for lesser asymmetry in males, but because of “slowing”of growth of the left side, rather than a trophic effect. The present evidence supports the Geschwind prediction, but, like the revision of the hypothesis in Galaburda et al.’s recent findings on the planum temporale (see above), the mechanism for testosterone action would have to be changed from a slowing influence to a trophic one, affecting a usually smaller structure such as the right planum or the medial preoptic nucleus of the hypothalamus. Evidence for a trophic effect of testosterone exists. For example, the destruction of the medial preoptic nucleus in the male abolishes copulatory behaviour, while implantation of male sex steroids reactivates this behaviour in the castrated male and produces male behaviour in operated females (Everitt and Hansen, 1983). In tissue culture experiments (Toran-Allerand, 1978), testosterone has a trophic effect resulting from regulation of the mechanism of competition between axonal populations of different origins for post-synaptic sites. It must be noted that similar effects as those described in rodents have not been reported in primates. Interestingly, however, morphologic sexual dimorphism has been reported in humans in another hypothalamic nucleus, the suprachiasmatic nucleus (Swaab and Hofmann, 1984), a nucleus involved in the control of circadian rhythms such as the sleep-wake cycles. The functional correlate of this finding, however, is not clear. Sex Hormones and Brain Asymmetries: Animal Models

The presence of brain structural and functional asymmetry in rodents has provided an opportunity for studying biologic factors influencing asymmetry as a model of human cerebral dominance (Sherman et al., 1982). Sex steroids, especially testosterone and its metabolite estradiol, have been demonstrated to alter functional as well as anatomical asymmetries. Effect of Sex Hormones on Functional Asymmetries. One of the most obvious evidence of asymmetric behaviour in rats is that these animals may turn in circles, either spontaneously at night, or after administration of drugs during the day. Although in a given population there are as many right- as left-biased animals, individual animals show a consistent tendency to rotate preferentially in one direction. It has been shown that the circling direction of females is

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determined in utero (at least in part) by the proportion of males in the litter, and thus it has been suggested that the amount of testosterone in the placental blood and amniotic fluid may prenatally influence subsequent laterality (Glick and Shapiro, 1984). Neonatal postural asymmetry, first demonstrated by Ross et al. (1981) as a preferential tail posture bias towards the left or right, proved to be an advantageous model for studying hormonal influences on laterality. Moreover, this tail bias is correlated with direction of circling preference of animals as adults. Consistent sex differences are demonstrable in both neonatal and adult asymmetry. For example, in Sprague-Dawley rats, females have an average tail bias to the right, whereas males are randomly right- or left-biased. Denenberg et al. (1982) found that in Wistar rats generally left-biased as adults, female neonates, but not males, show significantly more often a left-tail bias. Rosen et al. (1983) injected androgens to pregnant rats during the last gestational trimester, and found that female pups exposed to testosterone in utero had, in a majority of cases, a rightward tail bias. Taken together, these results strongly suggest that sex steroids may influence lateralized behaviours in these animals (Denenberg, 1984). That such findings can be transposed to human laterality is more debatable, but they undoubtedly lend support to the testosterone hypothesis of anomalous dominance and left-handedness. Another remarkable animal model of hormonal effect on a lateralized behaviour is provided by studies on singing birds. In most species of singing birds, males learn their vocal repertoire during a critical period of the ontogenesis. Nottebohm and colleagues (1980; 1984) discovered a highly specialized neural system controlling song learning and production consisting in part of a telencephalic nucleus known as hyperstnatuni ventralis, pars caudalis (HVc), and its efferents on the ipsilateral hypoglossal motor neurons, each innervating the ipsilateral vocal muscles. In canaries, song production is controlled by the left system, since destruction of either the left HVc or left hypoglossal nucleus abolishes singing behaviour, while destruction of the right produces few if any effects on singing. There are, however, no substantial leftright morphometric asymmetries to explain the striking functional lateralization. The lateralized system is strikingly sexually dimorphic, being much less developed in females, which cannot sing. This dimorphism is both functional and morphologic and is regualted by sex hormones. Early castration in male canaries both impairs singing behaviour and reduces the HVc volume. Females exposed to testosterone in adult life acquire singing behaviour close to the male one, while their HVc nucleus becomes larger. This increase in size is explained by dendritic proliferation under the influence of hormonal fluctuations, and the

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learning of new syllables may be accompanied by the generation of new neurons in adult life. Thus, in the adult male, both HVc size and vocal virtuosity undergo seasonal changes (lower in autumn, higher in spring) under control of fluctuations in androgen secretion.

Effect of Sex Hormones on Anatomical Asymmetries. Two different types of animal models have proved useful to study the relationship between steroid hormones and morphologic brain asymmetries. The bulk of our knowledge in this area comes from the work of M. Diamond and her colleagues. They first investigated Long-Evans rats for hippocampal size. In males, the right hippocampus was found larger than the left, whereas the reverse was found for females (Diamond et al., 1982). Also, the cortex of the posterior portions of the hemispheres was thicker in the right than of the left in males, while in females, the asymmetry was lacking or not significantly left-biased. Other experiments were devoted to the study of the effects of modification of the hormonal environment on the posterior cortical asymmetry. Thus, male rats were castrated at birth and some of the rightward asymmetries were converted to leftward asymmetries at 90 days of age. Likewise, females were gonadectomized at birth and the resulting pattern of asymmetry was the male type (Diamond, 1984). Pappas et al. (1979) showed that exogenous estrogen administered to adult female rats decreased cortical thickness. Estrogen, therefore, appeared to have the opposite effect of testosterone, that is, an antitrophic - atrophic - effect. This led Diamond to study cortical estrogen receptor concentrations in male and female rats. They reported that the estrogen receptor concentration is greater in the left hemisphere in males, and in the right in females between birth and 8 days of age (Diamond, 1987); by day 25, receptors have nearly disappeared, suggesting a critical period for the effect of this hormone. There thus seems to exist an inverse relationship between estrogen receptor concentration and the direction of the cortical asymmetry, with the increased leftsided receptors in males possibly explaining the rightward asymmetry in the posterior cortex, and the increased rightsided receptors in females explaining at least a trend toward leftward asymmetry in that gender. However, the situation may not be as simple in humans, since if this were to be the case in human brains, it should be possible to demonstrate larger left plana temporale in females and larger right in males, which has never, to our knowledge, been done. In fact, as Galaburda et al. reported (1987; see above), the left planum temporale does not seem to vary interestingly in the population, with all the individual differences in

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asymmetry relating to the size of the right planum instead. These authors, however, did not have gender data on their sample, and some interesting sex differences might have been present vis b vis the left planum temporale.

Left-Handedness and Sex Hormones Pathological Evidence. To date, only a few works have dealt specifically with the relationship of sex hormones to brain functional lateralization in man. Postulating that diethylstilbestrol (DES), a synthetic estrogen, has similar masculinizing effects as testosterone on genetically female animals, Hines and Shipley (1984) compared verbal ability, visuo-spatial ability and cerebral lateralization for verbal stimuli (on a dichotic listening task) in two groups of women: 25 women with prenatal exposure to DES and 25 of their unexposed sisters. All subjects, with one exception in a control woman, were right-handed (unfortunately, the strength of right-handedness is not reported). Overall they found that exposed women had performances different from those of their unexposed sisters and resembling those of normal men, suggesting that the prenatal exposure to this hormone had a masculinizing effect on their pattern of cerebral lateralization. More specifically dealing with handedness is the paper by Nass et al. (1987), who reported that females with congenital adrenal hyperplasia - a condition in which lack of the adrenal e q m e 21-hydroxylase results in excessive testosterone production - are more left-biased on a hand preference questionnaire than their unaffected sisters. While male patients, in whom testosterone levels are not modified in utero, have typical male handedness patterns. These results can be considered to be in support of the Geschwind hypothesis. Recently, Cappa et al. (1988), starting from a previous report (Hier and Crowley, 1982) of decreased spatial abilities in male patients with hypogonadotropic hypogonadism, a congenital disorder characterized by a deficit in androgen secretion, studied a group of 13such patients with verbal and spatial performance tests, laterality tests, and handedness assessment. They did not find any significant difference from a control group, neither in spatial abilities, nor in degree of lateralization. All patients were reported to be right-handed; the detailed scores obtained in handedness questionnaires, however, were not given. Effect of Alcohol Exposure. Several works have recently focused on the effect of mothers’ alcohol intake on brain lateralization in their children. For

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example, it was observed that neonates born to mothers who drink heavily have significantly more head-turns to the left, while the majority of unexposed neonates normally turn their head toward the right side (Landesman-Dwyer et al., 1978), a feature which has been shown to be related to the child's later hand preference (Gesell and Ames, 1947; Michel, 1981). Zimmerberg and Riley (1988) compared the performance of 75-day old rats prenatally exposed to alcohol and that of control animals, both groups conditioned to press a lever with either the right or the left paw. While most of the controls used one paw preferentially, experiment animals used both paws together or alternated paws. The authors concluded that prenatal alcohol exposure alters the normal development of behavioural laterality, and that this altered development persists into adulthood. More recently, Zimmerberg and Reuter (1989) studied the preferred tail bias on post-natal day 1 in rats prenatally exposed to alcohol. On post-natal day 3, animals were sacrificed and brains were analyzed for right and left neocortical and hippocampal volumes. Several interesting results emerged from this study: First, a correlation was found between sex, tail bias, and neocortical thickness asymmetry - left-biased male neonates having significantly larger right anterior neocortices - a result compatible with the above mentioned findings of Diamond (1984); moreover, alcohol-exposed male animals had both anatomical (anterior neqcortical thickness) and functional (tail bias) patterns of asymmetry that resembled female patterns, suggesting a "feminization"effect of prenatal alcohol exposure. The authors' conclusions were that prenatal alcohol may act on brain lateralization by modifying testosterone activity and, conversely, that these effects strongly support the role of prenatal hormones in modulating the development of asymmetries. An alternative explanation might also be entertained. Fetal alcohol exposure is associated with microdysgenesis of the neocortex consisting of deposits of abnormally migrated neurons in the pia and subjacent cortex and sometimes worse dysgenesis (Clarren et al., 1978). Perhaps a sign of damage, these developmental abnormalities may contribute to the syndrome of "pathologic left-handedness." Evidence from Special Populations. Geschwind's theorizing on the development of brain asymmetry was largely based on epidemiologic observations of biologic associations of the nonright-handed state; these included developmental dyslexia and other learning disorders, stuttering, autism, asthma, eczema and other allergic conditions, autoimmune diseases, juvenile onset diabetes mellitus, premature graying of the hair, blond hair, scoliosis, hare lip,

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depression, migraine, but also certain positive characteristics such as artistic and musical talents, and visuo-spatial abilities, including athletic abilities. Among these conditions, statistical support for an association with nonrighthandedness is available for some. We will focus here on two special populations. Mathematically and Verbally Gifted Individuals: There are several statistical accounts to the effect that mathematical ability is significantly associated with left-handedness, especially in males. It is generally proposed to explain this association, that mathematical abilities rely on more fundamental visuo-spatial aptitudes that may be enhanced in heft-handers. The better coordination between left and right hemisphere functions has also been suggested to play a role, within a concept of mathematics, as a special language-like code for representing aspects of the world that are otherwise “complex spatial images” (Annett and Kilshaw, 1982). Benbow (1986) studied a group of 416 extremely gifted and talented students who represented the top 0.01 percentile of their age group in mathematical and/or verbal reasoning ability. There was only one girl for every twelve boys in the group. The frequency of left-handedness was almost twice that in an unselected control group; 53 percent of the gdted group also had some evidence of atopic (allergic) disease, as against 25 percent of a control group. These results are remarkably consistent with Geschwind’s model. Moreover, the overwhelming male predominance in the group is suggestive, although not confirmatory, of a role of hormonal factors in the determination of such characteristics, since even in these enlightened times it is not entirely possible to exclude social factors in groups thus selected. Benbow and Benbow (1987) subsequently showed that mathematical ability in their subjects was associated with increased right hemisphere functioning and/or more bilateral representation of cognitive functions, which they proposed may be related to prenatal exposure to high level of sex hormones. Finally, they reported that the gifted students were born significantly more often than control subjects during one of the five months between February and June. Their interpretation of this finding is that circumannually variable exposure to daylight could have altered the pineal secretion of melatonin, which in turn has an inhibitory effect on gonadal hormonal secretion. Examination of their data shows that the 6th gestational month for births occurring between February and June corresponds to the period from November to March. These are precisely the five months in the year when the duration of daylight is less than twelve hours, leading to enhanced melatonin secretion and consequently a reduced level of sex steroids during the critical period for development of cerebral

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asymmetries. This is, however, not concordant with Geschwind’s hypothesis which would have predicted the opposite, i.e., increased testosterone level. Of course, given the extreme rarity of these gifted conditions, it is most unlikely that such a universally present factor could act alone or even predominantly in its causation. Homosexuality: Male homosexuality has been linked to the reduced effect of testosterone during the early cerebral development (McCulloch and Waddington, 1981). Prenatal determination of homosexuality is also suggested from studies by DOrner et al. (1983), showing that homosexuals, more often than heterosexuals, are born to mothers having sustained severe stress (such as war, bereavement, rape, etc.) during pregnancy. Recently it has been suggested that a common intrauterine factor may have determined both homosexuality and shift in hemispheric lateralization. In a study of 94 male homosexuals attending a venereal diseases clinic, Lindesay (1987) found a shift toward left-handedness in this group. Homosexuals not only tended to display strong right-handedness less often than heterosexuals, but also significantly more homosexuals than heterosexuals had a score between -85 and t85 in a handedness questionnaire derived from Oldfield’s Edinburgh Inventory (1971). The author relates this result to the Geschwind hypothesis, noting however that, again, the putative hormonal abnormality during fetal development is more likely a lesser rather than a greater effect of testosterone. The Geschwind hypothesis did consider that at a critical time the testosterone effect in individuals at risk for becoming homosexual may have indeed been higher, followed shortly thereafter by a permanently lower effect (see, for instance, Ward and Weisz, 1980, cited in Geschwind and Galaburda, 1985). This was felt to be akin to the situation in Klinefelter’s syndrome (genotypic XXY and related syndromes), whereby the early levels of testosterone are thought to be high, followed by testicular atropy and diminished levels later and an association to nonright-handedness (Netley and Rovet, 1982). McCormick et al. (1987) also explored this issue by assessing handedness in 70 homosexuals, 38 males and 32 females. The results show a significantly higher proportion of left-handers than in the general population in female, but not in male homosexuals. Here again the authors relate this finding to an increased prenatal level of sex hormones, which is explicitly more consistent with Geschwind’s hypothesis. Finally, one may cite the report of Bear et al. (1988) on a group of homosexual men with AIDS. They measured brain asymmetries on the CT-scans of 16 such patients and compared them to 31 healthy heterosexual men. The patients showed a significant increase in reversal of the usual frontal lobe asymmetry, that is, left frontal lobe predominance

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instead of the usual right predominance. One of the offered explanations was that anomalous frontal asymmetry could be the result, just as is homosexuality, of abnormal levels of intrauterine sex steroids.

Pathological Associations of Lett-Handedness One of the most compelling -- and most often put forward -- arguments supporting Geschwind’s views is the association of left-handedness and atypical laterality to two groups of pathological conditions: leaning disorders (mostly developmental dyslexia), and immune and allergic diseases. As another chapter of this book is devoted to the latter, we will mainly focus here on learning disorders. Learning Disorders as a Model of Prenatal Determination of Anomalous Dominance A link between dyslexia and anomalous cerebral dominance or interhemispheric balance has been suspected for a long time (Orton, 1925), mainly because of two characteristics of dyslexic children: the relatively frequent occurrence of left-handedness and the observation that reading and writing errors typically involve right-left confusions. Moreover, these children are predominantly, but not exclusively, male, which is one of the reasons for invoking a hormonal role. Yet, the first attempts at detecting brain differences in these subjects are quite recent. Hier et al. (1978) and Rosenberger and Hier (1980) first described, on CT-scans, a frequent reversal of the pattern of asymmetry of the occipital lobes, a finding not replicated later by Haslam et al. (1981) who, however, found a larger proportion of symmetric brains than in normal controls. More recently, Rumsey et al. (1986), using magnetic resonance imaging, confirmed a greater than usual incidence of reversed asymmetries in dyslexics. And there are the direct autopsy observations of Galaburda and coworkers (Galaburda and Kemper, 1979; Galaburda et al., 1985), which, in five male dyslexic brains, found consistent lack of asymmetry of the planum temporale. This absence of asymmetry has been seen in an additional male dyslexic brain and in three consecutive female dyslexic brains (Humphreys et al., submitted for publication), raising to nine the number of consecutively studied dyslexic brains showing symmetry of the planum temporale at autopsy. In addition, Galaburda and colleagues have reported abnormalities mostly of the

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cerebral neocortex consisting of ectopic collections of neurons in the superficial layer I, often associated with distortion (dysplasia) of the adjacent cortical layers. Less often, micropolygyria, abnormal blood vessels (microangiomata), primary brain neoplasms, and thalamic changes have been noted. Interestingly, the cortical changes are often more prominent in the left hemisphere, especially in the inferior frontal areas, but some degree of bilaterality of lesions is almost the rule and the lateralization pattern of the lesions does not seem to relate to the expressed pattern of handedness. Similar lesions may be found in apparently normal brains, but they do not reach the numbers and prevalence illustrated by the dyslexic brains (Kaufmann and Galaburda, 1989). In one recent case of socalled developmental dysphasia (Cohen et al., 1989), which can be viewed as exhibiting an extreme form of the linguistic anomaly seen in developmental dyslexia, neuropathological abnormalities were restricted to a focus of microgyria in the left temporal lobe and absence of planum temporale asymmetry. Although the exact pathophysiology of the cortical abnormalities seen in the brains of dyslexics is not known, the experimental studies of Dvor6k and colleagues (1978) show that similar abnormalities can be induced experimentally in the rat by injuring the cortex before the end of the period of neuronal migration. In the human, this would mean that the microdysgenesis can be attributed to a process taking place during the 6th gestational month. As to the absence of ordinary asymmetry in the planum temporale of dyslexic brains, it is not possible to determine at this stage whether this represents a genetic predisposition related to nonright-handedness in families with dyslexia, or whether it reflects the effects of injury also implicated in the cause of the cortical anomalies. Early brain injury, probably through its ability to modify neuronal numbers and axonal connections, has been associated with aberrant patterns of lateralization (Satz, 1973). One may wonder whether clinical symptoms in dyslexia are more likely the consequence of cortical dysgenesis or of planum symmetry. One of us (AMG) feels that symmetry of language areas represents a risk factor for linguistic anomaly, being derived from the presence of a large, perhaps noisy system of neurons with excessive connectivity (see earlier discussion of the mechanisms of brain asymmetry, above). In that case, the presence of microdysgenesis, with accompanying additional aberrant patterns of cell architecture and connectivity (Rosen et al., 1989), represents a further risk factor - perhaps interfering with compensatory strategies - that allows for the clinical expression of the disorder in most cases.

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The Immune Hypothesis

We have proposed that immunological dysfunction may account for the microdysgenesis and other forms of pathology seen in the brains of dyslexic individuals (Galaburda, 1987; Humphreys et al., submitted for publication). This hypothesis offered in a somewhat different form, was initially proposed by Geschwind following several lines of evidence. First, Geschwind and Behan (198% 1984) showed that autoimmune diseases (especially those involving the intestinal tract and the thyroid gland) and atopic diseases (e.g., asthma, eczema, hay-fever) are 2.5 times as frequent in strong left-handers as in strong righthanders. Moreover, among 1,300 families in which at least one member was (or had been) dyslexic, they found an abnormally increased incidence of atopic diseases, thyroid disorders, as well as three cases of childhood autism. These findings were essentially confirmed in several subsequent studies (e.g., Smith, 1987; Pennington et al., 1987). Observations of familial clustering of these conditions suggest a complex pattern of heritability to account for the fact that different features of the constellation may be present in different members of the family. It has been suggested (Urion, 1988) that families of language disabled individuals may be divided into two groups - one with associated increased nondextrality and immune related disorders; the other with, actually, a decrease in these characteristics. Urion suggested that the families with the cluster of associations may have specific ethnic origins. In addition to the epidemiologic link that exists between illnesses implicating immune dysfunction and anomalies of cerebral dominance, left-handedness, and dyslexia, there is now experimental evidence that immune dysfunction may be related to developmental brain anomalies. Thus, some strains of mice known to develop spontaneous autoimmune diseases and learning abnormalities, e.g., New Zealand Black (NZB) and BXSB mice, exhibit subpial collections of neurons very similar to human layer I ectopias (Sherman et al., 1985). Sex hormones appear to influence immune function. In experimental animals, testosterone is known to alter thymic activity, and castration of the male rat prevents thymic involution. Clinically, the immunosuppressive properties of testosterone are sometimes exploited. Several immune diseases are unequally distributed between sexes, so that one may suspect sex hormones to be involved in their clinical expression. All these data suggested to Geschwind that there could be a link between cerebral dominance, sex hormones, and immunity. Geschwind hypothesized that two processes, the development of cortical asymmetries and of the thymus, take place synchronously, and that during this critical period,

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hormonal factors (mainly testosterone) could lead both to anomalous cerebral dominance by slowing down the maturational rate of left cortical areas and to an impaired immune system by altering thymic development (Geschwind and Behan, 1982). Thus, either because of increased endogenous or exogenous testosterone levels or increased sensitivity to the hormone, there would be increased risk for left-handedness and dyslexia and for immune diseases. As an extension of the immune hypothesis, Galaburda (1987; Galaburda et al., 1985; Humphreys et al., submitted for publication) proposed that cortical abnormalities in dyslexia could be due to "immune attacks" during the above defined critical period. They noted that fetal organ injury can be the consequence of immune and allergic disease. For example, mothers with systemic lupus erythematosis carry autoantibodies that can injure the fetus and lead to various forms of the neonatal lupus syndrome. Thus, neonatal congenital heart block has been linked to the transplacental passage of anti-Ro antibody (Olson and Lindsley, 1987; Lockshin et al., 1988), which is known to react to cardiac conduction tissue. Behan (cited in Behan and Geschwind, 1985), studying 45 mothers of dyslexic children, found an increased rate of spontaneous abortions, cardiac malformations, and a significant rise in anti-thyroid and antiRo antibodies titers. A similar mechanism acting to produce fetal brain injury could then lead to the form of microdysgenesis described in the human dyslexics and in the immune defective mice.

Fetal Position, Birth Circumstances, and Handedness Some mention has been made of the role of fetal position on later handedness. For instance, Dareste (1885, quoted in Grapin and Perpere, 1968) noted that in most placental animals it is the embryo's left side that leans against the vitellus, thus permitting the "freer"development of the right side, which faces the amniotic fluid. Although this view is likely to be too simplistic, it is amenable to empirical testing by means of modern echographic techniques. At best, however, the fundamental question would be displaced back one step, and it would still behoove one to explain the preferred intrauterine position in the first place. Or, of additional significance, does the position in ulero reflect some asymmetry of axial rotation that attests to a pre-existing asymmetry in the nervous system? (See Geschwind and Galaburda, 1985). Two studies, both of them before the echographic era, have been published that provide information about fetal position at birth and lateralization. The

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first was carried out in 1956 by Grapin (cited in Grapin and Perpere, 1968), and reported that among 53 children born in the (most frequent) left occiput anterior (LOA3) position, 39 were considered right-handed and none left-handed when examined during the second year of life. Among 22 children born in right occiput anterior (ROA) position, there were no right-handers and 10 were lefthanded. The same authors added in a subsequent study that the rate of ambidextrous subjects was significantly greater in ROA birth. Their interpretation of the findings was the following: they contended that during the last three gestational months the fetal head is positioned in the inferior part of the uterus in 95 percent of cases. Thus, when the position at birth is LOA, the fetal head is leaning on the left side, especially when the mother lies in the supine position, favouring a relative hyperimia of the left hemisphere, which in turn leads to the greater development of that hemisphere. This explanation fails to account for the fact that mothers in the last months of pregnancy generally sleep lying on one side, and that in general, the final head position is actually determined only a few weeks or even days before term. Another report of an association between handedness and position at birth is that of Churchill et al. (1962). These authors studied 1,102 children at birth and at two years of age. Six hundred and two of them (54.6%) presented in the LOA position at birth and 500 (45.4%) in the ROA position. Handedness data showed a significant correlation between left-handedness and ROA, and conversely between right-handedness and LOA. Moreover, in order to evaluate a possible genetic factor, results were presented separately for the 979 children without parental left-handedness, in whom a similar relationship was found. Finally, in discussing the possible link between head position and later handedness, the authors envisaged two possibilities: (1) the asymmetric fetal posture is extended after birth by tonic head posturing and later hand preference. (2) Slight injury to one hemisphere may result from “forces operating upon the head of the baby at birth.” Although not further detailed, these hypotheses are reminiscent of two current trends (developed elsewhere in this book) present in discussions of the etiology of handedness, which implicate environmental influences and birth stress. The more conservative interpretation that neural asymmetry precedes fetal posture is preferred by these authors. Thus, as already mentioned, the direction

’

In the LOA position, the axis of the fetal skull is oblique, the occiput being placed under the left anterior wall of the maternal birth canal. In the ROA position, the fetal skull is placed along the right anterior wall of the birth canal.

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of head-turning in neonates has been correlated both to later hand preference (Gesell and Ames, 1947; Michel, 1981) and to intrauterine birth position (Michel and Goodwin, 1979). Although purely mechanistic theories still have some defenders (Myslobodsky et al., 1987; Konishi et al., 1987), it seems more likely that prenatal head position, neonatal head turning, and handedness are three successive manifestations of a unique biological tendency, which would be first expressed in the pattern of cerebral asymmetries, detectable as early as the 30th fetal week, i.e., clearly before any mechanical pressure could have exerted itself on the fetal skull and brain. Finally, theories emphasizing birth stress (Satz, 1973; Bakan, 1977) will not be discussed here. We will nevertheless note that they only pretend to account for a subgroup of sinistrals (“pathological left-handers“), and that this subgroup may be quite limited in number. Even at that, there is detracting evidence. For instance, Schwartz (1988), studying handedness two years after birth in 290 children, found no significant correlation between handedness and several indices of birth stress (birth order, mother’s age and smoking, maternal and hospital records of complications during pregnancy and labour, and child’s condition at birth). In either case, as emphasized by Geschwind and Galaburda (1985), birth stress and sinistrality may as well be two consequences of the same underlying cause as being related causally to one another. Indeed, maternal prenatal a’mormalities such as hormonal dysfunction, or maternal conditions justifying hormonal therapy, could conceivably both yield premature or pathological birth and brain developmental anomalies leading to sinistrality. A recent report by Van Strien et al. (1987) is compatible with this hypothesis. These authors investigated 422 students with a large questionnaire about personal handedness, maternal pregnancy, birth complications, birth order, maternal age, and developmental learning disorders. Among 23 possible birth stress conditions, only four were significantly correlated with increased left-handedness, all being more closely related to maternal status (e.g., high blood pressure during pregnancy or induction of labour) than to conditions susceptible to damaging the child’s nervous system (such as prolonged labour or anoxia at birth). The authors’ conclusion was that intrauterine conditions played a more important role than did trauma to the brain at birth. The last point we would like to emphasize in this review concerns premature birth. Ross et al. (1987) reported handedness as well as neurological and intellectual outcome in 98 children born prematurely with birth weights of less than 1,500g. At the age of four years, 63 percent of the children were righthanded, 17 percent showed mixed handedness, and 19 percent were left-handed,

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which differed significantly from a fullterm control group. No such handedness bias was found in the parents of these children (although no specific information was provided about handedness in the parents of the left-handed preterm infants). Moreover, a significant correlation was found between nonrighthandedness and lower IQ or language learning disorders, which was regarded as further suggesting early injury to the left hemisphere. The authors concluded that events occurring in utero as well as perinatal illnesses associated with prematurity are responsible for the observed shift in handedness. O'Callaghan et al. (1987) reported a follow-up study of 39 children with "extremely low birth weights" (ELBW, below 1,OOOg). At the age of four years, 54% were considered left-handed, while among children weighing 1,OOOg and more only 8% were lefthanded. Also, among three subgroups of non-ELBW (1,OOO-1,250g, 1,250-1,500g, and 1,500g and over) rates of left-handedness were similar. In discussing early brain injury explanations of sinistrality, the authors observed that they would have expected a gradation in increased nonright-handedness among groups with different risk factors for brain damage as assessed by birth weight, which was not the case. Noticing that all ELBW infants were born between 26 and 29 weeks of gestation, they preferred to suggest that premature delivery had prevented the development of normal brain asymmetries taking place during the 30th gestational week (see above). This statement deserves to be further tested in a larger sample of subjects. Also, the respective role of low weight and prematurity could be assessed by comparing ELBW infants of different gestational age. If the present results were to be confirmed, they could turn out to be of major importance, since they would demonstrate that handedness is more random when the developing brain is not subjected to environmental (including hormonal) factors during the critical periods of intrauterine life. In conclusion, even hand preference, which on the surface would appear to be a simple and straightforward behavioural characteristic, is biologically and environmentally complex. It is likely that we have not begun to understand even the behavioural ramifications of manual ability, let alone the anatomic underpinnings. For instance, hand preference may fall out behaviourally on the basis of the goal of the task being tested: the preferred hand for learning a fine motor task, for instance, may be different from the hand preferred for expressing an emotional state. In right-handed violinists the decision to make the left hand finger the strings and the right hold the bow may reflect subtle preferences dictated by the esthetics of violin music; and so on. So when we ask about handedness, we might ask "handedness for what?," akin to saying that the

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right hemisphere is preferred for some aspects of speech (prosody, for instance) and the left for others. In any event, there is here at least a complex interaction between biological predisposition, which includes genetic and (non-behavioural) epigenetic factors, some of which act in utero, and subsequent learning, which is influenced by the nature of the task to be accomplished and by social pressures. Handedness, for each specific task, may represent the statistical outcome of these influences, and as such may be modifiable according to the plasticity of each of the contributing factors. In regard to the social factors, we inhabit a world with right-handed hegemony, which, in the spirit of introspective science (and a touch of Panglossian weakness), could mean that the biological factors behind many activities that are socially adaptive in most people are left brained. It is these factors that we call "normal," and all others "aberrant."

Acknowledgements The writing of this chapter was supported in part by NIH grants HD 20806 and 19819, by a grant from the Carl W.Herzog Foundation, and by a grant from the Research Division of the Orton Dyslexia Society.

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SECTION 11: PHYSIOLOGICAL AND GENETIC FACTORS

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LEFT-HANDEDNESS Behavioral Implications and Anomalies, S. Coren (Editor) 0 Elsevier Science Publishers B.V. (North-Holland), 1990

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Chapter 5

Laterality in Hemiplegic Children: Implications for the Concept of Pathological Left-Handedness Merrill Hiscock University of Houston and Cheryl K. Hiscock The Methodist Hospital, Houston

Background In this chapter we address the question of how early damage to one side of the brain affects perceptual, motor, and structural laterality. We do this by describing certain characteristics of a group of children (i.e., right hemiplegics) who have acquired, very early in life, an extreme form of pathological lefthandedness. By studying these children, and comparing them to a group of pathologically reinforced right-handers (i.e., left hemiplegics), we uncover some clues as to the laterality characteristics most affected by early asymmetric brain damage.

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Hemiplegia

Hemiplegia and hemiparesis are terms used to describe loss or impairment of motor function on one side of the body. In adults, hemiplegia is a common consequence of cerebrovascular accidents (strokes). Strokes can also cause hemiplegia in children, but the incidence of strokes is much lower in children than in adults. A more common cause of hemiplegia in children is cerebral palsy, a clinical entity that entails movement abnormalities secondary to prenatal or perinatal damage affecting the motor system at the level of the cerebral hemispheres. If damage is largely confined to one hemisphere, there will be weakness and loss of motor control on the contralateral side of the body. In other words, the child will be hemiplegic. Since hemiplegia is an indicator of damage to either the left or right hemisphere, children and adults with hemiplegia have been an important source of information about hemispheric specialization for linguistic and cognitive processes. For example, the special role of the left hemisphere in language processing is recognized largely because of clinical studies in which adults with acquired hemiplegia on the right side were contrasted with those whose hemiplegia is left-sided. By observing children who become hemiplegic at different ages, it is possible to gain insight into the development of language and other cognitive functions within the left and right hemispheres. Yet, not much attention has been given to the hemiplegia itself. To what extent are the affected limbs incapacitated? To what degree, if any, do the children improve as they grow older? Are left and right hemiplegics equally impaired? Are there qualitative features that differentiate left and right hemiplegics'! When hemiplegic children learn to write, do they hold the writing hand in an inverted or noninverted writing posture? One can also address some interesting issues regarding visual and somatosensory functions. For example, by studying hemiplegic children we can determine whether sighting dominance (eyedness) is altered by lateralized damage to the motor system. In other words, do the factors that produce a shift in handedness also lead to a shift in eyedness? Moreover, we might ask whether hemiplegia is associated with asymmetries of somatosensory performance. Does damage to the motor system of one hemisphere alter perceptual functions, as well as motor functions, on the contralateral side of the body? At a more general level, a sample of hemiplegic children is useful in investigating neuroplasticity, or the ability of the nervous system, especially thc

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immature nervous system, to reorganize following injury (Fletcher & Satz, 1983; St. James-Roberts, 1981; Satz & Fletcher, 1981; Smith, 1983). Does the affected side of the body become less impaired as the child matures, or are perceptual and motor deficits relatively constant throughout childhood? These questions, all of which concern consequences of asymmetric brain damage occurring early in life, are directly or indirectly relevant to our understanding of pathological left-handedness. Therefore, before describing our data, we shall summarize the concept of pathological left-handedness. Pathological Left-Handedness Are left-handers disadvantaged in cognitive ability? This seemingly simple question has led to an intriguing paradox: The incidence of left-handedness is often reported to be elevated in clinical populations having impaired intellectual ability (e.g., Bakwin, 1950; Bradshaw-McAnulty, Hicks, & Kinsbourne, 1984; Gordon, 1920; Hicks & Barton, 1975; Silva & Satz, 1979) but left-handers in the general population appear to be as bright as right-handers (Hardyck, Petrinovich, & Goldman, 1976; Newcombe & Ratcliff, 1973; Roberts & Engle, 1974). The paradox can be resolved by invoking the concept of pathological lefthandedness (Gordon, 1920; Orsini & Satz, 1986; Satz, 1972, 1973; Satz, Orsini, Saslow, & Henry, 1985; Silva & Satz, 1979). Suppose that handedness is usually determined by genetic factors (e.g., Annett, 1985; Hicks & Kinsbourne, 1976) and that left-handednessper se has little or no impact on intellectual status. Suppose also that the number of left-handers in various clinical populations is increased substantially by the addition of genotypic right-handers who have become lefthanded as a consequence of early brain damage. Cognitive deficits in these socalled pathological left-handers could be attributed not to their left-handedness but, rather, to the brain damage that caused them to become left-handed. One would have to posit the existence of pathological right-handers as well but there would be fewer of them, primarily because there are so few genotypic lefthanders at risk for being converted by brain damage to right-handedness. Furthermore, the pathological right-handers would constitute a tiny minority group within the huge pool of "natural" right-handers. Though vulnerable to criticism on various grounds (McManus, 1983), the concept of pathological left-handedness is a potentially powerful explanatory construct. One of its primary limitations, with respect to both research and clinical purposes, is the lack of empirical criteria for discriminating pathological left-handers from natural left-handers. Recently, however, Orsini and Satz

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(1986) discovered three characteristics that differentiated a sample of individuals with suspected pathological left-handedness from other left-handers: an elevated incidence of left-ear superiority in dichotic listening, hypoplasia of the right foot, and deficient finger tapping performance in the nondominant hand. Other hypothesized indicators of pathological left-handedness--hypoplasiaof the right hand, selective depression of Performance IQ, and absence of familial sinistrality-were not found more frequently in the group with suspected pathological lefthandedness than in comparison groups. The Orsini and Satz (1986) findings suggest that it may be feasible to use neuropsychological criteria to differentiate pathological left-handers from natural left-handers. More specifically, the findings suggest that laterality characteristics might be especially useful as indices of pathological left-handedness. But what specific characteristics are most representative of pathologically altered laterality? What measures are most sensitive to the effects of early asymmetric brain damage? Orsini and Satz selected their tests to reflect characteristics that have been associated with putative cases of pathological left-handedness in the clinical literature (Satz et al., 1985). Other sensitive tests may have been overlooked simply because they have been used less often in clinical investigations. A plausible alternative is to select tests known from systematic research to be most sensitive to the effects of early lateralized lesions. However, as noted by Orsini and Satz (1986), such systematic research is lacking.

0bjectives Our first objective is to summarize our study of hemiplegic children and to describe the laterality characteristics that differentiate left and right hemiplegics. Following that, we discuss possible implications of the findings with respect to the concept of pathological left-handedness. Finally, we extrapolate our findings to suggest a profile of perceptual, motor, and structural characteristics that may typify pathological left-handers with more subtle brain damage.

The Study Subjects

We identified a relatively large sample of children with hemiplegia associated with cerebral palsy. This sample was well suited for addressing the questions of

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135

interest. First, the sample was relatively homogeneous insofar as spastic cerebral palsy was the diagnosis in all cases. Second, although the precise etiology was seldom known, the brain pathology in all cases was attributable to prenatal or perinatal events. In fact, the damage probably occurred well before birth in most instances (Towbin, 1978). Third, the most obvious effects of the early brain damage were motor signs and symptoms. In many cases, the damage appeared to be restricted largely to the motor system. The sample consisted of 57 children (28 girls, 29 boys) selected from an urban treatment center for developmentally disabled children, two urban orthopedic schools and one orthopedic classroom in a suburban public school. Various racial, ethnic and socioeconomicgroups were represented. Each of the selected children had been diagnosed by a pediatric neurologist or orthopedic surgeon as having spastic cerebral palsy with hemiplegia. Each child met standard diagnostic criteria: increased tone, hyperreflexia and pathological reflexes in the affected limbs (Bray, 1969). Prenatal or perinatal origin of the hemiplegia was inferred from the absence of a documented encephalopathic event occurring after birth. Children with significant bilateral involvement (spastic quadriplegia) were excluded from the sample, as were children with ataxic, athetoid or mixed cerebral palsy and children who were unable to complete the battery of tests. The 27 left hemiplegic children and 30 right hemiplegic children who contributed data ranged in age from 4 to 14 years (A4 = 8.2 years, SD = 2.3). Children within each side-of-hemiplegiagroup were divided into younger (4 - 8 years) and older (9 - 14 years) subgroups. Table 1 summarizes the mean ages and other characteristics of children within each subgroup. Left and right hemiplegic groups did not differ significantly in age or familial sinistrality. Younger and older groups did not differ in familial sinistrality. Methods Fifteen measures were obtained during individual testing of each child. Of these, six were measures of motor characteristics. These included measures of hand preference, manual skill, foot preference, and handwriting posture. Of the nine remaining measures, two were measures of hand and foot size, four were measures of visual acuity and sighting dominance, and three were somatosensory measures. The specific tasks within each category are listed in Table 2. Detailed descriptions of the various tasks and administration procedures may be found in the original research reports (Hiscock, Hiscock,

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Table 1: Characteristics of left and right hemiplegic groups

Group

Younger (N=12)

Older (N.15)

Younger (N=18)

Older (N=12)

Age in years'

Ma1e:Female ratio

Hand preference'

Fami 1 ial sin istra 1 ity rat io'S2

6.5 (1.1)

6:6

+11.7 (4.6)

.07 (.12)

10.3 (1.6)

10:5

+10.9 (6.7)

(.I71

6.3 (1.1)

10:8

-12.3 (3.1)

.14 (.24)

10.0 (1.4)

3:9

-13.0

.15 (.18)

(0.0)

.10

'Standard deviations appear in parentheses. 'Familial sinistrality data are based on N=50, as no data were available for seven children. The ratio is computed for each child by dividing the number of left-handed first degree relatives by the total number o f relatives.

Benjamins, & Hillman, 1989a, 1989b, 1990). Motor Findings Hand Preference. Surprisingly, three children failed to show the expected hand preference on the Raczkowski, Kalat, and Nebes (1974) handedness questionnaire. One right hemiplegic child, a 7-year-old girl, wrote with her right hand and showed no overall asymmetry on the questionnaire. Two left hemiplegic boys, aged 5 and 11 years, scored in the left-handedness range on the questionnaire. Although the 5-year-old wrote with his right hand, the 11year-old wrote with his left hand. All three of these children showed only small asymmetries on measures of manual skill. One might predict that anomalous hand preference would be associated with either minimal motor impairment on the paretic side or with substantial impairment on both sides. In other words,

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Table 2: Tasks administered to left and right hemiplegic children

Category

Subcategory

Task or Measure

Motor

Hand preference

Raczkowski, Kalat, & Nebes (1974) Handedness Questionnaire Finger tapping Purdue Pegboard Grip strength Ba 1 1 kicking Writing name

Manual skill Foot preference Handwriting posture Structural

Hand length Foot length

Measurement o f hands with tape-measure Measurement of feet with tape-measure

Visual

Acuity Eye dominance

Snellen chart Hole Test Pointing Test Asher lest

Somatosensory

Stereognosis

Identification of plastic geometric forms Identification of one or two touched fingers Identification of pattern drawn on back of hands

Finger Identification Graphesthes ia

one would expect the lack of asymmetry to reflect either a very high level or a very low level of competence in both hands. The data, however, failed to support this expectation. The three children with anomalous hand preference were diverse in their overall levels of manual skill. Each of the remaining 54 children showed a strong hand preference on the Raczkowski, Kalat, and Nebes (1974) handedness questionnaire. All of the right hemiplegics obtained scores of -13 (extreme left-handedness) and all but two of the left hemiplegics obtained scores of t 13 (extreme right-handedness). The other two left hemiplegics had scores reflecting moderately strong righthandedness. Thus, the large majority of children, with respect to hand preference, conformed to the stereotype of hemiplegia, i.e., these children seldom used the paretic hand for any common unimanual activity.

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Manual Skill. The manual skill data, however, provide quite a different picture of differences between the hands. Irrespective of task--finger tapping, Purdue Pegboard, or grip strength--intermanual difference scores were distributed in a manner approximating a bell-shaped curve. The contrast between hand preference and skill asymmetry is illustrated in Figure 1. The top panel of Figure 1 shows the frequency distribution of questionnaire scores and the bottom panel shows the frequency distribution of right-minus-left difference scores for finger tapping. Not only is there a pronounced central tendency in the finger tapping scores, but five of the children performed better with the paretic hand. Similar patterns were seen in the Purdue Pegboard and grip strength data. A second notable finding from the manual skill data is that left hemiplegic children tend to outperform right hemiplegic children. This tendency was statistically significant for finger tapping and grip strength but failed to reach statistical significance in the case of the Purdue Pegboard. The superiority of the left hemiplegic children in finger tapping was particularly evident when their right-hand performance was compared with the left-hand performance of right hemiplegic children. In other words, when the dominant hand of one group was compared to the dominant hand of the other group, the left hemiplegics showed clear superiority. A third finding of note arose from a correlational analysis of right-minusleft difference scores from the three skill measures. This finding is shown in Table 3. For the combined sample of children, the various asymmetry scores were correlated at the t.80 level. Correlations were similar for the left hemiplegic children alone. In contrast, when correlations were computed for the right hemiplegic children alone, the coefficients ranged from t -25 to + .39. Each of the coefficients for right hemiplegics was significantly smaller than the corresponding coefficient for left hemiplegics at p = .01. Foot Preference. Foot preference was defined as the foot used to kick a ball placed on the floor in front of the child as he or she sat in a chair. This procedure was intended to avoid the possibility that the child, if standing, would kick with the nonpreferred foot while using the stronger leg for support (Freides, 1978). Three trials were used to generate a foot preference score ranging from 3 (consistent left-footedness)to t 3 (consistent right-footedness). The resulting distribution of scores resembled the distribution for hand preference scores, i.e., most of the right hemiplegic children showed consistent left-footedness and most of the left hemiplegic children showed consistent right-footedness.

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30

139

kZTJ RIGHT HEMI LEFT HEMI

=

45

2o

10

0 -15

-10

-5

0

5

10

15

HAND PREFERENCE SCORE

'O

E B RIGHT HEMI LEFT HEMI

1

-60

-40

-20

0

20

40

60

TAPPING SCORE (R-L)

Figure 1: Frequency distribution of scores from the Raczkowski et al. Handedness Questionnaire (top panel) and contrasting frequency distribution of right-minus-left difference scores from the fingertapping task (bottom panel). Adapted from Hiscock et al. (1989). Copyright 1989 by Lawrence Erlbaum Associates, Inc. Reprinted by permission.

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Table 3: Correlations among laterality scores for each of the three measures of manual performance'

Finger tapping ( F T ) Purdue Pegboard (PP) G r i p Strength (GS)

Finger tapping (FT) Purdue Pegboard (PP) G r i p Strength (GS)

PP

GS

83

.80 * 80

.a3

.7a .83

29

.39 .25

*

~~~~~

R i g h t Hemip leg i c s On 1y Finger tapping ( F T ) Purdue Pegboard (PP) G r i p Strength (GS)

'The c o e f f i c i e n t s represent p a r t i a l c o r r e l a t i o n s from which t h e l i n e a r e f f e c t s o f age, sex, and t e s t i n g order have been removed.

Perhaps the most remarkable aspect of the foot preference data is the difference between groups in the incidence of anomalous foot preference. Only 1 of 27 left hemiplegic children showed a left-foot preference, but 7 of the 30 right hemiplegics showed a right-foot preference. This difference, which is statistically significant, remains significant when the three children with anomalous hand preference are disregarded. Handwriting Posture. It has been suggested that left-handers who write in an inverted fashion--with the hand above the line of writing and the pencil tip pointing toward the bottom of the page--are characterized by left hemispheric language representation and ipsilateral control of writing (Levy & Reid, 1976, 1978). With this hypothesis in mind, we examined the handwriting posture of the herniplegic children as they wrote (or, in some cases, printed) their names. The results were striking: There was not a single instance of inverted writing among the entire sample of 57 children. Although inverted writing is relatively uncommon in normal right-handers,

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it is observed frequently in normal left-handed children (Peters, 1986; Peters & Pedersen, 1978). The 0% incidence of inverted writing in our sample of lefthanders (right hemiplegics) differed significantly from the 27.1% incidence reported by Peters and Pedersen (1978) for Canadian school children and the 62.6% incidence reported by Peters (1986) for German school children. Structural Findings Measurements of hand and foot length, taken with a tape-measure, showed that the hand and foot on the hemiparetic side are significantly shorter than the opposite hand and foot. The average difference was 1.5 cm for the hands and 1.0 cm for the feet. Visual Findings Visual Acuity. A standard Snellen chart was used to assess visual acuity. Since many of the children were unable to name all the letters of the alphabet, the results are based on only 36 children. On average, these children had visual acuity of 20/30 in each eye. There was no difference between left and right eyes nor between left and right hemiplegics. Most importantly, there was no significant interaction between eye and side of hemiplegia. Eye Dominance. Sighting dominance was measured using three common procedures: the Hole Test, the Pointing Test and the Asher Test. Correlations among these three measures ranged from t .75 to t .85 in our sample of 57 children. Irrespective of the test used, left hemiplegics tended to show somewhat stronger right-eye dominance than did right hemiplegics, but the difference failed to reach statistical significance. After combining scores from the three eye dominance tests, we classified 45% of the right hemiplegics and 60% of the left hemiplegics as right-eyed. These proportions were not significantly different from each other. The availability of normative eye dominance data for elementary school children (Hebben, Benjamins, & Milberg, 1981) allowed us to compare our right hemiplegic children with normal left-handed children and to compare our left hemiplegic children with normal right-handed children. Neither comparison yielded a significant result. With respect to the frequency of right- and left-eye dominance, right hemiplegics were comparable to normal left-handers and left

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Table 4: Frequency of right- and left-eye dominance in hemiplegic children and in normal left- and right-handed school children'

Group

Number o f Chi ldren with Right-eye Dominance

Number of Children w i t h Left-eye Dominance

8

16 9

L e f t hemiplegics Normal r i g h t - h a n d e d

15 123

51

Total

159

86

Right hemiplegics Normal left-handers2

13

10

'From Hiscock e t a l . ( i n press). Copyright 1990 by Lawrence Erlbaum Associates, I n c . Reprinted by permission. 2Data from Hebben e t a l . (1981).

hemiplegics were comparable to normal right-handers. These data are summarized in Table 4. The negative findings, however, do not completely rule out the possibility that lateralized brain damage may alter eye dominance. We assume that most of our right hemiplegics are genotypic right-handers who have become lefthanded because of early damage to the left cerebral hemisphere. If these pathological left-handers are compared to normal right-handers, rather than normal left-handers, their incidence of right-eyedness is significantly lower than the incidence of right-eyedness in the comparison group (44.8% vs. 70.7%,p < .025). This suggests that eye dominance and motor asymmetry are not totally independent. Presumably, the early asymmetric brain damage that caused a shift of handedness also produced a shift of eyedness in some children. Somatosensory Findings Stereognosis. This task entailed palpating one of five plastic geometric forms with one hand or the other, and then identifying the form by pointing to one of six drawings on a response display. Analysis of correct scores for the left and right hands revealed only a significant interaction between side of hemiplegia and hand. Both right- and left-hemiplegic children performed markedly worse

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with the hemiparetic limb than with the relatively unaffected limb. Finger Identification. On this test, children were required to determine the finger (or two fingers) touched by the experimenter on each of 10 trials. The child’s hand was out of view during the tactile stimulation but not during the response, which consisted of pointing to the stimulated finger(s) with the opposite hand. As in the case of stereognosis, performance on the hemiplegic side was significantly inferior to performance on the opposite side. Graphesthesia. The examiner used the eraser end of a pencil to draw designs on the back of the child’s hand while the child’s vision of the hand was occluded. Then the child pointed with the stimulated hand to the drawing on a visual display that matched the pattern drawn on the hand. Although performance on the paretic side tended to be inferior to performance on the opposite side, the difference was not statistically significant. Correlation between Perceptual and Motor Asymmetries Partial correlations, with the linear effects of age, sex, and testing order removed, were computed as a means of relating each of the visual and tactile asymmetries to four measures of asymmetry (hand preference, finger tapping, Purdue Pegboard, and grip strength.) These correlations were computed first for the sample as a whole and then separately for left and right hemiplegics. For the entire samples associations between visual acuity asymmetry and motor asymmetry were small in magnitude and negative in polarity. However, correlations between eye dominance and motor asymmetry ranged from .25 to .44with a median correlation of .37, which suggests that motor asymmetry accounts for about 14% of the total variation in sighting dominance. Of the tactile asymmetries, stereognosis bore the strongest relationship to motor asymmetry. Correlations for the sample as a whole ranged from .68 to .76. Correlations between asymmetry of finger identification and motor asymmetry were somewhat more modest, with a range from .44 to .59. Correlations between asymmelry of graphesthesia and motor asymmetry fell between .20 and .25. When correlations were computed separately for each group, the only noteworthy pattern observed was a positive correlation between motor skill and asymmetry of stereognosis. The magnitude of these correlations was roughly .40 for right hemiplegics and .60 for left hemiplegics.

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Implications for Pathological Left-Handedness Assuming that right hemiplegia is an extreme manifestation of pathological left-handedness (and that left hemiplegia is an extreme manifestation of pathologically reinforced right-handedness) in the large majority of children, we summarize those aspects of our findings that might apply to children with more subtle early damage to either the left or right hemisphere. Motor Findings

Even though most children in the sample show the extreme hand and foot preferences that one might expect of hemiplegic children, the hand preferences of three children are markedly anomalous, as are the foot preferences of eight. Moreover, asymmetries of manual performance (finger tapping, Purdue pegboard, and grip strength) form approximately bell-shaped distributions with substantial overlap between left and right hemiplegic groups. Despite extensive asymmetric damage to the motor systems of these children, their motor laterality does not always reflect their brain pathology. Consequently, it seems unlikely that lesser degrees of early brain damage, as in putative cases of pathological left-handedness, would consistently produce the expected motor asymmetries, especially with respect to manual performance and foot preference. Another set of findings reveals some important differences between left and right hemiplegic children. Right hemiplegic children are significantly more likely to prefer the foot on the hemiparetic side. Right hemiplegics perform more poorly than left hemiplegics on some skill tests, and this inferiority is attributable primarily to the dominant hand. Whereas asymmetry scores from the three skill tests are highly correlated in left hemiplegics, the corresponding associations are significantly weaker in right hemiplegics. The manual skill data lead to two conclusions. From the performance of right hemiplegics (as well as their hand preference), we infer that left hemisphere damage may overcome the propensity to be right-handed. This supports the basic premise of pathological left-handedness. From the differences between right and left hemiplegics, however, we learn that the tendency to be righthanded is not completely overcome even when the motor system of the left hemisphere is severely damaged. When significant loss of right-hand functions occurs early in development, the left hand does not develop the degree of dominance enjoyed by the right hand following loss of left-hand functions. These data support the view that a predetermined pattern of cerebral specialization

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favours the right hand in the majority of children. Irrespective of whether this pattern is genetically determined (Annett, 1985; Hicks & Kinsbourne, 1976), it is present early in life and is resistant to the modifying influence of asymmetric motor system damage. Differences between left and right hemiplegics are not restricted to differences in skill level. As noted previously, correlations among laterality scores for manual skill are significantly higher for left hemiplegics than for right hemiplegics and the concordance between hand and foot preference is significantly greater for left hemiplegics. To what can these findings be attributed? One explanation is that, when motor control is transferred to the right hemisphere following damage to the left hemisphere, the transfer is sometimes incomplete. Depending on the severity and location of the damage, the left hemisphere may retain control over certain aspects of movement, producing an uneven pattern of asymmetry across motor tasks. Another explanation is that the left hand of right hemiplegics, being somewhat unskilled relative to the right hand of left hemiplegics, is more susceptible to environmental influences. Practice in manual activities during everyday life may generalize unevenly to laboratory tasks, causing the left hand of right hemiplegics to perform well on some motor tasks while performing poorly on others. These explanations are not mutually exclusive. The low correlations seen in right hemiplegics may reflect a combination of pathological factors and practice effects. Every one of the 30 right hemiplegic children is a noninverted writer. According to Levy and Reid’s (1976, 1978) hypothesis, the noninverted writing posture in left-handers indicates right hemisphere language representation and contralateral motor control. Taken at face value, this suggests that damage to the left hemisphere was sufficient in all cases to have shifted language to the right side. This inference, however, cannot be confirmed without direct assessment of language lateralization (cf. Ajersch & Milner, 1983). Each of the 27 left hemiplegic children also adopted a noninverted writing posture but this finding is less remarkable insofar as inverted writers among right-handers are relatively rare (Coren & Porac, 1979). In our sample of hemiplegic children, there is no significant age difference in asymmetry of motor skill. The inferiority of the hemiparetic limb neither diminishes nor increases in magnitude between the ages of 4 and 14 years. Moreover, there is little age-related improvement of motor skill on either side of the body. It appears that performance with both the dominant and hemiparetic hand falls farther and farther below age norms as the child matures.

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If these findings can be extrapolated to children with more subtle manifestations of pathological left-handedness, one would predict that, even though motor asymmetries will tend to remain constant throughout childhood, the motor development of pathological left-handers will be impaired, relative to other children. However, the validity of these extrapolations presumably depends on whether damage to the motor system is unilateral or bilateral. The poor overall motor development in the hemiplegic children may reflect bilateral motor system damage. Perhaps the milder damage occurring in other pathological left-handers would more often be restricted to the left cerebral hemisphere, thereby allowing normal development of motor skill in the unaffected (dominant) hand. Structural Findings Asymmetries of hand and foot size did not prove to be particularly accurate indicators of asymmetric motor damage. Although the hand and foot on the hemiparetic side are significantly shorter than the hand and foot on the dominant side for both right and left hemiplegics, the mean magnitude of the difference is only about 10% for hands and 5% for feet. If we classify children as left or right hemiplegics on the basis of hand length asymmetry, we find that three children show no asymmetry and two others are misclassified. A similar calculation on the basis of foot length asymmetry requires excluding five children who show no asymmetry and one whose hemiplegic foot could not be measured because it was deformed. Four of the remaining 51 children are misclassified. If the children who show no asymmetry are included in the calculations, the overall classification accuracy of these measures is 91% for hands and 84% for feet. Whereas these success rates are respectable, comparable classification accuracy is achieved using tests of motor performance, and somewhat better accuracy is achieved using the handedness questionnaire. In addition, it presumably is easier to differentiate right hemiplegics from left hemiplegics than to differentiate right hemiplegics (pathological left-handers) from normal lefthanders. Visual Findings Visual acuity is not useful in differentiating right and left hemiplegic children. Eye dominance not only fails to differentiate right hemiplegics from left hemiplegics, but it also fails to differentiate right hemiplegics (pathological lefthanders) from normal left-handers. Only when right hemiplegic children are

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compared with normal right-handed children does a significant difference in eye dominance materialize. Correlations between eye dominance and the various indices of motor asymmetry fall within the range of .25 to .44. The eye dominance findings, which at first seem somewhat contradictory, may be understood by considering the strength of the obtained relationships. The differential incidence of right-eye dominance in right hemiplegics and normal right-handers, as well as the association between eye dominance and motor asymmetry, suggests that early lateralized damage to the motor system can alter eye dominance in some individuals. The lack of complete independence between handedness and eyedness has theoretical significance. Perhaps damage to the left or right hemisphere of some children is not confined to the corticospinal motor system but extends to other systems, e.g., the oculomotor system, in the same hemisphere. Alternatively, congruence between eye and hand dominance may develop in some cases simply because such congruence is adaptive or convenient (Broer & Zernicke, 1979). The weakness of the association between hand preference and eye preference, however, makes it seem unlikely that eye preference would serve as a useful indicator of pathological left-handedness. If there is no significant difference in eye preference between right and left hemiplegic groups, or between right hemiplegic children and normal left-handed children, there is little likelihood that eye preference will allow investigators to distinguish individuals with milder manifestations of pathological left-handers from natural left-handers. Somatosensory Findings Tests of tactile perception yield no evidence of an overall skill difference between groups. Even though right hemiplegics tend to be more impaired than left hemiplegics on tests of motor skill, their performance is comparable to that of left hemiplegics on perceptual tasks. With regard to performance asymmetries on the three tactile tests, each of the outcomes requires a somewhat different explanation. Stereognosis and graphesthesia tasks yield markedly dissimilar outcomes. Stereognosis performance is lateralized in the expected manner within each group of hemiplegic children, but graphesthesia performance is not significantly asymmetric within either group. This difference can be attributed to differential motor involvement in the respective tasks: Stereognosis requires active palpation but graphesthesia does not. Poor stereognosis performance with the impaired hand probably stems from impaired movement control. The results for finger

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identification fall between the extremes of the strongly asymmetric stereognosis performance and the generally symmetric graphesthesia performance, not because finger identification requires an intermediate degree of motor activity but because it seems to depend on a finger schema consisting of both perceptual and motor components (Benton, 1959; Jackson & Zangwill, 1952). Even though finger identification requires no overt motor activity, defects of finger identification are so closely related to defects of finger praxis that the two kinds of impairment "can be conceived as representing a single basic neuropsychological deficit" (Benton, 1959, p. 44).

Hypothetical Characteristics of Pathological Left-Handers Insofar as right hemiplegia in children with cerebral palsy serves as a model for pathological left-handedness, our findings suggest four motor characteristics that one might expect to observe in pathological left-handers: (1) a strong preference for the left hand, (2) relatively poor skill in both hands, (3) low concordance of right-minus-left asymmetry scores across different tasks, and (4) a noninverted writing posture. However, since correlations among different measures of skill asymmetry in normal individuals tend to be modest (Barnsley & Rabinovitch, 1970; Eling, 1983), the third characteristic may less useful than the others in differentiating pathological left-handers from the general population of left-handers. Although motor asymmetry is unlikely to change much as the pathological left-hander matures, this lack of change seems to reflect slow development of motor skill irrespective of hand. Alternatively, one might suspect that poor overall motor skill development is characteristic only of children with bilateral brain damage. Hypoplasia of the hand and foot on the paretic side can be used to discriminate between right and left hemiplegic children and perhaps between right hemiplegics and natural left-handers as well. This conclusion is partially consistent with the results of Orsini and Satz (1986), who found that asymmetry of foot size, but not hand size, differentiated putative pathological left-handers from normal left-handers. Left-eye sighting dominance, though more frequent in right hemiplegics (pathological left-handers) than in normal right-handers, does not differentiate right hemiplegics from normal left-handers. Since graphesthesia performance fails to differentiate right and left hemiplegic children, this measure also is

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unlikely to discriminate between pathological and natural left-handers. Impaired right-hand performance on tests of stereognosis and finger identification may reflect primarily the diminished motor functioning of the right hand. Nonetheless, since correlations between asymmetry scores from these tasks and motor tasks are no greater than .40 for right hemiplegics, these two somatosensory tasks clearly are not redundant with the motor tasks. Consequently, stereognosis and finger identification cannot be ruled out as measures that might be useful in differentiating pathological and natural lefthanders.

Conclusions For children who otherwise would have become right-handed, right hemiplegia of prenatal or perinatal origin represents an extreme instance of pathological left-handedness and, as such, attests to the feasibility of the concept. Although most hemiplegic children--irrespective of the affected side-show strong preference for the nonimpaired (or less impaired) hand, they tend to be less asymmetric on tests of motor performance, foot preference, hand and foot length, visual acuity, eye dominance, and tactile perception. Some of these tasks fail to provide statistically significant differentiation of right hemiplegic children (pathological left-handers) from left hemiplegic children (pathologically reinforced right-handers) and even tasks that succeed in differentiating the groups nonetheless misclassify a substantial minority of children. Functional lateralization of the cerebral hemispheres does not appear to be a unitary phenomenon. Damage to motor system of one hemisphere may cause a partial shifting of functions to the other hemisphere. The resultant degree of laterality varies from one task to another and seems to be particularly variable in right hemiplegics. Comparisons between left and right hemiplegics provide information about the effects of early asymmetric brain damage on laterality but do not allow us to specify whether right hemiplegics differ from normal left-handers. Measures that differentiate most clearly between right and left hemiplegics are not necessarily the same measures that would provide the best differentiation between natural and pathological left-handers. From the tasks for which appropriate normative data for left-handers are available, we know only that right hemiplegics do not differ significantly from normal left-handers with respect to eye preference and that right hemiplegics are significantly more likely than normal left-handed children to write with a noninverted handwriting posture. Definitive criteria for

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differentiating pathological left-handers from normal left-handers remain to be developed. The data of Orsini and Satz (1986) provide a starting point. The present data, by showing which aspects of laterality are most affected by early asymmetric brain damage, suggest some additional possibilities.

Acknowledgements Preparation of this chapter was supported by a research grant to Merrill Hiscock from the Medical Research Council of Canada. The authors wish to acknowledge the contributions of David Benjamins and Stephen Hillman to the empirical work described in the chapter. The authors are grateful to Roxanne Inch for her assistance.

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Eling, P. (1983). Comparing different measures of laterality: Do they relate to a single mechanism? Journal of Clinical Neuropsychology, 5, 135-147. Fletcher, J.M., & Satz, P. (1983). Age, plasticity, and equipotentiality: A reply to Smith. Journal of Consulting and Clinical Psychology, 51, 763-767. Freides, D. (1978). On determining footedness. Cortex, 14, 134-135. Ghent, L. (1961). Developmental changes in tactual thresholds on dominant and nondominant sides. Journal of Comparative and Physiological Psychology, 54, 670-673. Gordon, H. (1920). Left-handedness and mirror-writing, especially among defective children. Brain, 43, 313-368. Hardyck, C., Petrinovich, L.F., & Goldman, R.D. (1976). Left-handedness and cognitive deficit. Cortex, 12, 266-279. Hebben, N., Benjamins, D., & Milberg, W. (1981). The relationship among handedness, sighting dominance and acuity dominance in elementary school children. Cortex, 17, 441-446. Hicks, R.E., & Barton, A.K. (1975). A note on left-handedness and severity of mental retardation. Journal of Genetic Psychology, 127, 323-324. Hicks, R.E., & Kinsbourne, M. (1976). On the genesis of human handedness: A review. Journal of Motor Behavior, 8, 257-266. Hicks, R.E., & Kinsbourne, M. (1978). Human handedness. In M. Kinsbourne (Ed.), Asyiitritetrical fiinction of the brain (pp. 523-549). Cambridge: Cambridge University Press. Hiscock, C.K., Hiscock, M., Benjamins, D., & Hillman, S. (1989a). Motor asymmetries in hemiplegic children: Implications for the normal and pathological development of handedness. Developmental Neuropsychology, 5, 169-186. Hiscock, C.K., Hiscock, M., Benjamins, D., & Hillman, S. (1989b). W-riting posture in right hemiplegic children. Cortex, 25, xxx-m. Hiscock, C.K., Hiscock, M., Benjamins, D., & Hillman, S. (in press). Eye dominance and somatosensory asymmetry in relation to motor asymmetry: Evidence from hemiplegic children. Developmental Neuropsychology. Jackson, C.V., & Zangwill, O.L. (1952). Experimental finger dyspraxia. Quarterly Journal of Experimental psycho lo^, 4, 1-10. Levy, J., & Gur, R.C. (1980). Individual differences in psychoneurological organization. In J. Herron (Ed.), Neuropsycliology of left-handedness (PP. 199210). New York: Academic Press. McManus, I.C. (1983). Pathological left-handedness: Does it exist? Journal of Communication Disorders, 16, 315-344. Orsini, D.L., & Satz, P. (1986). A syndrome of pathological left-handedness: Correlates of early left hemisphere injury. Archives of Neurolou, 43, 333337. Orton, S.T. (1937). Reading, writing and speech problems in children. New York: W.W. Norton, 1937. Peters, M. (1986). Incidence of left-handed writers and the inverted writing position in a sample of 2194 German elementary school children. Neuropsychologia, 24, 429-433.

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Peters, M., & Pedersen, K. (1978). Incidence of left-handers with inverted writing position in a population of 5920 elementary school children. Neuropsychologia, 16, 743-746. Porac, C., & Coren, S. (1976). The dominant eye. Psychological Bulletin, 83,880897. Raczkowski, D., Kalat, J.W., & Nebes, R. (1974). Reliability and validity of some handedness questionnaire items. Neuropsychologia, 12, 43-47. Rasmussen, T., & Milner, B. (1977). The role of early left-brain injury in determining lateralization of cerebral speech functions.Annals of the New York Academy of Sciences, 299, 355-369. Rudel, R.G. (1978). Neuroplasticity:Implicationsfor development and education. In J.S. Chall & A.F. Mirsky (Eds.), Education arid the brain. The seventyseventh yearbook of the National Society for the Study of Education (pp. 269-307). Chicago: University of Chicago Press. St. James-Roberts, I. (1981). A reinterpretation of hemispherectomy data without functional plasticity of the brain: I. Intellectual function. Brairi and Lariguuge, 13, 31-53. Satz, P. (1972). Pathological left-handedness: An explanatory model. Corta, 8, 121-135. Satz, P. (1973). Left-handedness and early brain insult: An explanation. Neuropsychologia, 11, 115-117. Satz, P., & Fletcher, J.M. (1981). Emergent trends in neuropsychology: An overview. Jounial of Cortsulting arid Clinical Psychology, 49, 851-865. Satz, P., Orsini, D.L., Saslow, E., & Henry, R. (1985). The pathological lefthandedness syndrome. Brain and Cognition, 4, 27-46. Semmes, J., Weinstein, S., Ghent, L., & Teuber, H-L. (1960). Sornutosensoty changes after pertetratirig bruin woitrids in man. Cambridge, MA: Harvard University Press. Silva, DA., & Satz, P. (1979). Pathological left-handedness: Evaluation of a model. Brain arid Language, 7, 8-16. Smith, A. (1983). Overview or "underview": A comment of Satz and Fletcher's "Emergent trends in neuropsychology: An overview." Journal of Consiilting and Clinical Psychology, 51, 768-775. Soper, H.V., & Satz, P. (1984). Pathological left-handedness and ambiguous handedness: A new explanatory model. Neuropsychologia, 22, 511-515. Towbin, A. (1978). Cerebral dysfunctions related to perinatal organic damage. Jounial of Abrionnal Psychology, 87, 617-635. Vargha-Khadem, F., O'Gorman, A.M., & Watters, G.V. (1985). Aphasia and handedness in relation to hemispheric side, age at injury and severity of cerebral lesion during childhood. Brain, 108, 677-6%. Winer, B.J. (1971). Statistical principles in experimental design (2nd ed.). New York: McGraw-Hill. Woods, B.T. (1980). The restricted effects of right-hemisphere lesions after age one: Wechsler test data. Neuropsychologia, lS, 65-70. Woods, B.T., & Teuber, H.L. (1978). Changing patterns of childhood aphasia. Annals of Neurology, 53, 273-280.

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The Neuroanatomy of Atypical Handedness in Schizophrenia Paul Satz, Michael Foster Green Steven Ganzell University of California, Los Angeles George Bartzokis Brentwood VA Hospital and UCLA Anthony Bledin Medical Diagnostic Imaging Joseph F. Vaclav University of California, Los Angeles Introduction A left hemisphere defect and/or anomalous form of lateralization has long been proposed, at least in a subset of schizophrenic patients (Marin & Tucker,

1981; Merrin, 1981; Seidman, 1983). One area that has attracted increasing interest in the past few years concerns patterns of atypical manual preference and/or motor domiitance in schizophrenia. The historic connection between manual and hemispheric speech dominance (Geschwind & Galaburda, 1985; Hecaen & Ajuriaguerra, 1964; Satz, 1979) represents only part of the reason for this interest. Asymmetries in manual preference are established early in childhood and, if pathological, may provide potential markers of early brain insult, including its neural substrate, long before the onset of schizophrenic symptoms (Seidman, 1983; Satz, Orsini, Saslow, & Henry, 1985). Furthermore, the effects of this insult on different brain structures probably remain silent until the targeted structure reaches maturation (e.g., young adulthood for frontal

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lobes and schizophrenia onset, Weinberger, 1987). Also, these asymmetries, in contrast to cognitive processes, are more easily assessed and are less influenced by age, socio-economic status, education and/or IQ. In light of these advantages, what does the literature tell us about patterns of atypical manual dominance, if any, in schizophrenia? For convenience, these studies can be divided into those that report (a) positive results (Le., a shift in handedness to the left, nine studies: Chaugule & Master, 1981; Dvirskii, 1976; Gur, 1977; Luchins, Weinberger, & Wyatt, 1979; Manoach, Maher, & Manschreck, 1988; Nasrallah, Keelor, Van Schroeder, & McCalley-Whitters, 1981; Nasrallah, McCalley-Whitters, & Kuperman, 1982; Oddy & Lobstein, 1972; Walker & Birch, 1970), (b) null results (five studies: Fleminger, Dalton, & Standage, 1977; Lishman & McMeekan, 1976; Merrin, 1984;Taylor, Dalton, & Fleminger, 1982; Wahl, 1976), and (c) paradoxical results (is., an increased shift in handedness to the right, two studies: McCreadie, Barron, & Winslow, 1982; Taylor, Dalton, & Fleminger, 1980). A study by Chapman and Chapman (1987) found an increase of mixed-handedness in a group of psychosis-prone subjects, which could be viewed as an additional positive result. It is difficult to critique these studies given the marked differences in subject selection, sample size, diagnostic criteria for schizophrenia, and assessment and clsification of handedness, as well as in interpretation of results. The primary problem is the differences in the assessment and classification of handedness. Although all studies used some type of preference questionnaire, there was substantial variation in the number of items used (range = 1-23, M = 10.6) as well as in the criterion used for handedness classification (range = 75%-100% in thirteen studies, unknown in three). If a conservative criterion is used ( k . , consistent unilateral preference across all items [loO%]), then the incidence of manifest right-handedness will vary inversely with the total number of items. In other words, the more items used, the higher the probability of nondominant hand preferences (Annett, 1985). Despite methodological differences across studies, the results support one general conclusion: there is a shift in the distribution away from the right hand in schizophrenia that is manifested as an increase in at least two subtypes: manifest left-handedness and/or mixed-handedness. Although both subtypes are observed in the normal population where the handedness distribution is typically J-shaped and shifted to the right (Annett, 1985), their prevalence (especially mixed-handedness) have been reported to be higher in schizophrenia. A raised incidence of manifest left-handedness (> 20%) in populations with kriowri brain

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insult has long been associated with early focal injury to the left hemisphere (before age six) that may result in alterations in handedness (pathological lefthandedness), hemispheric organization (preserved right hemisphere speech, impaired visuo-spatial ability) and/or limb development (right hemihypoplasia) [Satz, 1972; Satz et al., 19851. Mixed-handedness, however, has been more difficult to explain. One problem rests on how the construct is defined and measured. For example, the larger the number of tasks used to decide manual dominance, the higher the incidence of mixed-handedness. A second problem rests on the relatively unchallenged assumption that lateral preference for a skilled movement (e.g., hammer) is generally consistent over time. This view has tended to discourage further analysis of the nature of the pattern of mixed-handedness in humans. Hence we do not know if the variability is due to differences bemeen tasks (is., throw right, hammer left) or within tasks. The distinction is not trivial. The failure to adopt a consistent unilateral preference for a task requiring skilled movement may reflect a basic defect in the establishment of manual dominance that should be differentiated from the general population of mixed-handers. By varying the assessment procedure typically used in the assessment of human handedness, we have identified a subgroup of mixed-handers who show significant variability in their lateral preference for the same task. This identification is based on the administration of an eight item Hand Preference Demonstration Test in which each item is presented three times in a quasirandom order within a session. An atypical pattern of dominance has now been observed in a large subset (approximately 40%) of autistic and non-autistic mentally retarded patients (Soper, Satz, Orsini, Henry, Zvi, & Schulman, 1986; Soper, Satz, Orsini, Van Gorp, & Green, 1987), and in a smaller subset (approximately 20%) of two schizophrenic cohorts (Green, Satz, Smith, & Nelson, 1989; Nelson, Satz, Green, & Gaier, Note 1). Virtually all of the mixedhanders in the autistic and mentally retarded cohorts showed signs of withintask variability on three or more of the eight item handedness demonstration test, whereas only half of the mixed-handers within the schizophrenic cohorts showed this amount of within-task variability. We have defined this subtype as ambiguous handedness and have shown that its prevalence in the normal population, especially after age six, is rare (approximately 3%) [Green et al., 1989; Liederman & Healey, 1986; Satz, 19881. What neural substrate might account for this atypical form of manual preference? We had earlier suggested (Soper & Satz, 1984) that ambiguous handedness represents another pathological phenotype that unlike pathological

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left-handedness, arises from early brain damage of such severity that neither side of the brain is sufficiently intact for the establishment of manual dominance. The only support for this hypothesis, albeit indirect, rests on three autism studies which have observed lower IQ scores for this mixed-handed subtype compared to the more lateralized subgroups (Soper et al., 1986; Tsai, 1982; Fein et al., 1984). This explanation, however, seems more difficult to accept for a non-retarded schizophrenic cohort who, in contrast to autistic and mentally retarded samples, showed a lower incidence of ambiguous handedness (20% vs. 40%). Although one might still postulate a bilateral, though less severe type of early brain insult for the schizophrenic patients, the critical substrate would still have to involve the primary motor structures (e.g., pyramidal) on the left side of the brain. This is the only side that, if damaged, could shift the expected J-shaped distribution of manual preference away from the right (Soper & Satz, 1984). What remains unclear, however, is whether these gradations in nonright-handedness (mixed or left-handed) spring from primary unilateral (left) or bilateral asymmetric (left > right) perturbations to the developing brain. For this reason, the present study seeks to examine critical structures related to motor laterality in each hemisphere, with special attention to the left, that might correlate with these deviations in manual preference. To address this question we decided to focus apriori on only two anatomical measures in each brain side (i.e. ventricular brain ratios and hemisphere size) using quantitative measures of magnetic resonance imaging (MRI) in the coronal plane in a subgroup of lateralized and non-lateralized schizophrenic patients as well as matched controls. Although a few studies have looked at neuroanatomical correlates of left-handedness (Andreasen, Dennert, Olsen, & Damasio 1982; Katsanis & Iacono, 1983; Pearlson et al., 1989), the present study represents the first attempt to investigate anatomical substrates of atypical (mixed and ambiguous) handedness in schizophrenia.

Subjects Twenty-five schizophrenic subjects (22 male) from the inpatient units of Camarillo State Hospital were recruited from a study of handedness in schizophrenia (Green et al., 1989). All patients met DSM I11 criteria for schizophrenia based on an expanded version of the Present State Exam (PSE,

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Wing, Cooper, & Sartorius, 1974). All interviewers were trained to reliability of at least 85% for symptom presence with the Diagnosis and Psychopathology Unit of the UCLA Clinical Research Center (Robert P. Liberman, P.I.). The mean age for the patients was 35.3 (SD = 8.3), mean years of education was 11.6 (5.0), the average number of years since first institutionalization was 14.4 (6.5), and the average amount of medication (in chlorpromazine equivalents) was 1229.7 (744.8). Comparison scans of non-psychiatric control subjects were obtained from the data library of the imaging facility and consisted of scans from patients referred for headaches. These referrals had normal studies as determined by the staff radiologist (A.B.). Subjects were selected as control subjects after they were matched one-to-one to the patients for gender, age, and race. Potential control subjects were excluded if they had a history of head trauma or seizures, or if they had professional occupations. The average socio-economic status for the controls according to the Hollingshead scale was 4.9 (with seven as the lowest possible rating) which would not be expected to differ significantly from the patients. Brain Measures MRI of the head was performed using a 0.6 tesla instrument (Teslacon, Technicare Corporation). A spin echo sequence (TR 470 ms, T E 22 ms) was used to obtain T1 weighted images on a 256 x 256 display matrix and 7.5 mm slice thickness. It was decided apn'un' that the anatomical measure most relevant to our hypotheses would be the ventricular-brain ratio (VBR), as this measure controls for inter-subject variability. Studies of the VBR in schizophrenia often reveal that, as a group, schizophrenic patients have larger VBRs than normal controls (Weinberger, Wagner, & Wyatt, 1983). The finding of large VBRs is usually interpreted as an indication of brain atrophy or hypoplasia. The VBR measures lend themselves to the study of laterality when divided into separate VBRs for the left and right hemispheres. Also, in combination (total VBR) it provides a test for bilateral involvement. All measurements were taken from the coronal image that showed the maximum area of the third ventricle. The coronal plane seemed most appropriate for a study of manual dominance because the slices pass through the basal ganglia and would be expected to pass through a large portion of the sensorimotor cortex. In addition, the operational definition for choice of slice

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(largest third ventricle) allowed for inter-rater agreement of 100% for selection of slice. Eight X 7.5 cm. radiographic film images were used for area measurements. The Bioquant computer-assisted area measurement program was used to obtain all measurements. This system uses a graphics pad and a scribe with cross-hairs to trace the edge of the region of interest. The translucent graphics pad was placed in a frame over fluorescent lamps for best visualization of the images. Area measurements were made by a rater blind to the handedness of the subjects (S.G.). The reliability of the measurement procedures was determined by repeating measurements on a random selection of 15 scans. The results from the repeat measurements were consistently within plus or minus 5% of the original values. The following measurements were determined separately for the left and right hemisphere using the same coronal slice: 1)Lateral Ventricles. The area of the lateral ventricles was determined by tracing the area of the ventricles inscribed by the brain and the midline inscribed by the septum pellucidum. 2) Hemisphere area. The area of the brain was outlined by following the contours of the cortex. The sulci that were wider than the cross hairs of the measuring scribe were followed until they became so narrow that the cross hairs completely covered the sulcus. The area of the corresponding lateral ventricle was not included in the hemisphere measurement.

3) Ventricularbrain ratio in the Coronal Plane (cVBR) was determined by dividing the area of each lateral ventricle by the area of the corresponding hemisphere.

The following measurements were calculated from the measurements listed above: 4) Total cVBR was determined by adding together the areas of the right and left

ventricles and dividing by the area of the right and left hemispheres. 5 ) Difference cVBR was determined by subtracting the left cVBR from the right cVBR.

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Handedness Measures The Hand-preference Demonstration Test was administered to all subjects in a single session (Soper et al., 1986). The test requires the subject to demonstrate the use of eight items that involve a wide range of activities. The specific items are: (1) eating with a spoon, (2) drinking from a cup, (3) brushing teeth with a toothbrush, (4) drawing with a pencil on a piece of paper, (5) throwing a ball, (6) hammering with a plastic hammer, (7) picking up a piece of candy, (8) picking up a dime. The items were administered three times in a quasirandom order. The procedure yielded 24 unilateral manual responses from all subjects. The scores could range from right-handedness(using the right hand for all 24 responses) to left-handedness(using the left hand on all 24 responses).

Subjectswere considered as three groups: lateralized patients, non-lateralized patients, and normal controls. The lateralized patients showed consistent right hand preference on all 24 responses. In contrast, the non-lateralized patients showed at least one instance of inconsistent preference (i.e. switching hands for the same item). The areas for the left and right hemispheres for the three groups are shown in Figure 1. According to one-way analysis of variance, the groups showed no difference in area of the right hemisphere. The area of the left hemisphere showed differences (F = 5.13, df = 2,47,p < .01) because both patient groups had significantly smaller left hemispheres compared with normal controls (p c .05, Fisher PLSD). When comparisons were then made within the schizophrenic group (pooled for handedness), a smaller left than right hemisphere was observed in terms of mean area (t = 1 . 9 6 , ~< .05, one tailed) and frequency (21/25 patients showed left < right hemisphere size; compared with 14/25 controls). Figure 2 shows the areas of the right and left ventricles for the three groups. No significant differences were noted, although the non-lateralized patients tended to have smaller right ventricles compared to lateralized patients (p < .14) and normal controlsp < .07). Overall cVBRs (both hemispheres) did not differ among the groups (Means = 2.4, 2.2, and 2.3 for the lateralized patients, non-lateralized patients, and normal controls, respectively). When the left and right cVBRs were considered separately, no significant differences were found among the 3 groups. However,

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Figure 1: Mean Area Measurement of the Left and Right Hemispheres (with Standard Error Bars)

Figure 2: Mean Area Measurement of the Left and Right Ventricles (with Standard Error Bars)

Figure 3 shows that the non-lateralized patients tended to have larger left cVBRs and smaller right cVBRs than the other groups. In fact, when we consider the difference scores (right cVBR - left cVBR) the ANOVA is significant (1; = 3.85, df = 2,47, p c .05) The non-lateralized patients showed a significantly greater asymmetry (in the negative direction) than lateraliied patients (p < .03) and the normal controlsp < -02). Nine of the 10 non-lateralized patients showed larger left than right cVBRs. The normal controls and the lateralized patients, in

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3r 2.5

2 C

1.5

R 1

0.5

" A

Lateralized

Nonlateralized

=

Right Coronal VBR

Normal Controls

Lelt Coronal VBR

Figure 3: Coronal Ventricular-Brain Ratios for the Left and Right Hemispheres (with Standard Error Bars)

contrast showed little evidence of cVBR asymmetry.

Discussion The present results provide preliminary evidence for an anatomical asymmetry (left cVBR > right cVBR) that was primarily associated with the non-lateralized schizophrenic group. In fact, 9 of the 10 subjects in this subgroup revealed a pattern of left larger than right cVBRs. Furthermore, there was no evidence to suggest bilateral alteration in terms of hemisphere or ventricular size, or total VBR in either schizophrenic subgroup. The results provide initial support for a neural substrate that may account for the leftward shift in the distribution of manual lateralization (e.g., atypical handedness). An increase in VBR asymmetry (left > right) could be accounted for by unusually large left cVBRs or unusually small right cVBRs. To determine which of these factors were contributing to the asymmetry, the cVBRs for the nonlateralized patients were compared to the means for the normal controls. If we consider large deviations in VBR as scores beyond plus or minus 1.5 SD of the mean for normal controls, the non-lateralized patient group had only 1 subject with a large left VBR and only 2 with small right VBRs. Therefore, it is unlikely

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that the observed VBR asymmetry was due solely to unusually large left or small right cVBRs. One form of atypical handedness (ambiguous handedness) was postulated to have a bilateral neural substrate (Soper & Satz, 1984). However, the original suggestion of bilateral involvement with ambiguous handedness was based on severely retarded cohorts (autistic and mentally retarded). The nature of atypical handedness may differ between the retarded samples and schizophrenic patients. For example, only four of the schizophrenic subjects were inconsistent on two or more handedness items, while the mentally retarded subjects with ambiguous handedness were characterized by inconsistency on three or more items. This selection bias in the schizophrenic group was influenced by the lower base rate of ambiguous handedness in schizophrenia compared to autism and mental retardation (Soper et al., 1984, 1986, 1987) as well as the number of patients available for the MRI procedure (length of stay, informed consent, etc.). As such, the non-lateralized group may have been comprised of patients with less severe anomalies in the brain, but still sufficient to shift the distribution away from consistent right-sided preference. Another form of atypical handedness is pathological left-handedness where the putative neural substrate is postulated to be a left-sided focus (Satz, 1970, 1972; Harris et al., 1983). In contrast to pathological left-handedness, only 2/10 of the present non-lateralized patients were left-handed in terms of writing hand. This result suggests that the lesion substrate was either less severe and/or outside of the critical speech and motor zones on the left, but sufficient to shift the distribution away from consistent right-handedness. The finding of increased anatomical asymmetry in the non-lateralized patients is provocative, but it should be viewed with appropriate caution until the results are replicated on other cohorts of lateralized and non-lateralized patients. In addition, the non-lateralized group should include patients with greater withinitem inconsistency to determine whether the VBR asymmetry is associated with all degrees of inconsistent manual preference. This preliminary finding of an anatomical correlate of atypical handedness does not rule out a role for nonstructural factors in hand preference. An underlying neural substrate may interact with fluctuating psychotic symptomatology to produce the observed inconsistencies. We are currently assessing handedness in a sample of remitted schizophrenic patients to determine if manual preference (which is normally considered highly stable) might be staterelated in this population.

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Several studies have considered the correlations between neuroanatomy and left-handedness. A recent study (Katsanis & Iacono, 1989) reported that lefthanded schizophrenic patients had larger VBRs than right-handed subjects, and a previous report found a trend in the same direction (Andreasen et al., 1982). Consistent with these findings, Pearlson et al. (1989) reported that the presence of left-handedness was a significant predictor of larger VBR in a multiple regression analysis. It is difficult to directly compare our results to these previous studies because handedness in these studies was determined by a single item (writing hand) or by a questionnaire with no performance demonstration or assessments of within-item inconsistency. In addition, all of the previous studies used bilateral VBRs rather than left or right VBRs. Despite these differences, all studies report an association between types of nonrighthandedness (left or mixed) and VBR. Another finding of interest was the smaller left hemisphere in both patient groups (regardless of handedness) compared with normal controls. The left hemisphere was also observed to be smaller than the right within the schizophrenic group in terms of mean size and frequency. Such a finding (still preliminary) is consistent with an earlier computerized tomographic (CT) report of lower density in the anterior left hemisphere of schizophrenic patients (Golden et al., 1981). This anatomical finding, while unrelated to hand preference, is compatible with historical, as well as current interests regarding a left hemisphere abnormality in schizophrenia (e.g., Flor-Henry, 1976; Gur, 1978).

Acknowledgements The authors thank Norrie Shanonfelt, Lin Nelson PhD, and Donna Gaier for their help with the data collection. The sample was obtained with the excellent cooperation of the staff and administration of Camarillo State Hospital and Medical Diagnostic Imaging, Thousand Oaks, CA.. Funding for the project came from NINCDS Grant 22074-01 to Dr. Satz.

References Andreasen, N.C., Dennert, J.W., Olsen, S.A., & Damasio, A.R. (1982). Hemispheric asymmetries and schizophrenia. American Journal of Psychiatry, 139, 427-430.

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Annett, M. (1985). Lefi, right, hand and brain: The right shift theory. Hillsdale, NJ: Erlbaum. Chapman, J.P., & Chapman, L.J. (1987). Handedness of hypothetically psychosisprone subjects. Journal of Abnormal Psychology, 96, 89-93. Chaugule, V.B., & Master, R.S. (1981). Impaired cerebral dominance and schizophrenia. British Journal of Psychiatry, 139, 23-24. Dvirskii, A.E. (1976). Functional asymmetry of the cerebral hemispheres in clinical types of schizophrenia. Neuroscience and Behavioral Physiology, 7, 236-239. Fein, D., Humes, M., Kaplan, E., Lucci, D., & Waterhouse, L. (1984). The question of left hemisphere dysfunction in infantile autism. Psychological Bulletin, 95, 258-281. Fleminger, J.J., Dalton, R., & Standage, K.F. (1977). Handedness in psychiatric patients. British Journal of Psychiatry, 131, 448-452. Flor-Henry, P. (1976). Lateralized temporal-limbic dysfunction and psychopathology.Annals of the New York Academy of Sciences, 280, 777-795. Geschwind, N., & Galaburda, A.M. (1985). Cerebral lateralization; Biological mechanisms, associations, and pathology: I A hypothesis and a program for research. Archives of Neurology, 42, 428-459. Golden, C.J., Graber, B., Coffman, J., Berg, R.A., Newlin, D.B., & Bloch, S. (1981). Structural brain deficits in schizophrenia. Archives of’ General Psychiatry, 38, 1014-1017. Green, M.F.,Satz, P., Smith, C., & Nelson, L. (1989). Is there atypical handedness in schizophrenia? Journal of Abnormal Psychology, 98, 57-61. Gur, R.E. (1977). Motoric laterality imbalance in schizophrenia. Archives of General Psychiatry, 34, 33-37. Gur, R.E. (1978). Left hemisphere dysfunction and left hemisphere overactivation in schizophrenia. Journal of Abnonnal Psychology, 87, 226-238. Harris, L.J. (1983). Laterality of function in the infant: Historical and contemporary trends in theory and research. In G. Young, S.J. Segalowitz, C.M. Corter, & S.E. Trehub (Eds.), Manual specialization and rhe developing brain. New York: Academic Press, 177-247. Hecaen, H., & Ajuriaguerra, J. (1964). Lefi Handedness. New York: Grune and Stratten. Katsanis, J., & Iacono, W.G. (1989). Association of left-handedness with ventricle size and neuropsychological performance in schizophrenia. American Journal of Psychiatry, 146, 1056-1058. Liederman, J., & Healey, J.M. (1986). Independent dimensions of hand preference : Reliability of the factor structure and the handedness inventory. Archives of Clinical Neuropsychology, 1, 371-386. Lishman, WA., & McMeekan, E.R.L. (1976). Hand Preference patterns in psychiatric patients. British Journal of Psychiatry, 129, 158-166. Luchins, D.J., Weinberger, D.R., & Wyatt, R.W. (1979). Anomalous lateralizations associated with a milder from of schizophrenia. American Journal of Psychiatry, 136, 1598-1599.

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Manoach, D.S., Maher, BA., & Manschreck, T.C. (1988). Left-handedness and thought disorder in the schizophrenias. Journal of Abnomal Psychology, 97, 97-99. Marin, R.S., & Tucker, G.J. (1981). Psychopathology and hemispheric dysfunction. Journal of Nervous and Mental Disease, 169, 546-557. McCreadie, R.G., Barron, E.T., & Winslow, G.S. (1982). The Nithsdale Schizophrenia Survey: 11. Abnormal Movements. British Journal of Psychiatry, 140, 587-590. Merrin, E.L. (1981). Schizophrenia and brain asymmetry: An evaluation of evidence for dominant lobe dysfunction. Journal of Nervous and Mental Disease, 169, 405-416. Merrin, E.L. (1984). Motor and sighting dominance in schizophrenic and affective disorder. British Journal of Psychiatry, 146, 539-544. Nasrallah, H.A., Keelor, K., Van Schroeder, C., & McCalley-Whitters, M. (1981). Motoric lateralization in schizophrenic males. American Journal of Psychiatry, 138, 1114-1115. Nasrallah, H.A., McCalley-Whitters, M., & Kuperman, S. (1982). Neurological differences between paranoid and nonparanoid schizophrenia: Part 1. Sensory-motor lateralization. Journal of Clinical psychiatry, 43, 305-306. Nelson, L., Satz, P., Green, M.F., & Gaier, D. (1988). Atypical handedness in schizophrenia revisited. Submitted for publication. Oddy, H.C., & Lobstein, T.J. (1972). Hand and eye dominance in schizophrenia. British Journal of Psychiatry, 120, 331-332. Pearlson, G.D., Kim, W.S., Kubos, K.L., Moberg, P.J., Jayaram, G., Bascom, M.J., Chase, G.A., Goldfinger, A.D., & Tune, L.E. (1989). Ventricle-brain ratio, computed tomographic density, and brain area in 50 schizophrenics. Archives of General Psychiatry, 46, 690-697. Satz, P. (1972). Pathological Left-handedness: An explanatory model. Corieu, 8, 121-137. Satz, P. (1979). A test of some models of hemispheric speech organization in left- and right-handers. Science, 203, 1131-1133. Satz, P. (1988). Primate handedness: A paradoxical link to humans? Brain and Behavioral Sciences, 11, 4-5. Satz, P., Orsini, D.L., Saslow, E., & Henry, R.R. (1985). The pathological lefthandedness syndrome. Brain and Cognition, 4, 27-46. Seidman, L.J. (1983). Schizophrenia and brain dysfunction: An integration for recent neurodiagnostic findings. Psychological Bulletin, 94, 195-238. Soper, H.V., & Satz, P. (1984). Pathological left-handedness and ambiguous handedness: A new explanatory note. Neuropsychologia, 22, 511-515. Soper, H.V., Satz, P., Orsini, D.L., Henry, R.R., ZVi, J.C., & Schulman, M. (1986). Handedness patterns in autism suggest subtypes. Journal of Autism and Developmental Disorders, 16, 155-167. Soper, H.V,, Satz, P., Orsini, D.L., Van Gorp, W.G., & Green, M.F. (1987). Handedness distribution among the severely to profoundly mentally retarded. American Journal of Mental Deficiency, 92, 94-102.

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Taylor, P., Dalton, R., & Fleminger, J.J. (1980). Handedness in schizophrenia. British Journal of Psychiatry, 136, 375-383. Taylor, P., Dalton, R., & Fleminger, J.J. (1982). Handedness and schizophrenic symptoms. British Journal of Medical Psychology, 55, 287-291. Tsai, L.Y.(1982). Handedness in autistic children and their families. Journal of Autism and Developmental Disorders, 12, 421-423. Wahl, O.F. (1976). Handedness in schizophrenia. Perceptual and Motor Skills, 42, 9 4 - 9 6 . Walker, HA., & Birch, H.G. (1970). Lateral preference and right-left awareness in schizophrenic children. Journal of Nervous and Mental Disease, 151, 341350. Weinberger, D.R. (1987). Implications of normal brain development for the pathogenesis of schizophrenia. Archives of General Psychiatry, 44, 660-669. Weinberger, D.R., Wagner, R.L., & Wyatt, R.J. (1983). Neuropathological studies of schizophrenia: A selective review. Schizophrenia Bulletin, 9, 193212. Wing, J.K., Cooper, J.E., & Sartorius, N. (1974). The Measurement and Classification of Psychiatn'c Symptoms. London: Cambridge University Press.

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Chapter 7

Phenotype in Normal Left-Handers: An Understanding of Phenotype is the Basis for Understanding Mechanism and Inheritance of Handedness Michael Peters University of Guelph

To the person brought up in the positivist tradition, Feierabend yand Kuhn notwithstanding, the realization that some questions do not have a satisfactory answer comes as somewhat of a shock. I am beginning to suspect that the question of what causes a person to use a particular hand for a particular activity may not have a satisfactory answer. The standard solution to such a quandary has been offered by Gertrude Stein, in her reply to Alice Toklas' query as Gertrude was lying on her death bed: "Gertrude, what is the answer?" To which the reply was "what is the question?" Thus, it is useful to consider what we might accept as legitimate questions in the area of handedness. The first, and really most basic question concerns phenotype: how is handedness expressed in individualscomprising given populations? On superficial examination the question of what phenotypes exist is rather straightforward, and indeed, one would expect that a thorough exploration of phenotype should precede questions about mechanism and modes of inheritance. After all, before theories about mechanisms and inheritance can be addressed one has to know what range of phenomena have to be accounted for. Surprisingly, the amount of systematic attention given to phenotype in handedness is relatively small and

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attempts to deal with the inheritance of handedness have generally proceeded from a rather narrow base of phenotype specification. Two interacting factors are responsible for the general lack of enthusiasm for work that is explicitly concerned with phenotype. First, and this is quite obvious even to the casual observer, hand preference is rarely compelling in the sense that one hand absolutely cannot do the things the other can do. There may be limitations similar to the ones encountered in work with early hemispherectomies,where it does matter which hemisphere is lost, but not in any overwhelmingly obvious way, but these limitations have not been fully explored for handedness. The research paradigms that would have to be employed, for instance, tracking the acquisition of skills in persons who have lost the preferred hand, are confounded by the fact that so many years have been spent with a focus on the preferred arm. Nevertheless, in principle it should be possible to carry out work that is similar to that done in early hemispherectomies, only that in this case children who have lost the right or left arm early in life (particularly at ages where some preliminary hand preference was already manifest) would be studied. The second factor concerns cultural pressures. To be sure, cultural pressures will have developed in interaction with inherent preferences. Nevertheless, once the cultural norms have been established, they are bound to act back on phenotypical expression of handedness. Because of the existence of strong and complex cultural pressures, the relationship between a particular phenotype and a particular genotype (cf. Ardila, et al., 1989) is never clearly expressed and this must be a major factor in the unpopularity of work dealing explicitly with phenotype. Once questions regarding phenotype have been explored, the question of mechanism can be asked. What is the nature of neurological specializations that lead to hand preference? A full description of mechanism would allow very powerful predictions about most aspects of handedness. Current theories of handedness are in no position to address this question. It would be a mistake to view the descriptive/correlational model of Geschwind and Galaburda (1985) as addressing mechanism because the simple assertion that one or the other hemisphere matures sooner does not by itself constitute an account of mechanism. Assertions that neuroanatomical asymmetries (Witelson, 1980) have a causal relation to handedness must be viewed in the same light. For instance, the observation that pyramidal tract fibers from one side cross above the fibers from the other side at the pyramids in the brain stem cannot be confused with mechanism or causation. In analogy, the fact that there is more traffic between the cities A and B than between cities A and C is not necessarily due to the fact

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that A and B are connected by a multilane highway whereas A and C are connected by a two lane highway. After all, the multilane highway might have been built because there is more traffic; its existence does not permit causal arguments. It might be asked, quite legitimately, what a satisfactory description of mechanism and causation might be with regard to a heritable behavioural tendency. I can do no better than to refer to the work by Scheller et al. (1983), who have identified not only the gene that is responsible for a particular behaviour in aplysia, but also how the transmitters that are controlled by this gene act on a specific, delimited, neural network. There is no hope that in the work on the inheritance of handedness a mechanism that is as clearly and specifically described will be identified. In other words, by the standards of the physical and biological sciences, the question concerning mechanism is not likely to be answered in any satisfactory way. The third legitimate question is concerned with the heritability of handedness, and it is obvious that satisfactory answers must include an understanding of phenotype and mechanism. An Understanding of phenotype is necessary because one must know what it is that one looks for on a behavioural level in genetic studies, and an understanding of mechanism is necessary because mechanism is an essential link in the translation of genotype to phenotype. Of these legitimate questions, I will address only the question of phenotype, because, as stated, it has not received the determined attention it deserves. Three aspects will be discussed in separate sections. First, there is the general problem of whether in the classification of phenotype there is a place for the distinction between left-handedness that is caused by pathology and left-handedness that is not caused by pathology. It must be acknowledged at the outset that if it is difficult to document pathology in the causation of left-handedness, it will be equally difficult to argue that pathology is not involved. That is simply a matter of convention: once pathology has been implicated (however inadequately), an attempt to demonstrate lack of pathology is equivalent to proving the null hypothesis. The second section concerns the selection of questionnaire items in classifying phenotypically different left-handers. This discussion is basically one of how many ways there to slice the pie and how to justify that one method is more defensible than another. The third section concerns the determination of phenotype with the help of both questionnaire items and performance tasks. In this section, recent data that pose some problems for current theories of handedness will be presented and discussed.

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Pathology as Basis for a Phenotypical Distinction? A very thorough and well-thought out discussion of the literature of pathological left-handedness has been provided by Harris and Carlson (1988). Because the literature in general does not specifically address the initial question of what exactly one has in mind when talking about pathology, the present discussion will attempt to clarify what it is that one could mean by "pathology" in the context of handedness (cf. also McManus, 1983). When applied to physical disease, the term "pathology" is quite unproblematic. When "pathology" refers to mental disease or unusual behaviour, the story is different. Often it is the very fact that a behaviour is out of the ordinary that invites the term "pathological"although an organic underpinning is not established. In the case of left-handedness the line of argument that leads implicates pathology is complex and indirect. It begins with the observation that left-handers are far less common than right-handers and raises the question of why this should be so. In a world of right-handers the person who is left-handed must either not want to be like the rest, or he or she cannot be like the rest. The first possibility is not considered a serious candidate in the causation of lefthandedness, because it is an uninteresting proposition from the nwropsychological perspective, whereas the second one is. This implies that, in principle, everybody wants to be right-handed but something goes wrong that encourages use of the left hand as the preferred hand. The affected person does not want to be right-handed because it is easier to be left-handed. Lacking any plausible theory of handedness that includes mechanism, the evidence for "something going wrong" must, of necessity, be correlational in nature. An example would be the idea, backed by unimpressive evidence (Searleman, Coren & Porac, 1989) that birth stress produces damage which in turn leads to left-handedness. Where better data are available, as in the increased prevalence of left-handedness in the mentally retarded, the problem of cultural pressures again becomes significant. The more retarded a person is, the less likely is it that those who raise and educate the severely retarded child will make a determined effort to enforce right-handedness. Presumably, with the recognition that "something went wrong" comes greater acceptance of variation. Cultural pressure of this kind is the one recognized widely (Harris, this book), and is concerned with the active attempt to affect a person's handedness. A widely unrecognized factor that is likely of considerable impact is the need for persons to adhere to common norms. Thus, a child may not at all be forced to switch hands but will by observation of peers come to the conclusion that it does

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things differently. This puts a slightly different interpretation on sex differences in handedness. The traditional interpretation is that males and females are subjected to similar pressure to change but that males are more resistant to pressure. This may very well be true but an additional factor may be that girls are socially more mature and responsive than boys at an age where many hand choices become firmly entrenched and may be more sensitive to the fact that they are doings things differently. This 'heed to conform" factor will serve to reduce the phenotypical expression of left-handedness even in environments where there are no or on4 minimal external pressures against left-handedness and may serve to maintain sex differences even in the absence of overt external bias. In the case of retarded persons, not only is it likely that external pressures are less, but it is also likely that the severely retarded person is less likely to recognize that there is a general right hand preference and it is less likely that there is an explicit desire to conform in this respect. This can be said for all whose social perception is deficient. We do not know, therefore, how much of the left-handedness in the special subpopulations is organically caused and how much is simply due to the fact that this population is less susceptible to overt or subtle cultural pressures. I am not quite sure how far that sort of argument can be pushed but it is even possible to apply it to the subcategories of so-called pathological left-handers. Satz, Soper & Orsini (1988) make a case for the subdivision of pathological left-handers into those with ambiguous and those with consistent hand preferences. Ambiguous hand preferences can be interpreted as showing less concern for the quality of performance which usually (but, in the normal population, not always) suffers in the absence of specialization. Feuerstein (1980) in his analysis of why retarded persons do not perform well has noted that, among other things, the concern for accuracy and quality of performance can be said to be impaired. With this sort of problem, the desire to perform well is reduced because the definition of what is considered acceptable performance is reduced. As a result, and particularly with severely impaired individuals, ambiguous hand preference may be due not so much to the nature of any organic deficit but due to the lack of concern with quality of performance in reaching a movement goal. Of course, this point should not be overworked but it is clear that knowing the characteristics of this subset of the population makes a significant difference to interpretation. Tongue in cheek, it may be argued that because the mentally retarded are much less sensitive to social pressures thought to operate in depressing the manifestation of left-handedness, the prevalence figures in this population may be closer to the real thing than figures from the normal population.

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The "something went wrong" theory of pathological left-handedness is essentially a theory that views left-handedness as arising de novo; at most, it can be asserted that what is inherited is the greater chance of something going wrong in certain genotypes. Perhaps this latter possibility can be seen as the link between the pathological models and the models of those who see lefthandedness as a marker for the possibility of something going wrong (Irwin, 1985; Kinsbourne, 1988). Another approach to pathology in left-handedness is best introduced with reference to sickle cell anemia where a generally undesirable genotype is maintained in the population because it confers some advantages under some conditions. For instance, Annett and Manning (1989) have recently argued that extreme right-handedness is accompanied by a cost. Given her genetic model, the argument about the existence of left-handers is that genotypes that avoid extreme dextrality bring with them, unavoidably, the possibility for lefthandedness. Thus, it may be suggested that whatever costs are attached to being left-handed are outweighed by the overall benefit of not being strongly righthanded. In a similar vein, Benbow (1988) suggests that although there is a risk to left-handedness in terms of sensitivity to some kind of disease there are also benefits to being left-handed, in this case in the behavioural/cognitive domain. The argument that there is a cost/benefit ratio that can be attached to being right- or left-handed is attractive from the biological perspective because in most cases where relatively stable polymorphisms are observed in traits, one suspects that this is due to a balance of relative advantages and disadvantages of the different phenotypes in given types of environments. In other words, even if costs are attached to being left-handed, in the presence of identifiable benefits that are attached to genotypes that either avoid extreme dextrality or direct benefits accrued by left-handers in some environments, it is difficult to talk about lefthandedness within the framework of pathology. Finally, pathology can also be introduced within the framework of a developmental delay (e.g., Geschwind & Galaburda, 1985) model. Such models do not assert that left-handedness is a pathological condition. However, developmental processes cannot be delayed indefinitely without causing marked problems and at one point it becomes legitimate to link left-handedness to pathology. This brief discussion was intended to show that when pathology is mentioned within the context of left-handednessit is not entirely clear is meant by pathology (see McManus, 1983). On the whole, the evidence for an outright and direct pathological process in the traditional context of organic disease is surprisingly

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weak and the meaning of left-handedness as a marker for what might be the potential for truiy pathological conditions remains obscure. It has also been indicated that cultural factors, which might be expected to be of least importance in pathological left-handedness, can in fact not be ignored when arguments are made about the correlation between the prevalence of handedness and some other condition. Amidst all the concern with the label "pathology," the question of whether there is an identifiable pathological phenotype has been successfullyavoided. The answer, already reflected in the concerns raised by McManus (1983) is not encouraging. If we wish to compare the hand preferences and hand performance of pathological left-handers in order to compare them to nonpathological lefthanders, there is a problem if the pathological left-hander suffers from any overt neurological disorder because it will be this rather than an underlying dimension of hand preference and performance capacity that will serve as a basis of comparison. But if there are no overt neurological problems, why should one talk about a case of pathological left-handedness? Thus, although the distinction between pathological and nonpathological left-handers is viewed as a legitimate subclassification of left-handers in the literature, there is a severe problem of distinguishing meaningfully between the two groups (if such there are) along the very lines that are of interest: the motor behaviour of the hands. This really is a dilemma and we shall see how some researchers have attempted to solve the problem by recognizing a subgroup of "subclinical" (speak: subpathological) lefthanders.

Phenotype as Established by Questionnaire It is somewhat presumptuous to begin talking about questionnaires after so many researchers have done so many clever things with questionnaires and have said so many insightful things about them. Nevertheless, some questions merit continued consideration and the choice of questionnaire items is one of them. Handedness questionnaires are said to give us some idea about handedness and so the selection of items depends on our view of what handedness is. It is best, as Wittgenstein suggests, to begin with a simple common understanding of a concept before launching into elaborate definitions. When we talk about handedness we talk about a preference to do certain things with one or the other hand. Common understanding would further, if only implicitly, allow for a distinction between activities that matter from those that do not matter. For

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instance, if we reach for an apple on the table, it does not really matter which hand does the reaching because there is no cost involved in using the nonpreferred hand. Consequently, few people outside the research community would think of handedness in terms of reaching. Similarly, if we have to carry a heavy suitcase it is clear that we favour the stronger hand and arm but because we will tend to switch hands when carrying a heavy load, few people would think of handedness in terms of the hand that is chosen to carry a heavy object. Handwriting is different because it matters. As the objective is to write as clearly and quickly as possible, few people would want to write with a hand that is less adept because there is a cost involved. In our society, writing is very important and that is why most people would think of handedness in terms of writing. The same case can be made for the use of tools and throwing. As an aside, the issue of "does it matter?" is probably also of importance in the primate handedness literature (Harris, in press; MacNeilage, Studdert-Kennedy, & Lindblom, 1987). Classification of handedness should therefore proceed on the basis of acknowledging the distinction between important and inconsequential activities, and the distinction should be made on the basis of whether there is a cost/benefit decision to be made in choosing one or the other hand. No suggestion is made here to discount preferences for inconsequential activities altogether because such preferences may also tell us something. For instance, it well be argued that for a large range of activities there are no cultural pressures that prescribe directly the use of a particular hand and thus preference choices might afford a glimpse of uncontaminated hand preferences. Along similar lines it might be argued that many inconsequential activities, because the element of skill is not predominant, offer a look at hand preference in a situation where directional biases are expressed without direct reference to motor accomplishment. Nevertheless, an important reason for maintaining the distinction between important and inconsequential activities is that this avoids the confusion that results if preference choices in a questionnaire do not have a categorically equivalent status. For instance, the item of "opening the lid of a jar" is part of even the short list of a number of preference questionnaires (Ponton, 1987) and we find it quite unsatisfactory in terms of consistency (Peters, in press). If the actual behaviour of persons opening jars is observed, it becomes clear that in the case of a recalcitrant lid, persons will change readily from one hand to the other and back again. Worse, what a person will do also depends on the size of the jar to.- be held relative to the lid size. As a consequence, persons filling out a

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questionnaire may give answers that do not really relate to their actual behaviour even though in other cases (such as writing, drawing, throwing) the answers are quite in line with what is done in real life. Similarly, we find that when questions are asked about holding a broom (e.g. Oldfield questionnaire), subjects may answer in a certain way knowing full well that how they hold the broom will depends on how their body is oriented relative to the area to be swept, not to mention a fair number of college-aged subjects who actually may not have used brooms. The extensive work by some on the reliability of answers in a test/retest paradigm to judge the suitability of items (Porac & Coren, 1981) does not necessarily help here because the subject may answer consistently without behaving consistently. One of the most troublesome aspects of classification as based on preference items is the sensitivity of handedness categories to preference items that denote inconsequential activities. This problem is widely recognized but a solution to the question of how to deal with widely differing prevalence estimates based on the different questionnaires, and the procedures used with given questionnaires, is at present not forthcoming. If sufficiently large numbers of items are selected in the questionnaire, the point is reached where practically all but those who have lost an arm or are otherwise incapacitated, are mixed-handed. If shorter questionnaires are used, the way in which individual researchers categorize respondents still can vary considerably. Should one, as Witelson (1985) did, label all the subjects who give clear right-hand choices as right-handers, and call all others mixed-handers? Or should one select from a large sample all of those who show extreme right or left hand preference in order to get a clear separation, as Geschwind and Behan (1984) did? Does it make sense to separate right-handers and left-handers with strong and weak preferences in order to obtain four different groups, as Ponton (1987) did? All of these procedures yield different prevalence figures for right-handers, left-handers and mixed-handers and in the absence of any clear understanding of mechanism or even a proper phenotypical taxonomy it is quite difficult to argue that one method or the other is preferable in estimating population handedness. The problem is compounded by cultural pressures that invariably depress the estimated prevalence figures for left-handedness. The presence of cultural biases is not a problem that can be separated from other aspects of handedness because all prevalence estimates have to be seen relative to this factor. For instance, in the evaluation of questionnaire items, the concept of activities "that matter" was approached in purely value-free terms, because the only concern was with a movement goal and how it is attained. Superimposition of cultural pressures

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introduces another dimension of "what matters" because of the cultural constraints imposed on certain activities (Payne, 1987). As an example, where eating is considered an unclean activity, the left hand is used for eating but when the left hand is used for some acts of personal hygiene, as is the case for the Islamic world, it will not be used for eating. Faced with this situation, only minimal estimates as to the prevalence of left-handedness can be made. The basic assumption is that the fewer pressures there are against left hand use, the more individuals are free to choose whichever hand they please. Because there are indirect cultural pressures that stem from the desire to conform with the majority (see the chapter by Harris in this book), prevalence estimates even in liberal societies will be underestimates. The question of how many left-handers there really are is not only of interest in itself but interacts very directly with pathological models and models of how brain lateralization affects cognitive skills. In the section on pathological lefthandedness it was pointed out that the elevated prevalence of left-handedness in subpopulations with pathology might in fact be elevated only with regard to underestimatesof population prevalence figures in societies with pressures against left-handedness. In this case there is a possibility that the underestimate of prevalence figures in itself gives a faulty impression of how left-handedness relates to defect. The problem arises because it is not very likely that lefthandedness in special target populations such as the mentally retarded is underestimated to the same extent that it is in the general population. As a result, the large gap between prevalence figures in selected populations and the general population is likely to shrink if more accurate prevalence estimates in the general population become available. If the gap becomes very small, some of the arguments linking left-handedness to pathology will have to be revised or dropped. A similar argument can be made with regard to the other extreme of cognitive ability. Benbow (1988) has suggested that marked mathematical abilities are linked to brain organization and an important part of her argument is that left-handers are overrepresented in the population of those who are highly gifted in mathematics. I do not wish to become involved in that issue but from the point of classification it is interesting that Benbow's prevalence figures of lefthandedness in her highly gifted subsample are in the range of 15.1%. When contrasted with her comparison sample that is said to have a prevalence of 7.2%, the increase is very impressive indeed. The problem is that most prevalence figures of the current generation of North American students are in the range of 12% and higher. Spiegler and Yeni-Komshian (1983), for instance, give a

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prevalence figure of about 15% left-handed male writers in a college student population. Unfortunately, Benbow’s comparison groups are not well specified (Benbow, 1988, p. 180), and it seems that the parents as well as the mathematically less gifted siblings are thrown in together so that a direct comparison is not possible. Whatever is the case, the portion of Benbow’s argument that rests on the presumed increase in the prevalence of lefthandedness among the mathematically gifted must be viewed with some caution precisely because of the very unclear situation with regard to prevalence of lefthandedness in the general population at large. So much for the impact of classification procedures based on preference questionnaires and their relation to prevalence estimates, and lateralization models. Given the vagaries of the classification process as based on handedness questionnaires it is not surprising that many researchers have attempted to supplement information gathered with questionnaires with performance items. After all, one would want to have the certainty that questionnaire item preference has some meaningful relationship to behaviour.

Subclassification of Left-Handers by Preference and Performance Preference patterns in themselves cannot tell us about pathology or meaningful subgroupings in handedness. It is largely because of performance studies that researchers have come to the conclusion of that there are nonpathological left-handers (cf. the review by Harris & Carlson, 1988). This is one use of hand performance tests. Another has been to try and find out whether or not hand preference patterns and performance patterns can tell us something (however little) about the reasons why there is a hand preference (Todor & Smiley, 1984). As in the case of questionnaire items, where the choice of items has important implications, the choice of performance tasks is similarly important. This is best illustrated by a recent example. Bishop (1984) has suggested that among left-handers there should be a subpopulation who have significantly worse performance of the nonpreferred hand. While Bishop does not label this group as pathological left-handers because no overt neurological signs could be detected, pathology is implied because this is a group that presumably chose the left hand because something “went wrong;” they would otherwise have chosen the right hand as preferred hand. Bishop’s (1984) data do show such a subgroup.

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However, of the two performance tasks, match sorting identified the hypothesized group while a square marking task did not. Bishop has a plausible explanation of why the square marking task failed to produce the expected results but it is reasonable to await further corroborating evidence with other tasks before accepting her model. At any rate, her work illustrates the importance of choosing the right performance task. In choosing performance tasks it is important to sample a variety of movement categories (Bishop, 1989) so that the statements made about performance by the preferred and nonpreferred hand in various handedness groups do not rest on a base that is too narrow. Most importantly, it is important not to choose tasks that are replete with elements of overpractised and culturally lateralized tasks. For instance both McManus (1985) and Tapley and Bryden (1985) have chosen as performance tasks an activity that in all essential aspects contains the primary characteristics of writing. As a result it is not surprising that both studies show a distribution of performance relative to handedness that is identical to the sort of distribution one would expect when individuals are asked to write with the right and the left hand. In younger subjects one would expect some overlap between handedness group and hand performance, but as the years of practice with the writing hand mount, one would expect practically no subject performing better with the hand that is not used for writing. Harris and Carlson (1988) attempt to salvage the situation by suggesting that the Tapley and Bryden (1985) task "also includes the finger-tapping test (which bears little or no resemblance to writing) in that it requires rapid rhythmic up and down movements (p. 303). This salvage attempt fails because the rate of up and down movement in the dot marking test used by Tapley and Bryden is by an order of magnitude slower than the up and down movements in finger tapping. While it is clear that the particular choice of performance tasks is important, it is also clear that at this time the basis for choosing particular tasks rests on a foundation that is just as wobbly as the foundation on which the choice of preference items rests. This again is due to the absence of any clear idea of mechanism; if the mechanism were more clearly understood, the choice of performance task could proceed on a much more systematic basis. In the absence of such knowledge, the best one can do is to select as broad a band of performance tasks as possible. Apart from Bishop's attempt to delineate subgroups of left-handers on the basis of performance, there is a tantalizing suggestion by Ponton (1987) that performance can distinguish between subgroups of left-handers in a population of college students. Subjects from this population have the attraction that one I'

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can, at least to some extent, assume that pathological left-handers have been selected out before they reach this subject pool. Ponton's work has the additional attraction that questionnaire item data was directly related to performance items, so that the perennial question of whether "strong" or "weak" left-handers differ in performance could be addressed. Again, it has to be emphasized that while Ponton does not explicitly mention pathology, some subclinical deficit is implicated. Ponton, as others have done, assumes that if a left-hander is very consistent in left hand preference choices, in spite of living in a right-handed world, there must be some compelling reason for this, and this reason is presumably not of the wholesome variety. Left-handers with mixed preference choices, in contrast, are seen to respond flexibly to the demands of the external world, in spite of an inherent bias to perform some activities with the left hand. Ironically, Gutezeit (1983) begins his work with the question "do children with strongly expressed left-handedness show less learning disabilities than children with weakly expressed left hand preferences?" Gutezeit proceeds from the assumption that insufficiently developed hand preference is a sign of developmental problems (but he found no differences between the two groups). Because Ponton's (1987) study used a greater variety of performance tasks than are used in most studies, his claim of having found a meaningful subgrouping of left-handers must be taken seriously, and this claim formed the basis for our work. Following Ponton's methods, but using a somewhat greater variety of performance tasks which were chosen in order to represent particular aspects of movement, we were not able to replicate his results with regard to inferior performance of consistent or strong left-handers. However, by using his method of dividing left-handers into a group with consistent preference choices (CLH's = consistent left-handers) and a group with inconsistent choices (ICLH's = inconsistent left-handers) we were able to find some surprising and consistent performance patterns in the two subgroups (Peters & Servos, 1989: Peters, in press). Our Findings with Nonpathological Left-Handers Subject Selection Procedures and Classification: We began by asking for volunteers who considered themselves either right- or left-handed. We then classified our subjects in a way similar to that described by Ponton (1987): subjects who preferred the left hand for seven out of eight preference items (write, hammer, throw, unscrew lid of jar, use knife for cutting bread, use toothbrush, hold a match while striking it, hold a racquet), including writing,

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were classified as consistent left-handers (CLH's). Subjects with inconsistent hand preferences (ICLH's) were those who preferred the right hand for two or more of the eight items. The method of classifying subjects is quite arbitrary, but does have a certain logic. If the CLH's had been defined in terms of all left preference choices, subjects with quite strong and consistent left hand preference choices would have been included in the ICLH population and this would most certainly have changed the results. Because Ponton (1987), followingKimura's (1973) procedure included the "unscrew lid of jar" item which is not a good item, a good many consistent left-handers would have been classified as inconsistent because of this item alone. In principle, it would have been possible for a person to have expressed a preference for the right hand on six of the eight preference items without being considered a right-hander. The important aspect here is that subjects were self-selected and would not have put themselves down as lefthanders if they had been left hand writers. We do not hesitate to label a person who writes with the left "left-hander"even if other activities are done with the right hand because in our society the most obvious pressures for dextrality are directed at the writing hand and it would make little sense to label such a person as "right-handed." It is necessary to dwell on the method of subject selection for a moment. There are several disadvantages and some advantages to using self-selection methods. A clear disadvantage is that we cannot be sure of the population prevalence of left-handedness in the entire subject pool from which we are soliciting volunteers. We also cannot be sure about the proportion of subjects who do not sign up because they have difficulties classifying themselves as either left- or right-handed. Another dimension is the degree of interest shown by a subject in his or her handedness. All of us who work in handedness know that right-handers take handedness for granted. That is, few ever ask why it is that they are right-handed and relatively little interest is shown in the fact that the right hand is the preferred hand. Left-handers are often quite fascinated in the fact that they "are different" and they are more likely to actively seek out participation in experiments involving handedness (Thompson & Harris, 1978). Indeed, we occasionally have left-handed subjects from outside the undergraduate subject pool signing up as subjects because they are sufficiently interested in the issue. That never happens with right-handers.A clear advantage of self-selection is that the subjects who sign up as left-handed obviously consider themselves as left-handed, and there is no major difficulty of making the first classificationcut that differentiatesright-handersfrom left-handers. Also obvious

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is the fact that when extensive performance measures are taken, the very much larger numbers of subjects who would have to be tested in a random sample would make the task, labour intensive as it is, quite prohibitive in terms of cost and time. We would have had to test about 2000 subjects individually on more than ten performance tasks in order to arrive at a sample of left-handers similar in size to the one we worked with. As it was, the studies required more than 560 hours of individual subject testing. The subjects were tested on speed of writing, a square tracing task similar to that described by Bishop (1980), but including a condition were both hands had to trace a square at the same time, a movement sequencing task derived from Kimura’s (1977) procedure, but extended to be used on foot sequencing as well, all subtasks of the Purdue Pegboard test, a speeded finger tapping task, a bimanual 2 1 tapping task (Peters, 1985; 1987), a speeded counting task, two verbal fluency tasks and a throwing task in which subjects had to throw at a target (Peters, in press). Finally, measures of thumb and index finger size were also taken.

What We Found Left-Handers with Different Strengths of Hand Preference Did Not Differ in Quality of Performance and Performed on Par with Right-Handers. Altogether we tested 119 CLH’s and 95 ICLH’s, together with a group of 161 right-handers who served as a basis for comparison. We were not able to support Ponton’s conclusions: the CLH’s did not perform worse than the ICLH’s, on any of the performance tasks chosen. Left-handers performed as well as righthanders on all performance tasks overall. The only significant differences between left-handers and right-handers fell into a category that has previously been noted for populations of nonpathological left-handers: the preferred hand performance tends to be comparable to the preferred hand performance of righthanders (Harris & Carlson, 1988) while there is an edge to the nonpreferred hand performance of left-handers. This was the case for both ICLH’s and CLH’s. Our Sample of Left-Handers Could Be Labelled “Nonpathological.” We felt justified, therefore, in suggesting that our sample of left-handers could not be considered pathological in terms of the dimension along which they were defined, that is, hand use and performance. This did not mean, however, that the two groups of left-handers did not differ from each other.

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Table 1: Performance superiority in the three handedness groups, based on data from Peters and Servos (1989) and Peters (in press).

Cons 1 s t e n t Left-handers

ICLH ' s

RH's

S t rength/throw ing/hand s i z e

Hand s t r e n g t h Throwing accuracy Hand s i z e measure Kicking w i t h foot

L

R R R R

L L L

R R R R

Fine manual s k i l l , speed Square t r a c i n g Purdue Pegboard F i n g e r Tapping W r i t i n g hand

L L L L

R R R R

L L L L

Hand/arm o r f o o t sequencing task Kimura sequence hand Kimura sequence f o o t

L

-

-

R R

Allocation o f attentiodtiming Bimanual 2 : l task

Lo

R

' = Performance was b e t t e r when t h e i n d i c a t e d hand took t h e 2 : l task

R "2" c h a i n o f t h e

Our Sample of Left-Handers Did Differ Significantly in Terms of Performance Patterns. For the sake of convenience, the performance patterns of CLH's, ICLH's and RH's are summarized in Table 1. The directional signs denote significant differences at thep < .01 level. The quite arbitrary method of defining consistent and inconsistent left-handers created two groups that did not only differ in the pattern of their preference choices but also in the patterns of right/left superiority on the various performance tasks. In the Peters and Servos (1989) study, left-handers differed from right-handers in that they did not show any group differences in hand strength that might have favoured the right or the left hand, Right-handers showed a clear right hand strength advantage. A breakdown of the performance of left-handers showed that the consistent lefthanders had a stronger left hand and the ICLH's had a stronger right hand. As

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the strength advantage went in the opposite direction, the two groups cancelled each other out. This finding was replicated in the Peters (in press) study. We therefore were able to show that one of these two groups of left-handers had a strength advantage of the right hand (ICLH's) and the other had a strength advantage of the left hand (CLH's). When the ability of subjects to hit a target at different distances was examined, a corresponding dissociation was noted: ICLH's as a group threw more accurately with the right hand while CLH's as a group threw more accurately with the left hand. This came as no surprise since the majority of ICLH's preferred the right hand for throwing as opposed to CLH's who expressed a left hand preference. The right hand superiority of the ICLH's in tasks requiring strength and whole arm activity was further underscored by an anatomical measure that showed the thumb and index finger of ICLH's to be larger than the thumb and index finger of the left hand, as was the case for right-handers; the converse was found for CLH's. Perhaps the most perplexing results were obtained when subjects were tested on a bimanual 2 1 rhythm task, where subjects had to tap twice with one hand for every one tap in the other. Right-handers will do much better when the right hand performs the " 2 sequence and the left hand performs the "1"sequence (Peters, 1987; 1985). That task is of considerable interest in the delineation of handedness because the movements required by the two hands do not differ in any topographical sense and the speed of tapping even for the faster hand that does the "2" taps is well below the maximum speed of tapping for the nonpreferred hand; the two hands perform the very same kinds of movement and if asymmetries arise they can therefore not be said to be due to any motor aspects of handedness. The observed asymmetries have been attributed to an asymmetry of attention, attention being preferably directed at the nondominant hand in right-handers. For left-handers as a group, no lateral asymmetries were observed. In other words, performance was the same, regardless of whether the left hand or the right hand took the "2." In the Peters and Servos (1989) study, the right-handers did much better with the right "2," left "1"hand/task combination, supporting previous findings (Peters, 1987; 1985). Left-handers as a group did not show any significant performance asymmetries. However, when performance was analyzed separately for ICLH's and CLH's, it turned out that the two groups showed opposite asymmetries that cancelled each other out when the data were combined. ICLH's showed better performance with the right " 2 and left "1"combination while the CLH's showed better performance with the left "2," right "1"combination.

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The data show that the ICLH’s perform like right-handers in terms of strength, throwing ability and on the bimanual task that requires fine temporal integration of the two hands. In addition, ICLH’s prefer the right foot for kicking, This might be viewed as somewhat equivalent to the throwing task for the arm, and is consistent with the right hand throwing preference and performance advantage for the ICLH’s. CLH’s prefer the left hand for throwing and the left foot for kicking. Grouped together, there is again the faulty impression that left-handers show no group bias towards one or the other foot (Peters, 1988). A different picture emerges when the tasks involving fine skill of the hand, and hand/arm sequencing tasks are examined. The ICLH’s performed better with the left hand on the single square tracing task, as did the CLH’s. Of particular interest was the performance on the dual square tracing task. In this task, subjects had to simultaneously trace inside a 2 mm track around a 10 cm square (one square for each hand), trying to stay within the track as well as they could. Right-handers, CLH’s and ICLH’s did worse in the dual task that in the single tracing task, and all did worse with the nondominant hand (for ICLH’s, this was the right hand in this task). However, the between-hand differences for the ICLH’s were smaller than those for the RH’s and the CLH’s. The fact that the between-hand differences on this task were smaller for ICLH’s cannot be attributed to any differential practice with the right hand as writing hand because our sample of self-labelled left-handers all wrote with the left hand. We can only surmise that the fact that ICLH’s perform some activities with different hands allows them to be more flexible in allocating attention to one or the other hand than is the case for the RH’s and CLH’s. This is perhaps of advantage in the dual task, where. attentional resources are stretched, attested to by the fact that for all groups the between-hand differences were larger in the dual task than in the single hand tracing task. If so, the advantage must be seen as an example of nonspecific transfer because there is nothing we can think of that specifically predisposes ICLH’s to do better on such bimanual tasks. That there is something different about the way in which ICLH’s allocate their attention to the hands is seen by their divergent performance on the 21 task. The fact that the dual task exacerbated the between-hand differences for all groups is of practical interest, suggesting that dual tasks can be used when clear performance separation between the hands is wanted. ICLH’s performed significantly better with the left hand on the Purdue Pegboard task, as did the CLH’s. A tendency of both groups of left-handers to perform better than right-handers on the assembly subtask of the Purdue

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Pegboard test (Peters & Servos, 1989), while in agreement with data obtained by Annett and Kilshaw (1983) was not replicated in subsequent work. ICLH’s also performed the speeded tapping task significantly faster with the left hand, as did the CLH’s. Therefore, a consistent pattern emerges that shows ICLH’s to perform like CLH’s on tasks that require fine manual integrative skill, as in square marking and writing, skilled manipulative movement under visual guidance, as in the Purdue Pegboard task, and simple speeded movement of individual digits, as in finger tapping. On the Kimura sequencing task, where right-handers and CLH’s perform better with the preferred hand, ICLH’s show no performance asymmetries. When the Kimura sequencing task was performed with the feet, right-handers showed an asymmetry favouring the right foot while ICLH’s and CLH’s showed no asymmetries. The Kimura task differs from the other tasks in that a chain of movements with different topographies have to be sequenced, and whole arm and leg movement is involved. It is perhaps the fact that this task does not allow a functional distinction between hand and arm (or foot and leg) function that is responsible for the lack of any lateral asymmetries in the ICLH’s.

A Meaningful Subclassification of Nonpat hological Left-Handers? In summary, it is clear that CLH’s behave in many respects like righthanders, with the only difference that they prefer the left hand. Whatever smaller differences there are between CLH’s and RH’s in terms of CLH’s showing somewhat lesser between-hand differences on some tasks, and less consistency on others (such as hand strength measures) might well be attributed to lefthanders having to live in a right-handed world. It is also clear that ICLH’s show a dissociation in the hand chosen for fine manual skill and the hand chosen for strength and ballistic activities, such as throwing. In this, they are very different from the other handedness groups. Moreover, this separation of hand/task allocations appears to be accomplished without any apparent performance cost. The ICLH’s are inconsistent only with regard to their between task preferences; within task hand preference is quite consistent. Furthermore, there is a distinct clustering in their hand preference patterns. Tasks requiring fine manual skill, such as writing, tapping, peg placing and square tracing, favour the left hand. Tasks involving strength and whole arm

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movement favour the right hand. It cannot be said that current genetic theories of handedness (Annett, 1985; McManus, 1984) are unable to deal with the ICLH’s because precise predictions about performance of heterozygotes and those lacking any directional specificationsare not made. However, it is difficult to conceive of the behaviour of the ICLH’s in terms of these genetic theories unless further assumptions are introduced. These assumptions have to include a different specification of the mechanism of inheritance for fine manual activities and activities involving strength and whole limb movement. How this could be handled within any simple one gene - two allele model is unclear. What is clear is that the absence of any plausible idea of mechanism in the determination of handedness reinforces the weak status of handedness theories because they cannot predict patterns of handedness. At one point we might have been tempted to suggest that personality differences are involved in differentiating between CLH’s and ICLH’s. That is, in principle all nonpathological left-handers would have the option to use either hand for any activity, but that the hand preference pattern is determined by a personality variable that does or does not like consistency. Persons lacking the right shift factor, but having the tendency to be consistent would show up as either right-handers or as CLH’s, while persons with no such tendency would show up as ICHL‘s. We would further have speculated that it is the ICLH’s who have contributed to the popular stereotype of left-handers as somewhat unpredictable and nonconformist in their behaviour. We would also have attributed a marked resistance by some left-handers to write with the right hand to personality variables separating those who would rather fight than switch from those who would rather switch than fight. Such a hypothesis cannot be maintained in view of the nonrandom pattern of hand/task allocations in the ICLH’s. There might be a temptation to see CLH’s as those ”stubborn” lefthanders, in the sense used by Harris (see chapter this book), who either do not wish to change or who wish to change but cannot. However, considering that ICLH’s are already quite adept at using the right hand and refuse to consider using the right hand for writing, it may be argued that the ICLH’s also are very pointedly resistant to a switch with regard to the writing hand. Rather than concluding that ICLH’s are less strongly lateralized and more apt to be switched successfully, we suggest that their hand use patterns indicate a very strong and specific left hand preference for writing. ICLH’s do not suggest a lack of directionality in the sense of genotypes that lack the right shift gene, or individuals who are flexible in their hand/task allocations.

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It is interesting to note that Geschwind and Galaburda (1985) have mooted the possibility of a dissociation along the proximal/distal axis of the neuromuscular machinery. However, it is not quite clear how such a dissociation would be explained within the framework of the developmental lag in differentiation that forms the core of Geschwind and Galaburda's model. It is true that the machinery underlying the movements of the hand appears to mature later than the machinery that operates the arm and leg movements. In this sense a possible break in the developmental sequence that would lead to a dissociation between distal and proximal control system operation might be viewed as plausible. However, there is no particular reason to believe that comparable levels of motor control, depending on the complete myeliization of the pyramidal tracts, come any sooner in the proximal than in the distal musculature. Nobody would deny that arm and leg movements occur very early in life but there is no convincing evidence that precise control of the proximal musculature, comparable to that exercised in the distal musculature, predates that seen in the distal musculature. Quite the opposite; it might be argued that good control over individual finger movements is achieved at an age when control over limb movements (as evidenced use of the legs in running and dancing) is still not fully developed. As a result, some skepticism is advised when viewing the idea that differential rates of development in motor systems subserving the distal and proximal musculature might be responsible for a dissociation of the sort seen in the ICLH's. It might also be of interest, with reference to CLH's and ICLH's, to look at the idea that strength of hand preference rather than direction is the variable of interest in the heritability of handedness. This view has been developed, among others, by Bryden (1982). The question is: what is meant by weak lateralization? If the term "weak is based on handedness questionnaire responses that denote inconsistency of hand preference choices, the term is inappropriately used, because our ICLH's showed individually strong preferences - even though it is true that they showed somewhat more preferences denoted "mostly the left or right" rather than "always the left or right." If performance measures are used, the situation is even less clear because ICLH's are generally quite clearly lateralized in performance. Certainly there has been too little work with performance tasks to address the question of how individual CLH's and ICLH's differ with regard to between-hand asymmetries. Boles (1989) has shown that one and the same individual can show strong asymmetries on some perceptual tasks and weak asymmetries on others. There is no reason to believe that this cannot happen when CLH's and ICLH's are tested with batteries of motor tasks.

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For this reason, the correlation between the magnitude of between-hand performance differences and a measure of right:left choices on a handedness questionnaire, as summarized by AMett (1985) cannot as of yet be accepted as indicating a general principle because it is based on two performance tasks only. These considerations lead to the conclusion that the distinction between "strong" and "weak left-handers, as based on the consistency of responses to questionnaires, is not likely to be particularly informative. The term "weak lefthander would, by the preference criterion, be applied to ICLH's but the performance and preference patterns of such left-handers, while inconsistent, cannot be considered of a weakly expressed left hand preference. Rather, there is a left hand preference and performance superiority for some activities and a right hand preference and performance superiority for others. If there are problems with regard to the distinction between "strong" and "weak left-handers, as defined in the literature, there are also problems with the procedure of not recognizing subgroups of left-handers. For instance, Witelson's (1985) method of distinguishing between right-handers and mixed-handers will leave in the category of mixed-handers the two subgroups defined here. Is it reasonable to assume that CLH's and ICLH's have a similar or identical cerebral organization, as assumed by this procedure? Witelson used a twelve item questionnaire and because no person had a left hand preference for all items, no "pure" left-handers were identified. We agree with Ponton (1987), that it would make sense to allow persons with one or two right hand choices in the twelve items questionnaire to be classified as left-hander with consistent left hand choices. This asymmetry between defining "pure" left-handers and "pure" righthanders was defended by Ponton as reflecting the realities of left-handers living in a right-handed world. Amidst the concern with the subclassification of left-handers, the question of whether such a procedure would also not be appropriate for right-handers has been neglected. After all, are there not right-handers who write with the right hand and throw with the left hand? The fact that in our sample of right-handers only one such person was found does not necessarily answer the question because results can be expected to be different if subjects are drawn randomly rather than through selfclassification. In a sample of randomly drawn children, some can be found who are classed as right-handers by the experimenter but throw with the left hand (Annett, 1970). Unfortunately, one cannot view these two classes as equivalent. In the case of a person who writes with the left but throws with the right it is unlikely that cultural pressures, ineffective in changing the writing hand, have selectively caused the person to prefer the right hand for

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strength activities. In the case of a person who writes with the right but does other things with the left it is quite difficult to make a convincing argument that one is dealing with a right-hander who has chosen to do all other activities with the left hand, again because of cultural pressures that tend to foster the right hand as writing hand. Thus, while in our own sample there was no problem with such a group of persons, in larger and randomly drawn samples it is not at all clear that one can legitimately label right-handed writers who throw with the left as inconsistent right-handers.

Summary and Conclusions We have shown that nonpathological left-handers can be subdivided into two subgroups. Unfortunately, because of the sampling methods employed, it is not possible to state what proportions of the entire population these subgroups form. Nevertheless, assuming that these two subgroups of left-handers do not differ in their self-selection rates, it may conservatively be estimated that the ICLH’s constitute at least 30% of the population of left-handers, and perhaps as much as 50%. We are also hesitant, for the same reason, to estimate prevalence of the two groups among the sexes. This remains to be determined. Nowhere in our work did we address the question of whether patterns of cerebral lateralization differ between CLH’s and ICHL‘s. Left-handers as a group are considered notoriously variable in terms of a variety of characteristics. In terms of motor performance, our subclassification has brought some order into this variability with regard to the performance of CLH’s and ICLH’s on a variety of motor tasks. Because of this, it is not unreasonable to speculate about the possibility that the two subgroups will reduce inconsistencies along other dimensions, such as language dominance patterns. This would almost be too good to be true but it is worthwhile to consider phenotype as classified here and by Ponton (1987) when attempting to assess cerebral lateralization patterns of left-handers. Some will not accept the dichotomy established here, and there is very good reason to be suspicious of any all too convenient dichotomies. Neuropsychology has not had an impressive record in defending what in the beginning seemed to be clear-cut dichotomies. Nevertheless, the one aspect that does not fit into a continuous distribution of handedness model is the dissociation of the hand chosen for strength and fine manual skill. The classification used here gives a good, albeit not perfect, prediction of the likely characteristics of inconsistent

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left-handers. Perhaps a more rigorous approach will sharpen the distinctions further. In order to be consistent with Ponton’s criteria, we have defined our groups of CLH’s and ICLH’s as described. The present results encourage an approach where the classification of ICLH’s is based on defining only those subjects as ICLH’s who show a left hand specialization for writing and tapping and a right hand specialization for throwing and strength. Regardless of where this subclassification will lead, and whether it will survive the test of time, it is felt that the full exploration of phenotype is so basic to all aspects of research involving handedness that more attention must be devoted to this area of lateralization research.

References Annett, M. (1985). Left, right, hand and brain. London: Lawrence Erlbaum Associates. Annett, M. (1970). The growth of manual preference and speed. British Journal of Psychology, 61, 545-558. Annett, M., & Manning, M. (1989). The disadvantages of dextrality for intelligence. British Journal of Psychology, 80, 213-226. Annett, M., & Kilshaw, D. (1983). Right- and left-hand skill 11: Estimating the parameters of the distribution of left-right differences in males and females. British Journal of Psychology, 24, 269-283. Ardila, A., Ardila, O., Bryden, M.P., Ostrosky, F., Rosselli, M.,& Steenhuis, R. (1989). Effects of cultural background and education of handedness. Neuropsychologia, 27, 893-897. Benbow, C.P. (1988). Sex differences in mathematical reasoning ability in intellectually talented preadolescents: their nature, effects, and possible causes. Behavioral and Brain Sciences, 11, 183-232. Bishop, D.V.M. (1989). Does handedness proficiency determine hand preference? British Journal of Psychology, 80, 191-199. Bishop, D.V.M. (1984). Using non-preferred hand skill to investigate pathological left-handedness in an unselected population. Developmental Medicine and Child Neurology, 26, 214-226. Bishop, D.V.M. (1980). Handedness, clumsiness and cognitive ability. Developmental Medicine and Child Neurology, 22, 569-579. Boles, D.B. (1989). Do visual field asymmetries intercorrelate? Neuropsychologia, 27,697-704. Bryden, M.P. (1982). Laterulity. New York: Academic Press. Feuerstein, R. (1980). Instrumental enrichment. Baltimore: University Park Press. Geschwind, N., & Behan, P.O. (1984). Laterality, hormones and immunity. in N. Geschwind and A.M. Galaburda (Eds.) Cerebral dominance (pp. 211-224). Cambridge, Mass.: Harvard University Press.

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Geschwind, N., & Galaburda, A.M. (1985). Cerebral lateralization. Biological mechanisms, associations and pathology: 11. A hypothesis and a program for research. Archives of Neurology, 42, 521-552. Gutezeit, G. (1982). Linkshandigkeit und Lernstarungen? Prav's der Kinderpsychologie und Kinderpsychiatrie, 31, 277-283. Harris, L.J., (in press). Cultural influences on handedness: historical and contemporary evidence. In S. Coren (Ed.) Left-handedness: behavioral implications and anomalies. Elsevier Science Publishers. Harris, L.J. (in press). Handedness in monkeys and apes. In J. Ward (Ed.), Current behavioral evidence of primate asymmetries. New York: SpringerVerlag Harris, L.J., & Carlson, D.F. (1988). Pathological left-handedness: An analysis of theories and evidence. In D. Molfese and S.J. Segalowitz (Eds.), Brain lateralization in children (pp. 289-372). New York: Guilford Press. Ingram, P. (1985). Greater brain response of left-handers to drugs. Neuropsychologia, 23, 61-67. Kimura, D. (1977). Acquisition of a motor skill after left hemisphere damage. Brain, 100, 527-542. Kimura, D. (1973). Manual activity during speaking - left-handers. Neuropsychologia, 11, 51-55. Kinsbourne, M. (1988). Sinistrality, brain organization, and cognitive deficits. In D. Molfese and S.J. Segalowitz (Eds.), Brain lateralization in children pp. 259279. New York: Guilford Press. MacNeilage, P.F., Studdert-Kennedy, M.G., & Lindblom, B. (1987). Primate handedness reconsidered. Behavioral and Brain Sciences, 10, 247-303. McManus, I.C. (1985). Right- and left-hand skill: failure of the right shift model. British Journal of Psychology, 76, 1-16. McManus, I.C. (1984). Genetic of handedness in relation to language disorder. In: F.C. Rose (Ed.), Advances in Neurology, K 42,: progress in aphasioloey (pp. 125-138). New York: Raven Press. McManus, I.C. (1983). Pathological left-handedness: Does it exist? Journal of Communication Disorders, 16, 315-344. Payne, M.A. (1987). Impact of cultural pressures on self-reports of actual and approved hand use. Neuropsychologia, 25, 247-258. Peters, M. (in press). Subclassification of left-handers poses problems for theories of handedness. Neuropsychologia Peters, M., Servos, P. (1989). Performance of subgroups of left-handers, and right-handers. Canadian Journal of Psychology, 43, 341-358. Peters. M. (1988). Footedness: Asymmetries in foot preference and skill and neuropsychological assessment of foot movement. P&hological Bulletin, 103, 179-192. Peters, M. (1987). A nontrivial motor performance difference between righthanders and left-handers: Attention as intervening variable in the expression of handedness. Canadian Journal of Psychology, 41, 91-99.

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Psters, M. (1985). Constraints in the coordination of bimanual movements and their expression in skilled and unskilled subjects. Quarterly Journal of Experimental Psychology.37A, 171-1%. Ponton, C.W. (1987). Enhanced articulatory speed in ambidexters. Neuropsychologia, 25, 305-311. Porac, C., & Coren, S. (1981). Lateral preferences and human behavior. New York: Springer-Verlag. Satz, P., Soper, H.V., & Orsini, D.L. (1988). In D. Molfese and S.J. Segalowitz (Eds.), Brain laleralization in children (pp. 281-287). New York: Guilford Press. Scheller, R.H., Jackson, J.F., McAllister, L.B., Rothman, B.S., Mayeri, E., and Axel, R. (1983). A single gene encodes multiple neuropeptides mediating a stereotyped behavior. Cell, 32, 7-22. Searleman, A., Coren, S. & Porac, C. (1989). Relationship between birth order, birth stress, and lateral preferences. Psychological Bulletin, 105, 397-408. Spiegler, B.J., & Yeni-Komshian, G.H. (1983). Incidence of left-handed writing in a college population with reference to family patterns of hand preference. Neuropsychologia, 21, 651-659. Tapley, S.M., & Bryden, M.P. (1985). A group test for the assessment of performance between the hands. Neuropsychologia, 23, 215-221. Thompson, E.G., & Harris, L.J. (1978). Left-handers' sensitivity to hand usage: theoretical note on saliency in the self-concept. Perceptual and Motor Skills, 47, 833-834. Todor, J.I., & Smiley, A.L. (1985). Performance differences between the hands: implications for studying disruption to limb praxis. In E. Roy (Ed.), Neuropsychological studies of apran'a and related disorders (pp. 309-344). Amsterdam: Elsevier. Witelson, S.F. (1985). The brain connection: The corpus callosum is larger in left-handers. Science, 229, 665-668. Witelson, S.F. (1980). Neuroanatomical asymmetry in left-handers: A review and implications for functional asymmetry. In J. Herron, Neuropsychology of leflhandedness (pp. 79-113). New York: Academic Press.

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LEFT-HANDEDNESS Behavioral Implications and Anomalies, S. Coren (Editor) 0 Elsevier Science Publishers B.V. (North-Holland), 1990

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Cultural Influences on Handedness: Historical and Contemporary Theory and Evidence Lauren Julius Harris Michigan State University Right-handers comprise the vast majority of the adult population. Judging from depictions of hand use in works of art and from the analysis of the design of weapons, tools, and other historical artifacts, they also would seem to have been the norm since prehistoric time (Coren & Porac, 1977; Hollis, 1875; Spennemann, 1984). The very constancy of this finding reinforces the proposition that handedness is a fundamentally biological trait, that it stems from a certain inherent bias in the central nervous system. This does not mean that the environment is unimportant. Rather, it suggests that the environment at least provides the context and opportunity for the bias to be expressed and to develop into handedness. This may be what an early writer meant by his "nice distinction" that "though the [right-hand] superiority is acquired, the tendency to acquire the superiority is natural" (Humphrey, 1861, p. 203). In other words, handedness represents the end-product of a long and multifaceted biosocial process. So far as right-handers are concerned, the environmental contribution to this process is hard to gauge, since right-handers have fashioned their environment in their own dextral image. This means that the individual whose bias is to the right lives in a compatible world of right-handed objects and customs ranging from school desks, spiral notebooks, and fountain pens to scissors and sewing machines, from musical instruments and artist palettes to assembly lines and power tools, and from wristwatches, cameras, and place settings to handshakes, oaths, and salutes -- to name but a few.

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Into this dextral world, however, there occasionally comes a child whose predisposition is in the reverse direction, or at least not the modal direction. What happens to such a child? Does he adjust or not, and if he does, when and how and at what cost? If we knew the answers to such questions, we might better understand the role of the environment in the development of handedness. These are the questions addressed in this chapter. The environmental role can be studied in a number of ways. One way is through direct observation of the development of hand use in the context of the social and cultural environment. For example, by the end of the first year, most infants reach for objects predominantly with the right hand (see reviews in Young, Segalowitz, Corter, & Trehub, 1983). Some even cross over with the right hand when the object is placed to their left (Baldwin, 1890; Harris & Carlson, 1990). Direct observations might reveal whether parents are encouraging this preference (or discouraging the opposite preference) by their own actions. Observations also could be made at the preschool and school-age periods and, indeed, in every social situation throughout the life span. Of course, older children and adults also can be asked directly whether their hand preference has been encouraged or discouraged, and with what result, and inquiries can be made of teachers and school administrators as to their policies and practices. Cross-cultural surveys of the prevalence of left-handedness offer another way to measure the environmental role. The reason is that, although all cultures are designed for the right-hander, the rules are applied more liberally, or flexibly, in some cultures than in others. For example, in the more liberal societies, about 88-90 percent of the adult population are right-handed as indexed by hand usc for writing, and 10-12 percent are left-handed. In more conservative societies, will there be an increase in the percentage of right-handers and a corresponding decrease in the percentage of left-handers? The existence of many multi-cultural societies also allows the same question to be asked of persons from different cultural backgrounds but living within a single nation state (geographic boundary). Still another method is through cross-generational, or "secular-trend,"analysis. That is, assuming that the prevalence of left-handedness in a particular age cohort reflects cultural practices pertaining to handedness for that cohort, a comparison across cohorts could be used to measure both the efficacy of cultural practices and the occurrence of change or lack thereof. In recent years, all of these methods have been used, especially the last two. It is the studies using these last two methods that I shall draw on primarily.

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Historical Evidence Most Western countries today hold liberal views about left hand use, but in each case, there was a time when left hand use was forbidden or strongly discouraged for certain acts. Because such restrictions persist in many parts of the world today, we can better understand contemporary developments by viewing them within the context of the historical evidence. Writing In the West, and indeed throughout the literate world, the right hand was the hand routinely trained for writing. For example, in the Renaissance-era book The Petie Schole by Francis Clement, one of the earliest writing-books published in England, the student, guided by a drawing, was told to "hold the pen in your right hande" (Clement, 1587/1966, p. 55). The rules could be rudely enforced, as we learn from the left-handed Benjamin Franklin in his recollection of his schooldays in Philadelphia in the early 1700's. In "A petition of the left hand" addressed "To those who have the superintendency of education," Franklin let his left hand recount how it "was suffered to grow up in her [the right hand's] education. She had masters to teach her writing, drawing, music, and other accomplishments; but if by chance I touched a pencil, a pen, or a needle, 1 was bitterly rebuked; and more than once I have been beaten for being awkward, and wanting a graceful manner" (Franklin, 1779/1904, p. 369). In the 19th and early 20th centuries it was much the same. In 1879, right handwriting was even endorsed by the French Society of Public Medicine as onc of its recommendations for instruction in penmanship (Freeman, 1913, p. 825; see also Harris, 1983; Hertz, 1073; Jobert, 1885). In the United States, around the turn of the century, teachers were said to go to "great lengths" to insure right handwriting, even tying up the pupil's left hand during a writing exercise and giving "whacks" on the knuckles of any child caught writing with the left hand (Smith, 1903, p. 328). Eating The same rules were enforced for eating and handling food. As the physician Sir Thomas Browne observed:

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That the Greeks and Romans made use [of the right hand] hereof, besides the testimony of divers Authors, is evident from their customs of discumbency at their meales, which was upon their left side, for so their right hand was free, and ready for all service. (Browne, 1646, p. 18). Right hand training began early, judging from the testimony of the author (said to be Plutarch) of Pen Puidon Agoges (first English translation, The education or bringing up of children): "[Wle do accustome our chyldren to take meate with the ryght hande, & if they do put forthe the lefte hande, anone [at once] we correct them" (Elyot, 1533, Chapter 4, p. 16). Pen Puidon Agoges proved to be immensely influential in European educational theory and Renaissance humanist philosophy. In England during the late Renaissance, the chirologist and physician John Bulwer (1644/2974) therefore could say that the custom of right hand training is "drawn out of honesty itself, and nature, and hath ever beene in use in those Nations who have addicted themselves to humanity and good manners." It was the same in the 19th century. Custom dictated that children should not be allowed to "use the dinner-knife, spoon, &c." with the left hand (Jackson, 1880, p. 637). Reaching and Grasping Training was aimed primarily at hand use for writing and eating, but it could apply to the holding of any object. For example, any tendency by the infant or young child "to use the left hand in taking food or playthings is instantly corrected by the nurse refusing to give them unless the conventionally proper hand be held out" (Alcock, 1870, p. 557); "Perhaps no mother or conventional nurse ever fails to place an object for the infant to clasp except in its right h a n d (Kellogg, 1898, p. 356). A British physician, writing in 1902, was so impressed "[flrom seeing young infants grasp objects with either hand indiscriminately and from the frequency with which one hears the admonition 'Not that hand -- the other hand,' addressed to children somewhat more advanced in age," that he could not "help thinking that right-handedness is not innate and that it is in most cases the result of teaching" (Shaw, 1902, p. 1486). Were such practices effective? David Major (1906), a professor of education at The Ohio State University, tried to find out. Beginning when his son was three months old, Major observed the child's hand movements for a variety of actions. The developmental pattern shifted back and forth from no preference

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to a right hand preference until, early in the twelfth month, a "slight preference" for the left hand appeared, which "increased so rapidly" that by the end of the month, the child was "clearly what one would call left-handed." He picked up toys with the left hand, threw or tossed a ball with the left hand, and reached more with the left hand. Left hand movements were "surer and more graceful as well as more numerous" (Major, 1906, pp. 43-44). At this point, Major began to intervene. When the child reached for articles with his left hand, they were refused, and when toys or other articles were given to him, they were always placed in the right hand. Right hand preference began to re-emerge by the end of the 15th month, until, by 24 months, the child was "decidedly right-handed." He remained so through his 44th month (the last date given in the report), although Major was uncertain whether this was a direct result of the training or "the outcome of native tendencies." (Major, 1906, footnote, p. 45). Gesture

Sanctions against left hand use in conventional gestures of greeting, such as the handshake, are evidently universal, but some societies put similar restrictions on spontaneous gestures accompanying speech. In Ancient Greece and Rome, this was part of the training given students in the fundamentals of oratory, or public speaking (see Harris, 1989). The most influential of the rhetoricians, or teachers of oratory, was Marcus Fabius Quintilianus (Quintilian), Professor of Rhetoric early in the reign of the Roman Emperor Vespasian (Smail, 1938). Quintilian's rules for public speaking included detailed directions about use of the hands as well as other parts of the body. In the case of the hands, preference was to be given to the right. Quintilian said, "It is never correct to employ the left hand alone in gesture." It could be used only when its motion conformed to that of the right, "as, for example, when we are counting our arguments on the fingers, or turn the palms of the hands to the left to express our horror of something, or thrust them out in front." (Quintilian Book XI, p. 305, lines 114-118). Practically speaking, Quintilian's rules were required by the dress code of the day. The orator wore a toga, so wrapped about the upper body as to restrict movements of the left side, while leaving the right side free (Wilson, 1924, p. 126 andpassim). The style reflected an accommodation to the numerical superiority of the right-hander. That is, so long as one limb had to be constrained, the

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other free, the free side would be the right, not the left. That left-handers might have different sartorial needs seems not to have been considered. The rhetorical tradition continued through the Renaissance. By then, togas had passed from the scene, but the rules about left hand use remained. An example is Chirologia: or the Natural Language of the Hand, and Chironomia: or the Art of Manual1 Rhetoricke by the previously-mentioned John Bulwer (1644/1974). In this work, Bulwer repeated most of Quintilian's rules about hand use. Again, the rules applied to left- and right-handers alike, for, even though Bulwer acknowledged that some people are "more nimble and active in their left hands," the "utmost dispensation" he would grant to left hand movements in public speaking would be as "connivance in common actions" (meaning non-linguistic actions), but never "in matters of speech or ornamental gesture" (p. 236). The rules were much the same in the works of many 19th- and early 20thcentury rhetoricians, both in Great Britain and America (e.g., Austin, 1806; Bacon, 1875; Barber, 1831; Hyde & Hyde, 1893; Putnam, 1856; Ross, 1890).

Left Hand Duties With so many avenues closed to it, there seemed little for the left hand to do but to be a helpmate to the right. In fact, there were certain tasks exclusively reserved for the left hand. For instance, many societies strictly forbade use of the right hand for handling unclean material or for holding one's genitals or cleaning oneself after defecation. Such duties were for the left hand alone, and no less so for left-handers than for right-handers (e.g., Beidelman, 1973; Chelhod, 1973).

W h y Restrict Left Hand Use? The probable origins of the customs and practices that elevated the right hand at the expense of the left are various and complex, depending on the culture and the historical period. The following examples by no means cover all possibilities and are not meant to be mutually exclusive. In the most general sense, the customs and practices may be seen simply as the work of the majority group (right-handers) to create a physical and social environment to suit the greatest number -- themselves. It was (and perhaps remains) a reasonable practice in certain circumstances. For example, where sanitation was scarce and where people used their fingers to eat from the same

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bowl, it was a sensible public health measure to reserve food handling (as well as acts of social intercourse) for one hand and certain private functions for the other. The right hand being the dominant hand for the majority, the social functions understandably were reserved for the right hand for all individuals, the 'low' duties for the left. In some cases, use of the left hand was forbidden on the grounds that the left hand, by nature, was too clumsy to perform the action appropriately. Thus, when Bulwer (1644/1974) forbade left hand gesture in the accompaniment of speech, he implied that the left hand was less able to serve communication. How should it, he asked, make of itself a complete action since the action thereof is more contracted, inform, incomposed, and out-of-order? Whereas the actions of the right are free, frequent, continued, comparsed, and resembling the sweet cadences of numbers, and therefore hath the prerogative of eloquence in the body.(Bulwer, 1644/1974, p. 235). Bulwer may well have been correct where tagfit-handers were concerned (see Kimura, 1973a, 1973b). But having acknowledged that some people are "more nimble and active in their left hands," he could not appeal to inherent skill in defense of his universal right hand rule. Instead, he had to fall back on custom and tradition. In the case of writing, the justification likewise was the supposedly lesser skill of the left hand. The difference was that this reason was invoked even for the left-hander, as it was for Benjamin Franklin, whose (dominant) left hand was "beaten for being awkward and wanting a graceful manner" (1779/1904, p. 369). In America in the early 20th century, it also was supposed that the left-hander's problem was exacerbated by his writing style. For example, the supervisor of penmanship in the Cincinatti schools said that left handwriting "is, at best, difficult and awkward but especially so when not done in the right way; and probably three-fourths of the left handwriters write in the wrong way" (quoted in Selzer, 1933, p. 75). The "wrong way" was with the paper in the same position as for right handwriting, which "forces the writer to place his hand and pen above the line of writing and necessitates moving the entire arm eight to ten times in writing across the page." Such writing was "invariably cramped and unintelligible and ...p ainfully laborious." (p. 75). The "right way" was to place the paper "just opposite to the position for right-handed writing, thus making it easy and natural for the writer to do his work with the hand and pen below the

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line of writing" (p. 75). Other authorities justified right hand use for writing on the grounds of greater efficiency. According to the great French physiologist Xavier Bichat (1805/1809), this is why right handwriting came to be the conventional style in the first place. As he explained, "such are the wants of society" that a certain number of general movements must necessarily be executed by all persons "in the same direction, to the end that they should be understood by all." Because "[it] has been agreed that this direction should be from left to right," it was necessary to use the right hand, "which is better adapted than the left to the formation of letters in this direction, as the latter would suit the opposite mode of writing infinitely better" (p. 22). Bichat supposed that the requisite uniformity of movement also "determines armies to employ the right hand to seize their arms" and causes even "the most savage people" to use the right leg for principal motions in the dance (p. 22). Finally, even where specific reasons to forbid left hand use no longer applied, the old practices were still associated with and sustained by folklore beliefs about the left hand and side. These beliefs typically attached positive values to the right side and negative values to the left. For example, the right side was lucky, the left unlucky; omens on the right were auspicious, on the left inauspicious. Most people being right-handed, the right hand (and arm) normally were stronger and more muscular. Strength being a masculine virtue, the right side itself became linked with maleness, the left side with femaleness, and so the "right" side took on the social prerogative of the male himself. The lateral axis even figured in theories of procreation, starting with Eve, who was fashioned from a rib from Adam's left side. The left hand being less active, or more often idle and covered up by clothing, it also was supposedly more prone to use in thievery and other clandestine acts, and where it was used for unclean acts, it became 'dirty' by association. Values about right and left came to be expressed in language: To be "dextral" was to be skillful,just, fair, straight, true, direct, and honest; to be "sinistral,"with its etymological link to sinister, was to be the negation of all these qualities. The supposed awkwardness of the left hand is captured in the German "links" or "linkisch,"meaning clumsy, and in the French "gauche,"meaning both clumsy and impolite. The Greek words "iaeva"or "sinistral'were used to denote a thief, and one of the Greek words for left, "skaios," means ill-omened'. In Scotland, the

These particular associations are revealed in John Bulwer's rules for gesture. For example, for acting on the stage, the left hand could be used to designate a thief 'Toput fonh ihe leji Hand as ii were by stealth, is their significant endeavour who have an intent unseene to purloine and convey away something" (Bulwer, 1644/1974, p. 102). And the left hand thrust

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association of the left hand with private functions is seen in the name "cackhanded for the left hand, from "cack meaning excrement. From the European languages to the African is a long linguistic leap, but these different senses remain. For example, in the Bantu language, the name for left often implies inferiority or bad luck (Beidelman, 1973; see also Barsley, 1970, pp. 157-158; Harris, 1980, 1985a; Hertz, 1973; Lloyd, 1973; Needham, 1973; Wile, 1934, pp. 30-57). From this consensus about right and left, at least one nation has stood apart. The Chinese, like others, attribute unequal values to the two sides, but without dishonouring the left. As Granet (1973, p. 44) observed, "the diametrical opposition or polarity ...is not found in China." Nevertheless, the right hand remains the preferred hand for writing and eating, so that left hand use for these tasks is rare. Again, the sense is expressed in language. In China, the right hand is called the "food hand." Some Exceptions to the Rule In the face of all the restrictions and associated beliefs about the left hand and the left-hander, it is noteworthy that, for left-handers, certain left-handed acts were permitted and sometimes even encouraged. For example, left-handers always seem to have been free to hurl spears or missiles with the left hand. By one oft-quoted biblical account, they also could hit their targets with uncommon skill. These were, of course, the "seven hundred chosen men left-handed of the tribe of Benjamin, every one of whom "could sling stones at a hair breadth, and not miss" (Judges, chapter 20, verse 15). Left-handers also seem to have been tolerated well enough among men who drew sword (Harris, 1990d). Whereas early writing manuals, books on public speaking, and books of etiquette routinely forbade use of the left hand, fencing manuals just as routinely made allowances for the left-handed student, with statements of the form, "if the Scholar is left-handed, his left Hand is to be conducted with the same Instructions as the right." (Valdin, 1729, p. 17). But many fencing masters also showed special interest in left-handers because of the success they seemed to enjoy over their right-handed opponents. As Captain John Godfrey said, "I cannot help taking notice, that the left-handed Man has

forth with the palm turned backward, the left shoulder raised, and the head turned to the right could be used to signify refusal, abhorrence, or abomination of "some execrable thing" (pp. 186187).

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the Advantage over the right-handed, upon an equal Footing" (Godfrey, 1747, p. 16). The margin was enough so that, as he went on to say, "in both Small and Back-Sword, I would rather contend with the right-handed Man with more Judgment, than the other with less" (p. 17). The reason for the advantage sparked much interest. The average fencer seemed convinced that the left-hander was naturally superior, but the fencing masters agreed that the advantage was more-or-less an accident of number, the left-hander "exercisingoftener with Right-handed Men than a Right-handed Man with him" (L'Abbat, 1734, p. 86). The right-hander therefore was advised to practice more against left-handers -- "any difficulties...will be easily overcome by practice" (Roland, 1824, p. 126). Occasional "lessons from your master with his left h a n d were advised too (Roland, 1824, p. 126). Both the analysis and the advice were probably sound (cf. Wood & Aggleton, 1989). More recently, baseball, the "great American pastime," is the sport in which left-handers have made the greatest mark. In America, by the 188O's, as the advantage of the left-handed pitcher and hitter began to be recognized, their numbers started to increase (James, 1988, pp. 112-123). Ever since, they have been a dominant force, accounting for some 18 percent of all players who throw and bat with the left hand (Neft & Cohen, 1988). The left-hander had several advantages. For example, from the pitcher's mound, his fastball naturally tailed away from the right-handed batter. He also could keep a first-base runner closer to the base, giving the runner less chance to steal second or to advance to third in the event of another hit. At bat, as the journalist Grantland Rice pointed out, the left-hander was nearer first base (cited in Uhrbrock, 1970, p. 289). Another writer, speaking of Babe Ruth, the greatest and most charismatic of the lefthanded batters, remarked on the Babe's good fortune on being "brought up in an institution where nobody had time to object to his complete left-handedness" (Broun, 1920, p. 62). The writer was wrong on one point: although Ruth batted and threw left-handed, his left-handedness was not "complete." Officially classified as "incorrigible," he spent his formative years at St. Mary's Industrial School for Boys, a combination reformatory and orphanage in Baltimore run by the Catholic Order of Xavernian Brothers, and like probably every other boy there, he was taught to w i f e with his right hand. Judging from photographs, it was a life-long practice (Okrent & Lewine, 1979, p. 121 and dust cover; Ritter & Rucker, 1988, passim). In addition to the recruitment of left-handers into the game, a few righthanded pitchers in the early days even became "switch-pitchers'' and, for a few innings, would throw left-handed (James, 1988, p. 112). Even more right-handed

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batters became switch-hitters. Some sports-minded parents, fathers especially, also began to appreciate the advantages and to train their sons accordingly: "Fathers are interested in the development of left-handers, of course." (quoted in Selzer, 1933, p. 77).

Early Estimates of the Number of Left-Handers Where left hand use was restricted, did the restrictions work? Writing in 1909, the French anthropologist Robert Hertz seemed sure of it. While acknowledging that the "material bonds" had been abolished in Europe, in contrast to countries like the Dutch Indies, where native children reportedly "had the left arm completely bound" (Jacobs, 1892), Hertz observed that the psychological restraints were no less strong. As he said, "One of the signs which distinguish a well-brought-up [European] child is that its left hand has become incapable of any independent action" (Hertz 1973, p. 5). Hertz was being ironic. Still, his general point was supported by surveys of the number of left-handers in the population -- most set the figure at five percent or less. As can be true even today, the figures are uncertain because of the variety of criteria and decision rules used. Hand use for writing, however, was the usual measure. I have already referred to what may have been the earliest "statistical survey" -- the "seven hundred chosen men left-handed from the tribe of Benjamin. The tribe itself numbered "twenty and six thousand," and Wilson (1885, p. 130) noted that 700 is 2.7 percent of 26,OOO and that nearly the same figure was assigned by Wilhelm Hyrtl as the percentage of left-handed persons "based on observations made in one of the most civilized centres of modern Europe." Hyrtl was referring to Vienna, where he was professor of anatomy at the University. Hyrtl's (1860) estimate was based on the frequency of left hand use for writing. In Europe, during the last three decades of the 19th century, other studies came up with similar figures (two to five percent) using the same index (e.g., Biervliet, 1901; Delaunay, 1874; Jobert, 1885). In the early 20th century, much the same percentages were reported for schoolchildren. One of the first surveys was by Ballard (1911-1912), who submitted a handedness questionnaire to the head teachers of some of the elementary schools of South London. Of a total of 13,189 children, 545 (four percent) were judged to be left-handers (the judgment in this case being made evidently on the basis of their use of their left hand for "all the common dexterities,'' apart from writing). Of these left-handers, 146 (27 percent) were

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what Ballard called "pure sinistrals" (those whose left-handedness had never been interfered with), and the remaining 399 (73 percent) were "dextro-sinistrals" (meaning "a congenitally left-handed person who has conformed with social custom in writing with the right hand). In a new sample of 11,939 children, Ballard found that 322 (2.7 percent) were what he called "distinctly left-handed" (Ballard did not explain but said only that "all doubtful cases were excluded'), of whom there were 51 "pure sinistrals" and 271 "dextro-sinistrals." In summary, most children judged to be left-handed wrote with the right hand. That the total percentage of left-handers ("pure sinistrals" and "dextro-sinistrals") across the two surveys came to less than four percent also suggests, however, that Ballard's definition of left-handedness was overly strict. This may explain why Gordon (1921, p. 332) reported a higher figure -- 7.3 percent (241 of 3,298) in a study of elementary school children, although Gordon did not explain the basis for this determination. In perhaps the largest survey of all, Selzer (1933) sent questionnaires to supervisors of penmanship and supervisors of special classes in the 60 largest cities in the United States. Information was obtained on 230,156 schoolchildren. The results disclosed a wide variation in the percentage of left-handedness and suggested that the percentages reported were influenced by the criterion of judgment as well as by the amount of social pressure brought to bear in changing the left-handed child to right-handedness. In one sample, only one to two percent of the children were left-handed based on the hand used for writing as compared to nearly 29 percent based on strength of grip (Selzer, 1933, p. 35). Evidently, too, where training was applied, it did not carry over into all situations. The supervisor of penmanship for the Cincinnati Public Schools, mentioned earlier, recounted the results of a 1925 survey of all elementary school students to determine the number who wrote with the left hand. Of 34,316 children in the survey, 33,201 (97 percent) were reported to be right-handed, and 1,115 (3 percent) left-handed. Of the left-handers, 632 were reported to be "writing with the left hand in class, whereas the remaining 483 did "most or all of their writing" in class with the right hand. However, 889 of the left-handers were reported to be writing with the right hand "in the writing period (Selzer, 1933, pp. 74-75). This last statistic suggests that when the left-handed child was under the direct supervision of the penmanship instructor ("in the writing period) instead of in an ordinary class, use of the right hand increased. Age Differences. Several of the school surveys covered a wide age range (for example, Gordon's [1921] subjects ranged in age from four to fourteen years).

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On the view that the effect of training was cumulative, Bethe (1925) and Schaefer (1911; both cited in Peiper, 1%3, p. 258) looked to see whether the number of left-handers decreased with age. Bethe examined 95 German children ranging in age from one and a half to six years. Instead of using handwriting or drawing as an index of handedness, he used a variety of other measures, including handshake, eating with a spoon, pointing with the fingers, grasping for symmetrically placed candles, lifting objects from the floor, building with blocks, and cutting with a scissors. By these tests, Bethe ascertained that 40 percent of two- to four-year-olds were left-handed, 21 percent were either handed, and 38 percent were right-handed. By ages four to six, only 19 percent were lefthanded, six percent were either-handed, and 75 percent were right-handed. According to Peiper (1963, p. 258), Bethe concluded that many of the lefthanded children and those who had no preference became right-handed under the influence of school, and that afterwards, they are erroneously believed to be endowed with right-handedness. Schaefer's (1911) study, also of German children, provided further evidence of an age effect. Among 17,083 children, there were 692 left-handed children (4.1 percent), 37 ambidexters (0.2 percent), and 16,354 right-handers (95.7 percent). The number of left-handers continuously decreased from the youngest to the oldest group. For every group of 100 left-handed children in the lower grades, there were only 60 in the middle grades and 55 in the upper grades, which Peiper concluded "can probably be explained as resulting from education to right-handedness in school." Perhaps the most revealing age differences were found by Komai and Fukuoka (1934) in a study of more than 17,000 Japanese children from 20 primary schools, ranging from first-grade to eighth-grade. The children answered questions about the hand used for striking a match, holding a penknife for sharpening a pencil, throwing a ball, holding chop-sticks, and holding a writing brush. The percentage of children reporting left hand use for at least one activity dropped from approximately 16.5 percent in first grade to approximately nine percent in eighth grade, with the drop being greatest for the acts of writing and using chopsticks (under two percent for the eighth-graders), and least for the less socially important and therefore presumably less trained acts, for example, throwing a ball or using a scissors. Sex Differences. Early surveys also routinely indicated that left-handedness was more common in males than in females. For example, of the 11,939 children in Ballard's (1911-1912) second survey, 3.5 percent were left-handed

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boys, only 1.8 percent were left-handed girls. In one of Selzer's (1933) samples, the percentages of left-handed writers among the boys and girls were 7.1 and 4.4, respectively. Among adults, similar sex differences were reported (e.g., Ogle, 1871) (see also Wile, 1934, pp. 71-73). Recognition That Left Hand Use for Writing Underestimates the Number of Left-Handers. Recognizing the effectiveness of social training, several critics pointed out that defining handedness by the writing hand underestimates the number of left-handers. One of these critics, the French neurologist Fransois Moutier, therefore concluded that the hand used for writing "should never be used for the purpose of categorizing individuals, since, according to uniformly received education, we all learn to write with the right h a n d (Moutier, 1908, p. 119). Moutier himself proposed that the true ratio of left-handers was closer to one in ten. Selzer (1933) likewise thought that there was a "much larger percentage of left-handedness" than that indicated by writing hand (p. 31). In Selzer's opinion, if children were tested for functions that have not been influenced by custom, "we would find ten to twelve percent of left-handedness, and even this would perhaps represent only the extreme sinistral end of the distribution of innate left tendency" (p. 31). Wilson (1885, p. 130) made the same point in a different way by cautioning that in assessing handedness, "the civilized and cultured classes, affected by education, social habits, and many artificial usages, must be discriminated from those in whom nature has been left to operate with little constraint." The Supposed Effect of Right Hand Training on Cerebral Dominance for Language

Moutier's (1908) warning not to define handedness in terms of the writing hand was important for a question then -- and still today -- of much interest: the relationship between handedness and cerebral control for speech. Broca (1865) had proposed that the control for manual dominance was in the hemisphere of the brain opposite the dominant hand, thus in the left hemisphere for right-handers and in the right hemisphere for left-handers. The same was widely presumed to hold for speech as well. Today, we know that, although nearly all right-handers are left-cerebral dominant for speech, the reverse does not hold for left-handers. Instead, as many as 70 percent of left-handers are left-lateralized for speech, with the rest

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apparently having either bilateral or right-hemisphere speech (e.g., Rasmussen & Milner, 1977). It was nearly 80 years, however, before this new understanding

was reached, an understanding delayed, in part, by certain widely-held beliefs about the consequences of right hand training for the left-hander. The reason is as follows: To protect the "reversed-dominance" rule, early investigators had to explain the exceptional cases of what Bramwell (1899) called "crossed aphasia," wherein aphasia is associated with injury to the hemisphere ipsilateral rather than contralateral to the dominant hand. By the early 20th century, many such "exceptions" had been reported. Nearly all were left-handers who became aphasic after left-hemisphere injury, as indicated by the appearance of right hemiplegia (e.g., Gajkiewicz, 1911; Long, 1913; Tison, 1889). A few were righthanders who became aphasic after right-hemisphere injury with left hemiplegia (e.g., Raggi, 1917; Versiloff, 1908). Such exceptions were explained in a variety of ways, several of which took account of the practice of right hand training. For example, to explain crossed aphasia in right-handers, the British neurologist William Ogle suggested that these persons in fact might have been "persons with a natural left-handed tendency, in whom the bias is so feeble that its external manifestations become completely masked by education" (Ogle, 1871, p. 292). For individuals who were unquestionably left-handed, at least as indicated by left hand use for acts other than writing, another scenario was proposed. It was that writing with the right hand had shifted language from its original site in the right hemisphere to a new site in the left hemisphere. As a result, aphasia would be associated with left-hemisphere injury. An example is the case reported by Sir George Paget (1887). An elderly patient had suffered a series of seizures over a five and a half year period, in one instance culminating in left hemiplegia without aphasia, in another in right hemiplegia and severe aphasia. Paget's explanation for the "anomaly" was that the patient, "though left-handed for other actions, ...wrote with his right hand, and was in the habit of writing much (p. 1253). Paget saw his case as bearing on the question whether the localization of speech in the left hemisphere was due to a "congenital predominance" or to the education of the right hand. Paget thought both factors played a role but that his case: is an additional argument on the side of education, and it points to writing as the most potent of these educational influences. The ordinary rule of the organ of speech being in the same hemisphere that serves for the voluntary movements of the naturally predominant hand -- this ordinary

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rule was reversed, and the reversal is the more remarkable because the congenitally predominant hand had remained the preferential hand for all actions except writing. The habit of writing much seems the only assignable cause for this reversal. And it may be held to be an adequate cause, for the memory of words will naturally go with the movements of the hand that writes them; and the process is simplified by using one and the same cerebral hemisphere for recollecting the words and for directing the hand in setting down their symbols (Paget, 1887, p. 1259). Many others used similar reasoning, including Bastian (1898), Wernicke (1906, p. 268), and Bramwell (1899) (for further details, see Harris, 1990b). By the turn of the century, the notion that hand-training affects language lateralization had become so much the conventional wisdom that it was adopted as one of the rationales for an educational movement. The London-based Ambidextral Culture Society urged training both hands equally on the grounds that this would establish 'reserve' language centers in the right hemisphere, thereby providing a reserve area in the case of left-hemisphere injury (J. Jackson, 1905; see Harris, 1985b, 1985~).

"Stubborn Left-Handers" Moutier (1908) said that "we all learn to write with the right hand," although most of the early surveys, such as Ballard's (1911-1912), suggest that some lefthanders managed to escape this training. But even for those left-handers who were trained to write with the right hand, the lesson evidently did not stick in every case, or, as Selzer's (1933) evidence implied, it was effective only under the vigilant gaze of the penmanship teacher. It was this latter sort of evidence -that the training was not invariably successful -- that buttressed Broca's view that handedness (both left-handedness and right-handedness) stemmed from an "organic predisposition" rather than from social training and imitation. As Broca said, there was: a circumstance that does not permit attributing the choice of the right hand to imitation; for there are everywhere a certain number of individuals who despite all their efforts, all their perseverance, remain left-handed. For them, one must admit the existence of an inverse organic predisposition against which imitation and even education cannot prevail (Broca, 1865, p. 382).

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Ogle made a similar point: There are a vast number of individuals brought up under precisely the same conditions...with their fellows, who yet are what is called left-handed, and who remain so in spite frequently of their eager wish to change their manner and accommodate themselves to the fashions of their companions (Ogle, 1871 p. 283). Over the years, several others also took note of the lack of complete success of social training. Among them was the American behaviourist John B. Watson. As the champion of a social-training explanation of handedness, Watson was challenged and perplexed by the left-hander. Why, he asked, are there "5% of out and out left-handers and from 10-15% who are mixtures -- e.g., using right hand to throw a ball, write or eat, but the left hand to guide an axe or hoe, etc...."? (Watson, 1924, p. 102). Watson first called left-handers "those hardy souls who have resisted social pressure," then said that the answer "is not known." The British educational psychologist Cyril Burt (1937, p. 317) implied that lefthanders, at least certain left-handers, were "stubborn and wilful,'' and the American psychoanalyst Abram Blau (1946) declared that left-handedness was the result of "emotional negativism." Note that the assumption behind such characterizations is that, as Watson put it, left-handers actively resisted social pressure to use the right hand, whereas both Broca (1865) and Ogle (1871) were referring to left-handers who remain left-handed despite their own efforts to change. The implication is that there are at least two kinds of "stubborn" lefthanders: one who does not want to change, another who wants to but cannot. Change of Views I said that most Western countries today hold more liberal views toward lefthandedness than they used to. But when did they begin to change their views, and why? There had always been some who questioned the wisdom of the restrictive practices, who called them superstitious and even dangerous. In Great Britain, one such critic was the Victorian novelist Charles Reade, who said, "Impregnated with all these traditions, young mothers and nurses check infants, with superstitious horror, in the use of the left hand -- which, nota bene, the poor little victims invariably attempt -- and do their best to make a pagan tradition an

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immortal truth, and keep mankind one-handed and right-handed (Reade, 1878, p. 74). Reade was among those who regarded handedness itself to be unnatural, so that right hand training in his eyes constituted the imposition of an unnatural bias on a perfectly symmetrical system. This was what made the practice so repellant to him. "Every child," he said, "is even and either handed until some grown fool interferes and mutilates it" (Reade, 1878, 2 March, p. 175). In England, the followers of Ambidextral Culture were of like mind. In the United States, however, at least by the early 1 9 0 0 ' ~it~was the growing conviction that handedness was natural that led many experts to begin to criticize the imposition of universal rules, that is, rules for left-handers and right-handers alike. Often, too, it was the cruelty of the training, the "whacks" on the knuckles, that gave them pause (e.g., Smith, 1903). Even so, critics did not invariably condemn the goal behind the practice. Consider the 'mixed' advice of the Director of the Division of Hygiene of the Children's Bureau of the United States Department of Labor in 1922. A father had written to inquire about his 18-month-old daughter who "shows a strong tendency to be left-handed." Could the Children's Bureau "send any literature ...dealing with the methods of over-coming the tendency to be left-handed in a child of this age?" (Kassens, 1922). The Director replied that there was "very little available literature" on this particular question but quoted the advice given by a physician at Johns Hopkins University in response to a similar inquiry a few years earlier: I believe that it is much better in the case of the left-handed baby to take no serious steps to interfere with the free utilization of the left hand. The increased utilization of the right hand might not compensate for the interference with the natural organization of the individual child's activity. At the same time there is a certain advantage in being able to use righthanded tools, and in being free from the eccentricity of left-handedness, and it would be useful to make the child somewhat ambidextrous. This should be done rather by way of encouragement than by way of restriction. Where the position of the child makes a difference, as in being carried, or being near the light or being near certain objects, the mother might so arrange that there is a somewhat greater stimulus for the use of the right hand.

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It might console the parents to know that some of the most versatile among geniuses was left-handed, Leonard0 da Vinci (letter from Charles Macfie Campbell, 1916; quoted in Rude, 1922): Effect of Interference on the Development of Speech. What the physician did not say was what had already begun to worry many others, namely, that interference with the child's developing left-handedness could harm the development of speech. This concern was raised by several reports, between 1911 and 1918, of delayed or disturbed speech in "switched left-handed children and adults (e.g., Ballard, 1911-1912; Jones, 1915; Nice, 1918; Whipple, 1911). The most impressive of these reports was by Ballard (1911-1912), who carried out three separate studies. Reference has already been made to the first of these studies -- of the 545 normally left-handed children found in a sample of 13,189 from elementary schools of South London. Recall that of these 545 lefthanded children, 399 (73 percent of the total) were "dextro-sinistral," that is, had been required to write with the right hand. The proportion of stutterers in this group was 4.3 percent against only 1.1percent in the "pure sinistrals," those lefthanders whose hand use had not been interfered with. Making left-handed children use the right hand thus appeared to have multiplied the number of stutterers almost four-fold. Ballard's second study was on 944 mentally retarded children, of whom 882 were right-handers, Of the right-handers, 14 (1.6 percent) were stutterers; of the dextro-sinistrals, the figure was nearly 20 percent, meaning that right hand training had multiplied the chance of stuttering by 12. Finally, in the third study, also cited earlier, Ballard personally examined all of the 322 left-handers found among 11,939 children, 8 to 14 years of age. Of these 322 children, 271 were "dextro-sinistrals," 46 of whom stuttered at the time, and 24 others had stuttered previously and recovered. Of the 51 "pure sinistrals," not one child stuttered. The proportion of stutterers among the dextro-sinistrals therefore was about 18 times greater than among pure dextrals. Ballard offered a tentative neurological explanation of the effect, emphasizing the "intimate functional connexion of the writing centre with the system of word centres, and particularly with the speaking centre," and suggesting that "the dominant speech area is either robbed of some of its energy, or that some sort of competition takes place which tends to disorganize its function" (Ballard, 1911-1912, p. 308). The idea, in other words, was that cortical controls for

I am grateful to Robert and Alice Smuts for providing me with this document.

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speech and hand use were tightly linked, and that the link was disrupted by forced training of the 'non-dominant' hand, particularly for writing (a 'manual speech act'). For Ballard, the moral was clear: "Writing should always be done by the superior hand, and by the superior hand ewclusively" (p. 309; emphasis in original). Many came to agree, among them Lewis M. Terman, then an associate professor of education at Stanford University and already a leader in the field of testing and individual differences. Citing Ballard's report, Terman (1914) warned that the attempt to make right-handed children out of those who are normally left-handed "increases many times the liability of stuttering" (p. 346). Therefore, "Left-handed children should remain left-handed, for writing at least; the slight advantages that accrue from the change are entirely outweighed by the dangers to speech" (p. 346). Ada Hart Arlitt, Professor of Child Care and Training at the University of Cincinnati, agreed in the case of the older child. In her book on infancy and early childhood, published in 1928, Arlitt pointed out that although "a child may with perfect safety be conditioned in infancy to use the right hand," authorities agreed that "such conditioning should not be done after the sentence stage in speech has been reached, except by the advice of a specialist and under his care. There seems so close a relation between speech and hand movements that interference with left-handedness after the speech coordinations are fairly well developed may produce some forms of speech defect" (Arlitt, 1928, 1st ed., pp. 56-57). In the wake of the adverse publicity about the consequences of forcibly changing left-handedness, many began to speak out on behalf of left-handers, and, in America, in the early part of the 20th century, popular magazines started to publish articles with titles like Let Left-HandednessAlone! (Terrell, 1917) or The Crime Against Left-Handedness (Jordan, 1922). A stout defender of lefthanders was the physician George M. Gould (1908). Noting that in the United States there were "about six million originally or persisting left-handed persons," some of them "mental and even physical cripples from the injudicious antipathy of parents and teachers to the 'south-pawed' (p. 38), Gould urged, "Let the lefthanded child alone! Nature is quite as wise as the ignorant intermeddlers" (p. 39). Still, there was no strong consensus among the experts, so that teachers and school superintendents were uncertain what to do. This can be seen in the results of a questionnaire published in 1914 by McMullin. Asked whether the left-handed pupil must write with the right hand, and, if so, what was the

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expected benefit, most of the respondents (American school superintendents) expressed strong but differing views. About half favoured training the right hand (especially if the child was young and if left handwriting was not yet wellestablished), while the rest favoured letting the child use the left hand, although several remarked that practice in doing other things with the right hand would be to the child’s advantage because this is a “right-handed world.” Most of those allowing, or even favouring, left hand use found justification in physiological evidence purporting to demonstrate the ‘naturalness’ of left-handedness (for example, the notion that in the left-hander, more blood flowed to the right hemisphere than the left). Those advocating right hand training noted the many practical advantages of the right hand style, including the right-handed design of school desks and the presumed right-handed design (the left-to-right direction) of the English alphabet (the same reason given by Bichat in 1805). The views of supervisors of penmanship were solicited as well, and, judging from Selzer’s (1933) report, opinion was still mixed on whether to shift the lefthanded child. The decision, it seems, rested on several factors: as one respondent put it, on how “badly left-handed the child was because that would determine how easily the shift could be made, and to what extent it was believed that a shift would cause speech difficulties and other academic problems. This, in turn, was believed to depend on how the shift was accomplished. As another respondent said, “The use of force, scolding, criticizing, and nagging would cause any child, regardless of whether he is left- or right-handed, to become nervous and to have difficulty in learning to read, write, or do any other school work’ (quoted in Selzer, 1933, p. 76). According to figures quoted by Selzer, based on nine school surveys, approximately 30 percent of left-handers had been taught in school to write with the right hand. This educational decision, however, often rested with the individual school or school district. For example, on 20 November, 1922, a newspaper story, under the headline “Left-handedness is cured among pupils,’’reported: An intensive campaign to cure left-handedess among pupils in local schools here [Elizabeth, New Jersey] has resulted in a reduction from 250 to 66 since 1919. In the enrollment of nearly 13,000 this is slightly more than one-half of one per cent (quoted in Parson, 1924, p. 102). In the United States, the question whether left-handers should be converted continued to be vigorously debated at least through the 1950’s. Eventually, as

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Bakwin (1950) has written, more and more psychologists and pediatricians came to agree that coercion was ineffective at best and dangerous at worst, and articles in the popular press followed suit (e.g., Hicks, 1946). But if the experts agreed that coercion was the wrong way to proceed, they did not necessarily agree on the main question -- whether the left-hander should be converted but by other means. Several psychologists argued against any interference. One was the school psychologist Luella Cole, who said: a teacher [should] be totally indifferent in the matter of hand preference. She must literally not care in the least which hand a child uses. She must regard a preference for the left hand over the right as no more disturbing than the possession of blue eyes instead of brown (Cole, 1939, p. 437; emphasis in original). The teacher's job, therefore, was to use correct procedures for teaching the lefthanded child how to write, procedures that Cole herself developed and described in great detail. Another psychologist to reject converting the left-hander was Arlitt, in the third edition (1946) of her book on infancy and early childhood. Citing a new study by Bryngelson and Clark (1933) showing that forced conversion interfered with the child's academic work (reading or writing) as well as speech, Arlitt added to her original (1928) statement the warning that such researches "make it clear that the damage resulting from such conditioning is far too serious for any psychologist or psychiatrist to put himself in the position of advising reconditioning left-handed children" (Arlitt, 1946, p. 120). Taking the more traditional side was Gertrude Hildreth, a professor of education at Brooklyn College. For Hildreth, the "best rule" was "not to let the child get well-started in left-handedness for any skill he is likely to use steadily, that is, eating, writing, sewing, and using household tools and equipment" (1950, p. 104). Indeed, the nursery-school age child "should not be permitted to make his own choices in handedness for basic skills." Instead, "steps should be taken" to insure building right-handedness for those skills and acts in which most people are right-handed and that will be "consistently used through life and in which the environment favors right-handed patterns" (1950, p. 104). Hildreth offered many training suggestions, keyed to the age of the child, for example, "If a young child wants to use a hammer or other tool place it in his right hand (1950, p. 105). For the child whose left-dominance was already well-established, Hildreth recommended retraining for right-handedness if the left-dominance was not too

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firmly fixed and if the child was willing and cooperative. She also offered many suggestions on how to proceed, emphasizing the importance of a sympathetic approach, for example, by helping the child understand the purpose of the retraining, by being gentle and kind rather than dictatorial, and by not making the child self-conscious (1950, p. 112). Finally, for the left-dominant child for whom such approaches were ineffective or whose left-handedness for writing in particular was too well-established, Hildreth offered recommendations for helping him write more acceptably, including sloping the desk so that "he can see over his hand and avoid slanting his paper at an extreme angle" (1950, p. 122).

Current Evidence "Liberal"Countries Today As we have seen, in the early years of the 20th century, opinion in the West had begun to shift toward greater tolerance for left-handedness, although parents and teachers continued to get divergent advice on the matter. Today, even if the word "left" still conjures up the traditional dark images (see Domhoff, 19691970), this process of "liberalization" as measured by hand use for writing seems to have been, or is very nearly, completed. Levy (1974, Fig. 6.2, p. 133) plotted the reported percentages of left handwriters based on four separate surveys in the United States from 1932 to 1972 and found a negatively accelerated function, which, by 1972, reached about 99 percent of asymptotic value at 11 percent. Subsequent surveys of students in the United States as well as in Australia, Canada, Great Britain, New Zealand, Sweden, and the Netherlands continue to yield estimates in the 9-12 percent range (e.g., Brigs & Nebes, 1975; Bryden, 1977; Levander & Schalling, 1988; Tan, 1983), with one survey of American college students reporting a 13.8 percent figure (Spiegler & Yeni-Komshian, 1983). In these countries, the hand used for writing evidently can serve as a valid measure of handedness, contrary to what would have been so in the past.

Secular Trend Analysis. In the case of Great Britain and the United States, we traced the beginnings of the change in practice to the period from approximately 1915 to the early 1930's, largely on the basis of statements by psychologists and educators of that era. As mentioned earlier, another, more direct index of this change is through cross-generational, or "secular trend," analysis. That is, if we assume that the prevalence of left-handedness in a

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particular age cohort (such as that for the students in the studies cited above) reflects cultural practices pertaining to handedness in operation for that cohort, then by comparing cohorts, we can chart temporal changes, including starting and finishing points. This technique has been used in several recent studies. Fleminger, Dalton, and Standage (1977) tested a British sample and found that left-handedness for writing was inversely related to age, ranging from 2.9 percent in 55- to 64-year-olds (born from 1912-1922) to 10.8 percent in 15- to 24-year-olds (born from 1953-1962). Two other studies of hand use for writing, in the Netherlands and in Australia and New Zealand, have yielded similar findings while suggesting that the liberalization began as late as the 1940's in the Netherlands (Beukelaar & Kroonenberg, 1986), meaning that it has taken only 20 years to complete, whereas in Australia and New Zealand, it may have begun as early as the 1880's but has taken a far longer time to complete (Brackenridge, 1981). Beukelaar and Kroonenberg (1986) suggest that the difference may have something to do with the size of the country and/or its labor mobility. Other secular-trend analyses have compared generations within families. For example, Smart, Jeffrey, and Richards (1980) examined hand use for writing in three generations of Britons: grandparents, parents, and six-year-old children. The percentage of left-handed writers was 6.2 percent in the grandparents, 10 percent in the parents, and 17.5 percent in the children. The authors suggested that their result probably reflects the "mellowing of school teachers' attitudes toward sinistral writing from being punitive some 50 years ago to relatively liberal now" (p. 82). They added that there therefore may be many "covert lefthanders" among the parents and grandparents who are right-handed writers but do not recollect their left hand being discouraged. Another implication of the secular trend data, of course, is that in liberal societies today, use of the right hand for writing is a more valid indication of right-handedness (nonlefthandedness) in younger than in older generations. Tambs, Magnus, and Berg (1987) assessed hand preference for writing in three generations of Norwegians -- grown children and their spouses and siblings, their parents, and their own children. The percentage of left handwriters rose across the three generations, from 1.2 percent for the oldest generation to 8.7 percent for the youngest generation. (This latter figure being on the low end of contemporary estimates for Western countries, it suggests some residual cultural bias against left handwriting.) Each generation itself also was divided into three age groups at ten-year intervals, yielding a total of nine age cohorts, ranging from the oldest group, born about 1895-1905,to the youngest group, born about

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1975-1985. A classification of the entire sample, applying ten-year-intervals of estimated age, showed a smoothly increasing prevalence from about 1920 to 1960. Two more studies compared only two generations -- parents and their children. In a study of Australians, of 508 subjects comprising the parental generation (over 35 years of age), only 3.7 percent reported writing with the left hand compared with 12.3 percent of the 917 younger subjects (Tan, 1983). The subjects in the other study (Ashton, 1982) were Americans living in Hawaii. Three different ancestral groups were tested: European, Japanese, and a third category consisting of Chinese, Korean, Hawaiian, and part-Hawaiian. For the parents but not the children, the percentage of left-handed writers was higher for the European than for the other groups (due mainly to the very low frequency of left-handed writers in the Japanese adults), but, irrespective of ethnicity, the filial generation reported a significantly higher proportion of left-handed writers.

"Conservative"Countries Today Elsewhere in the world, many of the traditional "conservative" practices remain in force. The countries include Italy (Salmaso & Longoni, 1983, 1985), Germany (Peters, 1986), the Soviet Union (Louis, 1983), Japan (Hatta & Nakatsuka, 1976), and many nations of Africa (Brain, 1977; Dawson, 1972; Payne, 1981; Verhaegen & Ntumba, 1964), especially nations where Islam is the predominant religion (Payne, 1987). (I know of no studies, however, of the Islamic nations of the Middle East.) Again, certain public acts, especially writing and eating, remain the prime targets so that, in the case of writing, left hand use is reported for only one to five to six percent of the population, including the younger generation. Anecdotal evidence also suggests that, at least in certain countries, the right hand training starts well before the school years. For example, in Tanzania, where giving or receiving anything with the left hand is considered to be insulting, one witness recalled seeing a woman smack the hand of a twelve-month-old infant who extended his left hand to receive a banana (Brain, 1977, p. 186). In the Orient, too, despite often pronounced moves toward Westernization since the 1950's, cultural practices pertaining to hand use have remained largely in place. For example, among Chinese living in Taiwan, only 0.7 percent of elementary school children and college students report that they use their left hand for writing (Teng, Lee, Yang, & Chang, 1976, 1979), the same low percentage reported by Komai and Fukuoka (1934) for Japanese 8th-graders in

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1934 (see also Dean, Rattan, & Hua, 1987). For Chinese-American children, however, the percentage is 6.5 percent (Hardyck, Goldman, & Petrinovich, 1975), indicating a significant if still incomplete relaxation of the cultural restrictions. The authors of the report on Taiwan could see "no trend of increasing permissiveness towards left-handedness." (Teng et al., 1979, p. 47), but in certain other traditional societies, some of the cultural bias may be on the wane. For example, in Nigeria, Payne (1981) surveyed a large population on the single measure of hand-use for writing. In a sample of 56,779 primary schoolchildren, 4.5 percent used the left hand, a figure at least twice as high as had been found in earlier reports from Nigeria and other African nations (Bakare, 1974; Dawson, 1972; Verhaegen & Ntumba, 1964). Payne (1981) credited the difference to increasing urbanization and rapid expansion of "Western-type'' primary education, both of which presumably act to break down traditional socialization systems in an agricultural society (pp. 234-235). That the prevalence figures found are still well below Western figures suggests, though, that the "Westernization" process is incomplete. In Nigeria, it also would seem to be proceeding faster in some ethnic sectors than in others, in particular, faster among Christians than among Muslims. One indication is the reports Payne received of pupils being allowed to use the left hand in their state school while being forced to write Arabic with the right at Quranic school. Another indication comes from a comparison of Muslims' and Christians' attitudes about handedness (Payne, 1987). When Nigerian Muslim and Christian college students were asked which hand they themselves would prefer to use to perform each of 60 different tasks (based on Provins, Milner, & Kerr, 1982), the mean scores for right hand preference for the Muslim students were higher for 47 of the 60 items but reached statistical significance in only six instances. However, when asked which hand they thought should be used, the Muslims' scores were higher for 56 items, significantly so in 37 cases. In other words, the Christians for the most part did not see use of the right hand as obligatory even though they themselves tended to perform most tasks with the right hand, whereas the Muslims not only reported performing tasks with the right hand but also thought it important for all people, including left-handers, to do so. Payne (1987, p. 255) also cautioned that the views of the general Nigerian population are likely to be far more strongly influenced by social norms and taboos than those found for Nigerian university students, who must be considered even more atypical of wider society than their western counterparts. Further evidence of cultural influence is revealed in studies of handedness and ethnicity in Israel. Kobyliansky, Micle, and Arensburg (1978) compared

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the prevalence of left-handedness for writing in a large sample of young (17-25 years) Israeli Jewish males divided into four groups according to their own or their parents' place of birth: Eastern Europe, Central Europe, Middle East, and North Africa. The results showed that 13.3 percent of the subjects of European origin wrote with the left hand, compared to 9.8 percent of the North African group and only 4.2 percent of the Middle Eastern group. When the Eastern European sample was further divided into those born in Eastern Europe and those born in Israel of parents who had immigrated from Eastern Europe, the percentage of left handwriters was nearly three times greater (15.5 percent) in the Israeli-born than in the European-born group (5.9 percent). The authors credited the difference to liberal Israeli educational policies that do not hinder the development of left-handedness in children. Silverberg, Obler, and Gordon (1979) have reported a similarly high prevalence of left-handedness in young native-born Israelis, irrespective of ethnic background.

Agricultural vs. Hunting Societies. Dawson (1972) has hypothesized that the distinction between "liberal" (non-traditional) and "conservative" (traditional) practices is associated with local economic conditions and means of production. As evidence, Dawson (1972) pointed to his finding a higher percentage of lefthandedness (around ten percent) in Australian Aborigines, Chinese boat-people, and Canadian Eskimos (Inuit), who live in permissive hunting and fishing societies, and a lower percentage (one to three percent) in the Temne of Sierra Leone and in the Hakka Chinese, who live in agricultural societies with harsh socialization customs pertaining to right hand training (cf. Littlejohn, 1973). Higher percentages of left handwriting also have been reported among the Kwakiutl Indians (a group of closely-related North American Indian tribes who traditionally have inhabited Vancouver Island and the adjacent mainland of British Columbia). In three age groups of 60 persons each -- four to six years, ten to twelve years, and adults (ages unspecified) -- the percentage of left handwriters was 21, 14, and 15, respectively, with an additional 17, 3, and 7 percent reported to be "ambihanded for writing. The figures for a Caucasian sample, matched for sex and geographic location, were only seven percent for all three of the same age groups, and with virtually no cases of "ambihandedness" (Marrion, 1986). Marrion and Rosenblood (1986) suggest that the true number of left-handers in Kwakiutl society may be even higher inasmuch as depictions of hand use in late 19th-and early 20th-century Kwakiutl totem and house poles show 24 percent left-handedness, 56 percent bihandedness, and only 20 percent right-handedness. They see the difference as a reflection of the extent to which

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contemporary Kwakiutl society has incorporated the right-handed culture of the West, with its English graphology, specialized industrial tool use, and greater requirement for writing during the compulsory periods of formal education. The authors thus see their results as challenging the conventional wisdom that only about ten to twelve percent of the population is left-handed. Their implication would seem to be that, even in "liberal" societies, this figure represents an underestimation because of the influence of a right-handed culture. If so, this might square with other indirect evidence (from artifacts and artistic representations) suggesting that in certain geographic regions and in certain historical periods, the prevalence of left-handedness might have exceeded the 'conventional' upper limit (cf. Brinton, 1896; Mortillet, 1890 [cited in Harris, 1980, pp. 9-10]; Spennemann, 1984). Whatever may prove to be the case on the "upper limit" of left-handedness, the premise that the societal permissiveness that marks the hunting and fishing culture invariably boosts the incidence of left-handedness has lately come under challenge. Using a five-item questionnaire adapted from Bryden (1977), Ardila and his associates (1989) compared the handedness among Tucano adolescents with that of several control groups. The Tucano people, who live in the jungle of the Colombian Amazon, are a hunting and fishing people. The culture is "highly permissive and puts no pressure on the child's hand preference, either before or during his time at school" (p. 894). Nevertheless, left-handedness among the Tucano was significantly lower than that of several control groups, including groups from urban areas (Ardila, Ardila, Bryden, Ostrovsky, Rosselli, & Steenhuis, 1989). Sex Differences

Recall that early studies found left-handedness to be more prevalent (by one to three or even four points as a percentage of the total population) in males than in females. In contemporary studies, although the sex difference has not appeared invariably (e.g., Ashton, 1982; Dennis, 1958; Levander & Schalling, 1988; Salmaso & Longoni, 1983), it is by far the more frequent outcome, in both Western and Non-Western countries, and in adults (e.g., Brito, Brito, & Paumgartten, 1985; Bryden, 1977; Dawson, 1972; Maehara et al., 1988; Oldfield, 1971; Porac & Coren, 1981; Tapley & Bryden, 1985) as well as children (e.g., Annett & Turner, 1974; Calnan & Richardson, 1976; Clark, 1957, pp. 189-190; Douglas, Ross, & Cooper, 1967; Gillberg, Waldenstrom, & Rasmussen, 1984; Hardyck, Goldman, & Petrinovich, 1975; Hildreth, 1950; Maehara et al., 1988;

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Peters, 1986; Peters & Pedersen, 1978; Porac & Coren, 1981; Verhaegen & Ntumba, 1964). One explanation of the sex difference is that it reflects biological differences. There is evidence that the male develops hand preference, or at least right hand preference, later than the female (Archer, Campbell, & Segalowitz, 1988; Carlson & Harris, 1985; Humphrey & Humphrey, 1987). This might be a correlate of the male’s overall slower rate of physical development or perhaps a specific result of the later functional maturation of the left hemisphere (Shucard, Shucard, Cummins, & Campos, 1981). The latter could account for the male’s slower development not only of manual praxis but of other lefthemisphere skills such as speech (Moore, 1967; Wells, 1979). Geschwind and Galaburda (1985) link the developmental difference to the higher doses of fetal testosterone in the male. According to their model, fetal testosterone, either by slowing development of the left hemisphere or (as new evidence suggests) by directly promoting growth of the right (Galaburda, Corsiglia, Rosen, & Sherman, 1987), gives the right hemisphere the lead, thereby raising the likelihood of lefthandedness (right-hemisphere dominance for manual praxis). Still another model links the sex difference to evidence that the neural mechanisms underlying praxis and speech are less focally organized in the left hemisphere of men than women (Kimura & Harshman, 1984). Kimura (1983a) suggests that this may explain not only why right-handedness is less prevalent in men than in women but why, in right-handers, the margin of right hand superiority in motor performance is weaker in males than in females (e.g., Annett & Kilshaw, 1983). The possibility that the sex difference has biological roots does not, of course, preclude a further and even substantial role for training and cultural pressure. This is what Selzer (1933) seems to have had in mind when he suggested that the sex differences revealed in his own surveys might have been insignificant if handedness had been based on some self-learned function instead of on hand use for writing. Selzer’s suggestion wins some support from Komai and Fukuoka’s (1934) study of Japanese school children, mentioned earlier. Among the 1st- graders, 16 percent of the girls and 17 percent of the boys reported using the left hand for at least one of the six activities named. By 8th grade, girls had dropped to six percent, the boys to only 9.5 percent. The difference lay more in the presumably less socially controlled acts of throwing a ball, using a scissors, and striking a match, and less in the more trained acts of writing and using chopsticks, or using a penknife, where left hand use dropped to nearly equally low levels in both sexes (see also Dean et al., 1987).

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If these data imply a role for socialization in the generation of sex differences, they do not tell us exactly what that role is. At least two different possibilities can be contemplated. Inasmuch as socialization pressures are generally applied more strongly to females than to males (Barry, Bacon, & Child, 1957), the same could hold for social pressures against left hand use (e.g., Dennis, 1958, p. 209). Alternatively, perhaps social pressure is applied equally to both sexes, but girls are more likely than boys to make the necessary accommodation. Recent evidence leans more toward the second possibility. Shimizu and Endo (1983), in a study of Japanese high school students, found a higher percentage of conversion to right hand use in left-handed girls than in left-handed boys but no evidence that the girls had been subjected to more social pressure. Porac, Coren, and Searleman (1986) surveyed 650 undergraduates at a Canadian university and found 88.4 percent of the group to be right-handed, 11.7 percent left-handed as measured by a four-item questionnaire that did not include questions about hand use for writing or eating. The subjects also were asked whether they had experienced any pressures to change their hand preference in any way (including from right-to-left as well as from left-to-right). Seventy three persons (11.2 percent) reported having had such experiences, in 52 of whom the pressure had been to switch hand preference to the right. The likelihood that an individual had experienced pressure was not related significantly to sex, but the success of the hand change varied with sex, with 61.3 percent of the females reporting success in making the shift compared to only 26.3 percent of the males (S. Coren, personal communication, Sept., 1989). The results also revealed that of all the successful shifts, 80 percent were initiated prior to grade 3, suggesting a strong effect of age. What is surprising is the 23 individuals reporting pressure to shift to the left. The circumstances were not disclosed, but one wonders whether they had to do with sports for which both left- and right hand skills are favoured. The subjects being Canadian, the sport most likely would be hockey. If so, the targets presumably were more often males than females. If the second scenario is correct, we still must explain why girls accommodate more than boys to right hand training. At least two perhaps related reasons come to mind: girls are generally more socially compliant than boys -- "more ready to bow to convention" (Clark, 1957, p. 12) -- or they are inherently more capable of complying, either because of their greater motoric maturity or because of underlying sex differences in neurobiological organization of the sort mentioned earlier.

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Males vs. Females as Agents of Social Transmission Females not only might accommodate more than males to right hand training, they also might be more important than males in influencing the development of hand preference in others. In teaching the school-age child to write (with the right hand), this would be inevitable and theoretically uninteresting because most primary and secondary school teachers are women (although we must keep in mind that in some countries today and in certain historical periods, schoolmasters, not schoolmistresses, ruled the classroom; also, certain orthodox Islamic societies, such as Saudi Arabia, practice strict sexual segregation in education, so that boys are taught only by men, girls by women). Although teachers have been, and continue to be, forceful instruments for right hand training in the classroom, evidence suggests that when the teacher is lef-handed, the result can be an increase of left hand use by the pupils. This may occur through imitation. Many years ago, Scheidemann (1930) reported that in the second-grade classroom of a certain American school, 16 of the 34 children were left handwriters. Both the first- and second-grade teachers were left-handed, and Scheidemann estimated, from additional behavioural tests, that perhaps as many as twelve of these children were actually right-handed and therefore had come to write with their left hand through imitation of the teachers. Whether the female teacher is necessarily the more salient model remains to be seen, but what we can assume is that the crucial imitation episodes occur when the teacher is writing at the blackboard or sitting next to the child, since these are the only occasions when the child can directly map his own movements onto those of the teacher; the teacher who modeled left handwriting while facing the child would be more likely to elicit mirror-image imitative movements, meaning right hand movements (cf. Harris, 1990~). Apart from being models for left hand use, left-handed teachers perhaps are overtly more sympathetic to left-handedness and therefore less likely to discourage it in their students. Where their own children are concerned, women also might be expected to be more influential social agents because of their earlier, more intense, and prolonged relationship with the child. There is evidence for this possibility in recent studies of handedness patterns within families. The evidence is statistical rather than from direct observation, and it is derived only from studies in "liberal" societies. McGee and Cozad (1980) examined the incidence of left hand preference among offspring from reciprocal matings from six different studies, including one of their own. They concluded that there was a consistent, highly

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significant maternal effect on left-handedness such that there were more lefthanded offspring when the mother was left-handed and the father was righthanded than when the father was left-handed and the mother was right-handed. Ashton (1982) reported similar differences, although they were not statistically significant (see also Annett, 1972; 1978; Falek, 1959; and Leiber & Axelrod, 1981). All of the preceding studies have been of school-age children, but the process might begin much earlier. Harkins and Michel (1988) found a similar maternal effect in six- to thirteen-month-old infants, which they suggest might have come about through direct imitation of the mother’s left-handedness. Harkins (1987) previously had found that eight- to nine-month-old infants imitate their mother’s hand-use during social interactions and play with toys. Psychological Effects of Forcible Shifting of Left-Handedness The evidence shows that training the left-handed child to write with the right hand can be highly effective. Less certain is whether there are any other psychological consequences of the kind proposed by early investigators. Speech and Language. The notion that the training would cause stuttering or otherwise disrupt the development of speech and language -- ironically the very possibility of which helped to weaken the practice -- has long been a bone of contention. Even in the early days, there were critics who took a dim view of the hypothesis, both on empirical and conceptual grounds (see Burt, 1937, pp. 323-330). Contemporary critics are also dubious, although, judging from recent reviews (e.g., Rosenfield, 1984), we seem to know little more on this question, pro or con, than was known in the 1930’s. The reason is that new studies of the relationship between handedness and speech disorders have ignored the question of hand training and have focused on the broader question whether speech disorders are more common in left-handers generally. Here, with a few exceptions (Geschwind & Behan, 1982; Geschwind & Galaburda, 1985; Morley, 1957), the evidence fails to show any relationship (for a review, see Homzie & Lindsay, 1984; see also Records, Heimbuch, & Kidd, 1977; Webster & Poulos, 1987). The new cross-cultural studies offer another opportunity to assess the hypothesis, but, except for Brain (1977), the issue seems not to have been addressed. What Brain found was a 1.13 percent incidence of stuttering in Tanzanian children against estimates of 0.6 -- one percent in American children

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(Snyder, 1960). Brain, who takes a more sympathetic view of the hand-shift-stuttering hypothesis than most contemporary writers, has suggested why the effect is trivially small, effectively absent, in the African child. He observes that in the "liberal" societies of the West, much of the formal right hand training for the left-handed child is unlikely to occur until the child is about six or seven years old and is taught to write (Brain seems to be implying that right hand training for writing was still the practice as of the mid-1970's). By contrast, the African child, as a teacher quoted by Brain pointed out, must know the difference between his hands "by the time he is old enough to walk across the room and hand something to a guest," meaning by the age of two or three years at the latest. Brain, therefore, suggests that one of the reasons the African child might be spared the problem is the very early age at which handedness training commences. Recall that earlier writers likewise supposed that handedness could be changed without incident if the training begins early enough. On balance, the evidence looks weak, even accepting early and impressive statistical reports, such as Ballard's (1911-1912) or Bryngelson and Clark's (1933). Nevertheless, it is not beyond the realm of possibility that forced right hand training is one of several factors that put certain left-handed children at risk for speech problems. At greater risk might be those left-handers already having language or speech problems, especially perhaps those left-handed children with highly discrepant skill favouring the left hand, which Bishop (1984) sees as a possible sub-clinical sign of pathological left-handedness stemming from lefthemisphere dysfunction (Bishop, 1980, 1984; see also Harris & Carlson, 1988, pp. 354-356). Males also would be expected to be at greater risk than females, males already being three to four times more likely than females to develop stuttering, regardless of handedness (Homzie & Lindsay, 1984, p. 238). Poor bimanual coordination might be another sign of risk (cf. Vaughn & Webster, 1989). Finally, particularly in the presence of other risk variables, common sense suggests that training (or retraining) that is severe and punitive, as opposed to kindly and gentle, as Hildreth (1950) recommended, ought to make negative effects more likely. Some early evidence appears to bear this out (Haefner, 1929, pp. 69 ff.). Psychological Adjustment. What of the effects of hand-retraining on psychological adjustment generally? Again, they could hardly be salutary if the training is punitive and severe. Consider the story told by Alexander Veyn, a leading Soviet psychologist, writing in Literatuniaya Gazeta. A boy whose lefthandedness showed at the age of three or four years was forced by his parents,

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on their physician's advice, to hold his spoon and pencil in his right hand. Any failures were punished. The situation worsened when the boy started school and his teachers insisted that he write with his right hand; his parents even tied his left hand while he did his homework. According to Veyn, the child's schoolwork suffered and he gradually became psychologically disturbed (Louis, 1983). Anecdotes, of course, are not scientific evidence. Some corroboration, however, is provided by empirical studies, although, unfortunately, there are precious few to draw on. What may still be the most suggestive report is now over twenty years old. Young and Knapp (1966) compared the personality characteristics of two groups of thirteen- to fifteen-year-old left-handed boys. One group of boys lived in Italy, either South (Palermo), Central (Rome), or North (Florence). All of these boys wrote with the right hand but showed themselves to be left-handed by other, less socially-targeted acts. The second group were American boys living in Boston, all second-generation of Southern Italian descent. All wrote with the left hand and, presumably, had grown up in an atmosphere relatively free of pressure against left hand use. There also were two control groups of right-handed boys, one in Italy, the other in Boston. On the Cattell High School Personality Questionnaire, the Italian left-handers, in comparison to the American left-handers or to the control right-handers, proved to be significantly more demanding, impatient, subjective, dependent, and hypochondriacal. Cattell regards this constellation of traits as having an association with neuroticism but not anxiety, and as being strongly affected by the environment. On other parts of the test, there also were indications of regional differences, the left-handed boys from Palermo generally being more "schizothymic," lower in ego strength, and higher in "desurgency" (less enthusiastic and happy-go-lucky) than other groups. Young and Knapp reasoned that the duress of forced conversion in the Italian left-handers was the primary determinant and also noted that the cultures of Sicily and Southern Italy traditionally imposed greater penalties on the left-handed child than did the cultures of Central and, especially, Northern Italy. They also cited similar findings in a study of Polish children (Sielicka, Bogdanowicz, Dilling-Ostrowska, Szelozynska & Kaczenska, 1963). Perhaps no less crucial than the method of hand training is the larger cultural context in which the training takes place. In Italy, especially in the South, as Young and Knapp (1966) have noted, the left hand (and the left-hander) by tradition is scorned and reviled. Left-handedness is considered to be both a "moral and personal defect and regarded with widespread suspicion" (Young & Knapp, 1966, p. 36). This may be why right hand training was so effective (that

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is, tiom of the left-handed boys in the Italian group wrote with the left hand). In countries with more moderate, if still conservative, views about lefthandedness, any psychological effects might be weaker or absent. This might be especially so in China, where, as mentioned earlier, the left hand itself is not dishonoured even though its use is proscribed for writing and eating. Cognitive Skills. The evidence suggests that the forcible conversion of hand use can affect emotional development, at least under certain circumstances. Whether it could have any further effects on cognitive skills is even less clear. The scant amount of evidence on this question appears to cover every possibility. For Young and Knapp’s left-handed Italian schoolboys, it would not be surprising if, like the Russian schoolboy (Louis, 1983), the psychological duress to which they presumably had been subjected were to have hindered their academic performance. No such outcome was mcntioned, but evidence to this effect has been reported in a study of British school children born in 1946, a time when most were converted to right handwriting (Douglas, Ross, & Cooper, 1967). Likewise, Clark (1957, pp. 185-187) found that left-handed Scottish school children who had been forcibly converted from left- to right hand use did much less well on school achievement tests (arithmetic and English) than right-handers paired for sex, school grade, and intelligence. Clark concluded that the effects could not be attributed to the influence of speed and style of writing because these factors were removed in this assessment (p. 187). By contrast, no ill effects were evident among the “forced left-handed adults in Teng et al.’s (1979) study of left-handers in Taiwan. After all, all of these individuals were students at public universities of high academic standing, the main requirement for admission to which was scoring within the top three percent on comprehensive entrance examinations. Nor was there any difference between the handedness distribution of these college students and the schoolchildren in the same survey. These converted left-handers therefore would seem to be no different from the tiorz-converted left-handers in liberal societies, who, likewise, show no evidence of lower intelligence or academic success in comparison with right-handers (e.g., Hardyck & Petrinovich, 1977; Hardyck, Petrinovich, & Goldman, 1976). Here, again, if forced use of the right hand has any consequences for academic performance, the more important factors may be the training method and the larger cultural context in which the training takes place. If requiring left-handers to write with the right hand were to have any consequences for cognition, standardized achievement and intelligence test scores may not necessarily be the best measures. Both draw on what Cattell (1971)

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calls "crystallized intelligence," that is, knowledge or automaticized skills, vocabulary and reading in particular. The more appropriate measure may be tests of "fluid intelligence," which calls for active problem-solving, such as the manipulation of figural or spatial content as in a three-dimensional mental rotation task. On spatial tasks like these, differences between normal lefthanders and right-handers have been found, with left-handers more often at an disadvantage (McKeever, 1986, in press; see also this volume), although complex interactions involving sex and reasoning ability also occur (Harshman, Hampson, & Berenbaum, 1983; Lewis & Harris, this volume). If at least some normal (that is, unconverted) left-handers already may be weaker on such measures, the possibility arises that forcible conversion (in a context of duress and general disparagement of left-handedness) might exacerbate the existing weakness. A recent study appears to support this possibility. Ardila, Correa, Zuluaga, and Uribe (1988) tested three groups of adults, all living in Bogota, Colombia. Included were eight "forced left-handers" (that is, forced to write with the righf hand but left-handed for three other activities (using scissors, throwing a ball, and lighting a match), eight control left-handers (use of left hand on all four tasks and no report of having been forced to write with the right hand), and eight right-handers, none of whom presumably had been subjected to social pressure of any such kind. All the subjects were matched for age, sex (five men, three women in each group), and educational background. All subjects took the sphtial relations subtest of the Differential Aptitude Test (DAT), a measure of "mental rotation" skill (Bennett, Seashore, & Wesman, 1959). The result was that the average DAT score for the forced left-handers was less than half that of the control left-handers, who were slightly but not significantly below the right-handers. The forced left-handers also presented "outstanding difficulties in everyday spatial activities," including manipulating spatial information, spatial orientation, ordering numbers, following routes, and moving a car backward. In contrast to these results, Bakare (1974) found that Nigerian children from the rural and lower social classes scored significantly higher on a battery of leftright discrimination tasks than did children from the middle class, although, eventually, they were surpassed by them. Bakare's suggested explanation for the rural and lower-class children's initial advantage was that they have earlier opportunities to practice the concept of right and left because their parents gave them more independence and opportunities for exploration of their spatial environments. What Bakare did not consider was his additional finding that nonright-handedness was also higher among the rural and lower-class children,

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a difference that he attributed to the relaxation of pressure among the middleclass. The more important contribution to the rural and lower-class children’s early spatial skill therefore could have been earlier and more intensive training in hand use. As Brain (1977, p. 186) has pointed out, the African child must know the difference between his hands much earlier than his Western counterpart, so that by the time most African children reach school, they have learned that the right hand is the correct hand to use for every social purpose. It is not unreasonable to suppose that such children might develop concepts of left and right earlier than their Western counterparts and that the difference also will be reflected between social classes within African society. By this interpretation, of course, we face a problem in understanding why Bakare’s effects were ‘positive’(albeit only initially), whereas Ardila et al.’s were ‘negative.’ More specifically, even supposing that ‘normal’(that is, unconverted) left-handers are already weaker in certain spatial skills, why would right hand training exacerbate the problem rather than mitigate it as Bakare’s findings suggest?

Further Questions Given the evidence for left-handers, it seems clear that social-cultural practices can play an important role in molding hand preference. Still, many questions remain. Several have been mentioned already. In this last section, I want to raise a number of others. When Does Training Begin, and How Does It Work? One question is, in “conservative”societies, what does social training consist of? How early and in what ways do parents begin to influence their children to use the right hand and not the left? Where writing is concerned, formal right hand training presumably begins when the child enters school and is instructed in penmanship, but informal instruction could start much earlier, at least by the preschool years when the teacher (or parent) places the crayon in the child’s right hand, or even in infancy through the infant’s imitation of the parent’s own hand use (Harkins, 1987). Similarly, early anecdotal reports indicate that instruction in table manners can begin as soon as the child is old enough to reach for food or to hold a spoon, so that parents could place objects (food, toys, utensils) closer to the child’s right hand or into the right hand directly. This has

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been shown in a longitudinal study of American children from nine to fifteen months of age whose parents expressed no ambition, much less determination, that their child shall be right-handed (Wang, Fitzgerald, & Harris, 1989), so one expects that the practice would be all the more deliberate and insistent in conservative societies. But how much of the parent’s behaviour is planned and how much of it is in response to subtle cues sent out by the child himself? In either case, how effective are such early actions, either in enhancing any right hand preference already present, or in suppressing left hand preference? To date, we have little more to go on than anecdotes and single-case reports, such as Major’s (1906), described earlier. A prospective cross-cultural longitudinal study could help to answer these questions. We also need better information about the day-to-day problems facing the left-hander that might contribute to changes or adjustments in hand use. Some provocative, although still indirect, evidence on this point recently has been reported. In a survey of college students in a Canadian university, left-handers were significantly more likely than right-handers to report having an injury requiring medical attention during the last two years (Coren, 1989). The categories included injuries at work and in the home, and while playing sports, driving a vehicle, and using tools or implements. For the left-hander, covert dextral biases in the environment evidently can make the environment not only inconvenient but downright dangerous. An interview study by Falek (1959) also suggests that left-handers experience certain social problems associated with their left-handedness, including unpleasant jokes at their expense. The more serious problems facing the lefthander, however, were vocational and economic. For example, the left-handers noted that manufacturing jobs involving assembly lines were set up for righthanders. Falek’s data also raise the possibility that the drawbacks of lefthandedness are seen as more serious in some social-economic circles than in others. This possibility is hard to gauge with college students, the usual research subjects, because of their greater social homogeneity.

What Mechanisms Underlie Sex Differences in Agency of Social Transmission? If, as the family pattern data suggest, the mother is the more important agent of handedness than the father, more particularly of left-handedness when she herself is left-handed, the mechanisms remain to be explained. Does her own left-handedness make her more alert to signs of left-handedness in her own child

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(cf. Etaugh & Brausam, 1978; Thompson & Harris, 1978), making her either more likely either to encourage this development or less likely to discourage it? Or is the more important factor the model of left-handedness that she provides her child for imitation, a possibility already demonstrated both in the case of the mother with her infant (Harkins, 1987) and the teacher with the primary school pupil (Scheidemann, 1930). However the question is answered, because all of the data supporting a "maternal transmission" factor come from the analysis of intrafamilial handedness patterns in "liberal societies," can we suppose that similar patterns will emerge in "conservative societies," such that the mother now plays the leading role in suppressing left hand use? To complicate the picture further, there may be circumstances, within the "liberal" society, when it is the father who plays the larger role in promoting left-handedness. I alluded to this possibility earlier in suggesting that the father may be the one to promote left-handedness, or, to be more precise, left hand orientation to specific tasks, in order to give his child an advantage for certain sports. Alternatively, if the father himself is left-handed and has personally experienced its liabilities in the workplace, he might be particularly keen on discouraging left-handedness in his child (cf. Falek, 1959). All such parental roles also might be related to the sex of the child. Finally, in a conservative society, sex differences might be further related to social class. In the study, cited earlier, of British schoolchildren born in 1946, a higher proportion of working class than of middle class children were found to have inconsistent or left hand preference, with the usual sex difference (more lefthandedness in males than females) being confined to the working class sample (Douglas, Ross, & Cooper, 1967). Which Left-Handers Do Not Shift? As we have seen, even in very conservative societies, some left-handers write and eat with the left hand when most others are using the right. This has been so in societies that are liberal today but conservative in the past as well as in societies that remain conservative today. For example, recall that 27 percent of the left-handers in Ballads (1911-1912) first survey, and 0.7 percent (of the total sample) in Teng et al.'s (1979) study, were left-handers who wrote with their left hand, or recall the "intensive campaign to cure left-handedness" among schoolchildren that still left "one-half of one percent" left-handers in the school population (Parsons, 1924).

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Who are these individuals? Of course, some might be those left-handers who have not been taught to write with the right hand, a reasonable enough possibility in the less rigid conservative societies or in societies in transition where right hand training is no longer universal (as in America in the 1920's where a left-handed child's likelihood of being "cured might depend on the school he attended; Parson, 1924). Alternatively, these left-handers might have been deliberately passed over because they showed stronger, more stable left hand dominance, leading parents and teachers to regard them as less promising candidates for right hand training and to focus their efforts instead on children with weaker preferences. Perhaps the ones passed over belong to a subgroup of "consistent" left-handers recently described by Peters (1990). These individuals were consistent in preference and performance within tasks, in contrast to another subgroup who were consistent within but inconsistent between tasks. Remarkably, the individuals in this latter group were actually stronger and threw better with the righf hand than the left. Because all of the subjects were of a young generation (college students) from a "liberal" country (Canada), it is understandable that both groups also wrote with the left hand. In a conservative society, however, right handwriting might be more common among "inconsistent" left-handers because their inconsistency marks them as better candidates for right hand training. A tendency to apply right hand training selectively to children with weaker or less stable hand dominance also might be magnified in those conservative societies where absolute prohibitions against left handwriting are starting to break down. That is, as tolerance to left-handedness increases, perhaps this tolerance is extended more to the child who is more strongly or consistently left-handed than to the child with weaker, less consistent preference. This question could be explored by comparing countries (or regions) where change is underway with countries where change has not yet begun. All this may seem straight-forward, but, if so, who are the "stubborn" individuals who are subjected to rigorous right hand training and who remain left-handed despite all efforts to change them, or who, once changed, revert to left hand use when the pressures against left hand use are relaxed (for example, after leaving school)? And are they different from those persons who, as Ogle (1871, p. 293) said, remain left-handed "in spite frequently of their eager wish to change their manner and accommodate themselves to the fashions of their companions?" The latter are individuals for whom, let us say, the "trait" is stubborn even if the will is not. If there are two such groups of "stubborn" lefthanders, current cross-cultural data suggest that, together, they comprise from

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10 to 20 percent or more of the left-handed population. The evidence, mentioned earlier, that males and females are equally often the target of right hand training but that females make the adjustment more easily (Porac et al., 1986; Shimizu & Endo, 1983) suggests that proportionately more of the "stubborn" left-handers are males.

Neuropsychologically Distinct Subgroups. What lends these speculations greater interest is the possibility that the analysis of the generic "stubborn" lefthander will advance our understanding of the relationship between handedness and neuropsychological organization. For example, left-handers who are trained to write with the right hand but who revert to left hand use when they can (e.g., after social pressures are relaxed) could be neuropsychologically different from those who continue to use the right hand. This possibility is suggested by a study carried out in Vienna, Austria by Gloning, Gloning, Haub, and Quatember (1969). They examined the records of 114 patients who had died of cerebral lesions suffered in adulthood. Fifty-seven had been strongly right-handed (RH); 57 others were either left-handed or ambidextrous (NRH). The authors noted (p. 43) that none of the NRH patients showed any evidence of early brain damage of the sort "that could have led to a shift of cerebral dominance" (the reference presumably is to either or both cerebral control for language and for manual dexterity). All, furthermore, had been forced to write with the right hand during the period of elementary education (as was customary for this age cohort). However, 17 of these individuals had changed to writing with the left hand some time after leaving school, whereas the remaining 40 individuals continued writing with the right hand. Neuropsychological testing indicated that the verbal behaviour of the NRH brain-damaged patients, taken as a whole, was different from that of RH patients with similar lesions. This much would be expected in light of other research. But it also indicated that, among the NRH group, the 40 persons who continued to write with the right hand after leaving school were neuropsychologically different from the 17 who changed to writing with the left hand. In both groups, both left- and right-side lesions were associated with speech disorders. However, in the 40 right handwriters, lefthemisphere lesions were associated with agraphia, alexia, and dyscalculia (as it is known to be for right-handers with left-hemisphere lesions), whereas in the 17 left handwriters, these same disorders were associated with right-hemisphere lesions. In summary, among the NRH subjects, lateralization for speech did not predict lateralization for reading, writing, and calculation, and the nature and extent of this dissociation was different for the 17 nonright-handers who came

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to write with the left hand versus those 40 who continued to write with the right hand. These findings suggest that, although both groups of left-handers can learn to write with the right hand, the task is simpler (and more enduring) for left-handers whose writing skills (along with reading and calculation skills) are co-lateralized, along with speech, in the left hemisphere, that is, the hemisphere contralateral to the current writing hand, but more difficult for left-handers whose writing (and other linguistic) skills are lateralized in the right hemisphere, meaning that these skills are laterally dissociated from speech skills.

Hand Posture and Manual Regulation for Writing. Short of studying cognitive loss after unilateral lesion, how might one identify a left-hander's subgroup membership? As Levy (1982, 1986; Levy & Wagner, 1984) has pointed out, left-handers whose regulatory routes for manual control (including control for writing) are specialized in the ipsilateral left-hemisphere would, by definition, have unusual regulatory routes. She also suggests that this unusual control may be correlated with unusual hand postures during writing, namely, the left-inverted (LI) posture. In consideration of the findings reported by Cloning et al., does the LI posture mark the individual as one of those for whom a shift in hand use for writing could be accomplished more easily? Cross-cultural research could help to answer this question by comparing the frequency of the LI posture among those left-handers in conservative societies who still write with the left hand with that in left-handed writers in liberal societies.

W h y Might Forced Right Hand Training Have Further Cognitive Effects? Assuming that forced use of the right hand is related to psychological development, the mechanisms remain to be found. To return to the study by Ardila et al. (1988), the authors themselves explained the lower spatial performance in their forced left-handers (left-handers forced to write with the right hand) by invoking Levy's (1969) hypothesis that the greater bilateralization of language functions in left-handers (at least some left-handers) compromises right-hemisphere spatial functions. "Forced left-handers," they suggest, would represent "an extreme in this trend (p. 149). This interpretation seems to imply that forcing the left-hander to write with the right hand is somehow responsible for the greater bilateralization of language function and thus, indirectly, for the poorer spatial performance. Nothing in Levy's (1969) model, however, suggests that left-handers are less lateralized for linguistic functions us u result of being forced to write with the right hand.

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(Levy's left-handed subjects, in any case, wrote with the left hand; they were the counterparts of Ardila et al.'s control left-handers.) The explanation also trades on the old explanation of "crossed aphasia," mentioned earlier, that forcing lefthanders to write with the right hand creates auxiliary language centers in the contralateral (is., left) hemisphere. Although protracted use of one limb undoubtedly has certain consequences for functioning in the contralateral hemisphere, the consensus today is that the induction of cortical specialization for language is not among them3. Instead of supposing that forced left-handers are different from control lefthanders because they have been forced to write with the right hand, consider the possibility that they were different to begin with -- in other words, that forced left-handers and control (non-forced) left-handers come from different populations of left-handers. In connection with Ardila et al.'s (1988) findings, the study by Cloning et al. (1969) raises two possibilities. One is that the forced left-handers in Ardila et al.'s study represent only one subgroup of forced-lefthanders, the other forced left-handers having returned to use of the left hand for writing once they were out of school (a subgroup not sampled in this study). The other possibility is that the neuropsychological organization of Ardila et al.'s forced left-handers was already different from that of this second subgroup. If so, the spatial deficits of Ardila et al.'s forced left-handers might have more to do with their prior neuropsychological organization than with hand training. That is, it may be this different neuropsychological organization that has

In any case, this now-discredited model does not quite fit with what Ardila et al. seem to be suggesting. As I said, the old idea was that forcing left-handers to write with the right hand would create left-hemisphere language centers. Because most left-handers are left-hemisphere dominant for language functions to begin with, or at least for speech functions (e.g., Rasmussen & Milner, 1977). it would seem that any effect of right-hand training would be only to further consolidate language functions in the left hemisphere. This would make these forced lefthanders less bilateral, not more bilateral. The only way it could make them more bilateral would be if left-handers were (as used to be thought) right-hemisphere dominant for language. Then, assuming again that forcing the left-hander to write with the right hand can create lefthemisphere language centers, the result would be supplementary left-hemisphere centers in addition to the primary right-hemisphere centers. If this is the scenario Ardila et al. had in mind, perhaps they were confusing Levy's version of the "cognitive crowding" model with Lansdell's (1969). Levy was postulating the outcome of a specific cerebral-organizational pattern in 'natural' left-handers. Lansdell was postulating the outcome of a pathological process in brain-injured patients, namely, that the compromising of spatial processing in the right hemisphere was the result of a pathologically induced shift in language functions to the right hemisphere following early damage to left-hemisphere language zones in genotypic right-handers. A related consequence of this same early injury would be pathological left-handedness (see Harris & Carlson, 1988; Satz, Orsini, Saslow, & Henry, 1985). This presumably is not the case for the forced left-handers studied by Ardila et al. (for further discussion, see Harris, 1990a).

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compromised their spatial ability, while making it easier for them to accommodate to right handwriting. Supporting this extrapolation from Gloning et al.'s clinical data is evidence, from normal left-handers, of lateral dissociation for different verbal tasks (McKeever, Van Deventer, & Suberi, 1973). We therefore would want to compare Ardila et al.3 forced left-handers' spatial scores (as well as their performance on tests of lateralization) not only with those of the control left-handers (those who report never having experienced pressure to write with the right hand) but also with the scores of this second subgroup of forced left-handers. What Are the Further Effects of Hand Training on Manual Preference and Skill?

Aside from questions about the effects of forcible shifting of hand use on the left-hander's spatial ability, speech fluency, and emotional development, we need better information about the effects on manual preference and skill directly. For instance, do left-handers, trained to write with the right hand, write as well as right-handers do, or as left-handers do with their left hand? Anecdotal reports suggest that some do and some do not (see Clark, 1957, pp. 187-189), but, in light of the previous discussion, it seems likely that any such comparison will be confounded by already-existingneuropsychologicaldifferences be tween subgroups of left-handers. Another question is, does right hand instruction for writing and eating (and other specifically targeted actions) have any broader effects on hand use? The evidence on this point looks mixed. Some of it suggests that the effects are very circumscribed. For example, recall that Paget's (1887) left-handed patient wrote with his right hand but used his left for all other actions. Babe Ruth likewise wrote with his right hand, but, again on the evidence of photographs (Ritter & Rucker, 1988), ate, drank, played pool, boxed, and "bone-rubbed his bat with his left, and, of course, threw and batted left-handed. This "single-case'' evidence is supported by recent questionnaire data. In their study of Chinese elementary school and college students living in Taiwan, Teng et al. (1976) found that those left-handers who switched to the right hand for eating and writing continued to use the left hand for such tasks as striking a match, hammering a nail, brushing teeth, cutting with scissors, and throwing a ball. A similarly selective effect is indicated in the study by Tambs et al. (1987) of three generations of Norwegians: In the population born before World War 11, writing-hand was weakly related to general handedness (as measured by a

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twelve-item questionnaire, including hand use for writing), suggesting that hand use for writing was the measure chiefly affected. By contrast, in the younger population the writing-hand preference explained about 75 percent of the total variance in general handedness. Against this evidence, other reports suggest that the effects are broader and more extensive. For example, although Tan’s (1983) comparison of two generations of Australians showed that the largest differences were for actions involving writing and drawing (the actions presumably targeted specifically for right hand training), it also revealed lower percentages of reported left hand use for each of twelve other items on the handedness questionnaire (Crovitz & Zener, 1%2), including hammering (9.7 percent for the older generation vs. 14.6 percent for the younger generation, using a toothbrush (6.7 percent vs. 12.9 percent), and holding a glass (5.2 percent vs. 10.4 percent). (Komai and Fukuoka’s [19341 study of Japanese schoolchildren, cited earlier, provides further supporting evidence.) Tan (1983) therefore concluded that during the older subjects’ formative years, left hand preference for a “wide variety” of manual behaviours had been reduced by cultural pressures to conform to the dextral norm (p. 872). For another example, Fleminger et al. (1977) found age-cohort differences in the same direction for other hand-use items (based on responses to a questionnaire; Annett, 1970a) that they found in hand-use for writing. It seems hard to reconcile the two kinds of reports. Unless we are willing to suppose that, say, Tan’s (1983) older subjects experienced a wider range of active (or passive) restraints on left hand use than Teng et al.3 (1976) subjects, it is difficult to account for the lesser selectivity shown in Tan’s subjects’ responses. Presumably, the influence will be more likely the more similar the untrained actions are to the trained actions, but to find out will require formal analysis of the similarity between trained and untrained actions (see Beukelaar & Kroonenberg, 1983), perhaps along the dimension of skill required (see Steenhuis & Bryden, 1989), as well as better information about the actual timing and nature of training provided. It may be important whether the training is explicit (didactic instruction on left hand use for writing, eating, and other socially important skills) or implicit (through imitation of others or through accommodation to dextrally-oriented objects and customs). However this question about the range of the effect of hand training is resolved, other evidence strongly suggests that the fundamental difference in hand skill itself cannot be laid to practice per se. Annett (1970) tested righthanded children ranging in age from three and a half to fifteen years on a pegmoving test and found that although overall speed increased across the age

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groups, absolute differences favouring the right hand actually were smaller in the older than in the younger children. Annett suggested that this was a result of the negative exponential shape of the curves and, therefore, concluded that the distributions of hand preference and skill on this unimanual novel task are fairly constant across this age range and that the greater skill of the right hand does not come from practice (Annett, 1985, pp. 338 ff.). Tupper (1983) drew a similar conclusion after assessing peg-board performance in a group of three- to eightyear-olds. Like Annett, he found greater speed with age but no change in the margin of dominant-hand advantage. At all ages, the non-dominant hand functioned at about 90 percent of dominant-hand performance. Experimental studies of the effects of sustained practice over brief time periods also suggest that performance asymmetries are relatively resistant to practice effects (see Annett, Hudson, & Turner, 1974; Peters, 1981).

Do Secular Trends Represent Cultural Change or Developmental Change? So far, we have attributed the secular trends in the prevalence of lefthandedness to changes in cultural conditions, and where hand use for writing is concerned, to educational changes in particular. Could the secular trends also reflect, in part, a genuine age change in laterality, such that left-handedness is less frequent among older persons because dextrality becomes stronger over the lifespan (thus diminishing left hand preference itself)? Fleminger et al. (1977) raised this possibility in connection with their own finding, mentioned earlier, that age-cohort differences were in the same direction for other hand-use items as for hand use for writing, so that over the whole age span, the percentage of subjects who were fully dextral (for all twelve questionnaire items) rose by 23 percent (see also Maehara et al. [1988] and McGee and Cozad [1980]). Porac, Coren, and Duncan (1980) have made a similar proposal from an analysis of 34 normative studies of hand preference conducted between 1913 and the mid-1970’s. The studies chosen were restricted to those with Western, Caucasian adult samples given a variety of questionnaire-based measures of hand preference like those used in previous studies (Coren & Porac, 1978; Raczkowski, Kalat, 8i Nebes, 1974). Porac et al. then produced a scatterplot of the reported percentage of right-handedness as a function of the date of publication of the study. The correlation between the two variables was negative (r = -.28), as we would expect from the secular-trend studies cited earlier, except that it was not statistically significant. Furthermore, the slope of the observed decrease in the reported prevalence of dextrality was .05 percent, which was only

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25 percent of the rate of change found in a secular-trend analysis of their own. Porac et al. therefore concluded that changes in overt social pressure could account for only a small portion of the observed change in handedness. In support of this conclusion, they drew on their own finding that age increases in dextrality were accompanied by similar changes in foot, ear, and eye preference. They thought it unlikely that overt social pressures had been applied to these other measures of laterality (certainly not to the same degree as for hand preference). Porac et al. therefore proposed two alternative “developmental”hypotheses to explain the secular trend data: One is that covert environmental pressure toward right-handedness (of the sort brought about through right-handed objects and customs like those mentioned in the current paper) gradually strengthens the development of right-handedness throughout life (and weakens the expression of left-handedness). Noting that similar covert biases for use of the right foot (e.g., foot control pedals in automobiles) and right eye (e.g., cameras and other sighting devices) could explain the further finding of an age trend toward right-footedness and right-eyedness, these authors suggest that the change, rather than representing a leftward shift with age, represents a loss of dextrality and a regression toward an unbiased population (equal numbers of right- and left-lateralized individuals). The other hypothesis, also suggested by Fleminger et al. (1977), is that the trend reflects genuine developmental maturational processes. Here, as Porac et al. (1980) note, although most studies indicate that neural development and myelination are complete by the end of the second decade (Flechsig, 1920), there are reports that myelination in some cortical regions continues through the fourth or even fifth decade (Kaes, 1907; Yakovlov & Lecours, 1967). As Porac et al. (1980) acknowledge, the problem with this second analysis is that it is inconsistent with evidence from dichotic listening, dual-task paradigm, and other studies, which fail to show any age changes in either the direction or strength of the effect (for a review, see Hiscock, 1988). The best evidence thus indicates that cerebral lateralization is age-invariant in contrast to the hypothesis that it continues to develop throughout the life span (cf. Brown & Jaffe, 1975; Lenneberg, 1967). This means that if we accept a developmental maturational analysis of the secular trend evidence, then we must consider the possibility that more than one physiological mechanism is responsible for manifestations of laterality, with different mechanisms contributing differently to different aspects of laterality.

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The data now available may not allow a definitivejudgment on these different developmental hypotheses. I agree with Beukelaar and Kroonenberg (1986), however, that the increasing prevalence of left-handers writing with the left hand is better explained by educational changes than by a genuine age change in laterality (increasing dextrality) caused by the accumulated exposure to a righthanded world. If the latter hypothesis were true, then, as Beukelaar and Kroonenberg point out, their own results on Dutch subjects indicate that this environmental cause takes 13 years to become effective, that is, only at the age of 19 years would the influence become measurable given that writing has been learned at the age of six years. As they note, their questionnaire was administered in 1979, and only for left-handers 20 years old and older (born in 1955 and before) were there significant departures from 100 percent left-handed writing. Likewise, after 40 years, the environment "would have succeeded in converting all left-handers to writing with their left hand. Such an overwhelming and perfect penetration of the environment seems hardly likely" (Beukelaar & Kroonenberg, 1986, p. 302). Tambs et al. (1987, p. 167) raise a similar objection to the "genuine age change" explanation in noting that consolidation of writing hand usually occurs during the first years of schooling, and that, on the contrary, one would expect an increase in left-handedness with age after the restrictive influence of education has subsided. Finally, Beukelaar and Kroonenberg (1986) argued that in the study by Porac et al. (1980) cited earlier, the non-significant correlation found between date of study and percentage of left-handers in the 34 twentieth-century studies would constitute evidence against the educational-effect hypothesis only if in all studies writing hand had been used as the sole measure of handedness. In fact, as already mentioned, a variety of measures were used. Finally, recall the evidence mentioned earlier that, at least on novel tasks, such as peg-moving, hand differences do not increase over age (Annett, 1970; Tupper, 1983) or over experimental periods of prolonged practice (Annett et al., 1974; Peters, 1981). Where hand-use for writing is concerned, the maturational hypotheses would seem to fail on similar statistical grounds. Where they still might be valid is for explaining increasing dextralization within a right-handed population (cf. Maehara et al., 1988), but even here another possibility occurs. The age effect, instead of representing increasing left-hemisphere specialization across the life span, could mean that left-hemisphere functions are better preserved than righthemisphere functions in the older-adult period. This is what Weller and Latimer-Sayer (1985) concluded from a study of performance on a standardized pegboard task by right-handed subjects ranging in age from 16 to 87 years.

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Although there was no sign of an age effect as indexed by scores on a handedness quzstionnaire, on the pegboard task right hand performance was preserved relative to left hand performance with increasing age. (See also Meudell & Greenhalgh, 1987.)

When and How Are Cultural Beliefs Translated into Hand-Training? As we have seen, right hand training has been nourished over the centuries by a variety of attitudes and beliefs about the left hand and left-handedness. Anthropological studies have made an important contribution to our understanding of the cultural origins and nature of these beliefs and practices (e.g., Needham, 1973), but this information has yet to be incorporated into a developmental psychological analysis. The result is that in societies with traditional views about handedness, or in societies in transition from traditional to liberal views, we still barely understand when and how cultural beliefs get translated into actual hand-training practice. For example, when do children in such societies incorporate these attitudes into their own belief systems? Does the attitudinal development precede, accompany, or follow the behavioural change? What happens to the self-concept (or self-esteem) of the left-hander who accommodates to the cultural mode? What about the left-hander who cannot or will not?

What If Right-Handers Lived in a Left-Handed World? Because ours is a "right-handed world," our strategy in trying to understand cultural influences on handedness has focused on the left-hander. Strictly speaking, this strategy tells us only about whether and how the left-hander adjusts, and not about the process in general. Until we can study the righthander in a "left-handed world," we cannot extrapolate our results to all individuals. But even if such a study were possible (for example, a prospective longitudinal study of children growing up in a 'left-handed micro-environment' complete with left-handed customs and tools, and myths of sinistral honour and dextral degeneracy!), we should not expect the results to mirror those found for left-handers. There are several reasons, but all reduce to the same principle: left-handers as a group are not the mirror-image of right-handers. As already noted, left-handers instead comprise several, possibly distinct sub-groups with different neuropsychological characteristics having perhaps crucial implications for the process of adjustment. Whether and how the left-hander adapts to a

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right-handed world therefore may say more about certain left-handers than about the adjustment process per se. To mention but one possibility, which sub-group of left-handers is the better model for predicting the right-hander's adjustment: those who learn easily to write with the right hand and continue to do so throughout their lives, or those who cannot learn and who, following a period of enforced (and laboured) right handwriting, revert to use of the left hand? More generally, to the extent that left-handers as a group show a greater measure of bilateral organization for manual praxis than right-handers (Kimura, 1983b), perhaps they will be constitutionally more capable of accommodating to a dextral environment than right-handers would be if the situation were reversed. Judging from accident data (Coren, 1989), the left-hander's adjustment is not entirely without cost, but perhaps for the right-hander, the price would be even higher. Left-handers and right-handers also may not be equally susceptible to environmental influences on hand use. According to Annett's "right shift" genetic model of handedness (1985), approximately 85-90 percent of the population inherit a "right-shift"factor (RS t ) that predisposes left-hemisphere specialization for speech and right hand dominance. The rest of the population lack this factor (RS-), so that chance determines the side of cortical control for speech and handedness. According to the theory, left-handers are more likely than righthanders to be drawn from the RS- group, in which case, as Tanner (1978) might have said, the developmental pathway for handedness in the RS- individual will be less "canalized (in the sense of being a less clearly and less directly specified outcome of the developmental process) than in the RS t individual. This would make left-handers inherently more susceptible than right-handers to environmental influences on hand preference. Finally, consider the reported positive associations between left-handedness and an odd assortment of "symptoms,"including certain psychiatric disorders (e.g., Chayette & Smith, 1981; Lishman & McMeekan, 1976; but see McCreadie, Crorie, Barron, & Winslow, 1982); the use of licit stimulant and depressive drugs such as nicotine and alcohol (e.g., Harburg, 1981; Lee-Feldstein & Harburg, 1982); responsiveness to other neuroactive drugs (Irwin, 1985; Irwin & Fink, 1981); anxiety (Hicks & Pelligrini, 1978); "emotionality" (Orme, 1970); neuroticism (Mascie-Taylor, 1981); and difficulty in sleeping (Coren & Searleman, 1987). All these have been reported in "liberal" societies and, in several instances, involving young people who, presumably, have not been targets of anti-sinistral attitudes or practices. If severe, punitive methods of hand-retraining can precipitate certain forms of psychological maladjustment, as the (admittedly sparse) evidence suggests, the risk perhaps will be greater for the left-hander than for the right-hander if, as

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these data suggest, the left-hander is already at greater risk for psychological dysfunction. In conclusion, we have seen that the story of the cultural influences on handedness, both historical and contemporary, has many facets. This review has only laid out the main plot, steered us through some of the more obvious twists and turns, and pointed out a few others. The whole story remains to be told. It is a story well worth pursuing, for, through the unfolding of the roles of culture, religion, sex, personality, cognition, and neuroanatomy, among other variables, it promises to greatly enrich our understanding of the development and scope of human laterality.

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LEFT-HANDEDNESS Behavioral Implications and Anomalies, S. Coren (Editor) 0 Elsevier Science Publishers B.V. (North-Holland), 1990

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Chapter 9

Switching Hands: A Place for Left Hand Use in a Right Hand World Clare Porac University of Victoria Laura Rees Carleton University and Terri Buller University of British Columbia

"Right-handedness is woven in the brain; to change the pattern you must unravel its tissues. My own conviction is that, as regards righthandedness, our best policy is to let well alone and to stick to dexterity and the bend sinister." (Crichton-Browne, 1907, p. 652) This quote expresses the conventional wisdom that humans are an inherently right-handed species living in a right-handed world, a notion that still carries impact today. Over the last 40 years, interest in brain-behaviour relationships has promoted a search for "pure" handedness types; a non-overlapping dichotomy of left- versus right-handers is the goal. These groups then demarcate the independent variable in experimental studies of hemispheric function. The intense research interest in right and left hemisphere lateralization continues to

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foster the tendency to dichotomize human populations into a dominant righthanded majority and a small, but consistently present, left-handed minority. Bryden (1989) argues that investigations of individual differences in lateral preference and hemispheric lateralization have diminished because experimental results from studies of right-handers, especially consistently right-handed males, are less ambiguous than those derived from studies of other groups (namely, females and left and mixed-handedness types). He asserts that our current knowledge of brain lateralization is biased inappropriately by this emphasis. We know something about the lateralized cerebral functions of strongly right-handed males, who comprise about 62% of the population (Lansky, Feinstein, & Peterson, 1988), but know little about that of other groups. Other researchers express similar opinions. For example, Lansky et al. (1988) conclude: Perhaps right-handedness is no more ‘natural’ a state for our species than is white maleness, or white male college ‘sophomoreness.’ We suggest that with careful analysis some important parallels might be drawn among sexism, racism, and what might be labelled ‘handism’...‘handism’ also implies that there are systematic patterns of social arrangements and behaviours which reinforce the belief. Such behaviours would include a bias that may have affected scientific work and theorizing about handedness. (p. 474) Such statements acknowledge the wide variation in manifest hand preference behaviours, both within and between individuals, and criticize the reliance on physiological explanations of human handedness, which make sacrosanct the right hand, left hemisphere connection as the anatomically preferred form of human lateralization and preference. Deviations from a neural reductionist approach to human handedness exist in the research literature. Hand preference is treated as a complex of behaviours where, for example, specific deviations from right hand use may be studied in individuals categorized as right-handers (Annett, 1985; Beukelaar & Kroonenberg, 1983; Bryden, 1982; Chapman & Chapman, 1987; Healy, Liederman, & Geschwind, 1986; Koch, 1933; Oldfield, 1971; Plato, Fox, & Garruto, 1984; Salmaso & Longoni, 1985). Studies of handedness item classification concordance are often prompted by the search for performance and questionnaire items that discriminate, in a non-overlapping fashion, rightfrom left-handed individuals. Frequently, their goal is to refine handedness

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classification schemes so that the separation of groups into right- versus lefthanders has increased behaviourial validity. Thus, the philosophical view of handedness remains one that assumes a natural dichotomy, which is clouded by the persistent presence of mixtures of handedness types. An example of this approach is the debate over using one item, usually the writing hand, as a valid overall predictor of typical hand use. Some researchers argue for its use either because writing hand is the best predictor of a total laterality score, or because it has the highest loading on a hand preference factor in statistical approaches to categorization (Annett, 1985; Bryden, 1977; McManus, 1985; Roszkowski, Sacks, & Snelbecker, 1982; White & Ashton, 1976). Others argue that it is a poor predictor of overall handedness, especially for left-handers, since it is often the hand use behaviour most subject to environmental alteration (Beukelaar & Kroonenberg, 1983,1986; Payne, 1987; Salmaso & Longoni, 1985). Studies of handedness item classification reveal the ambihanded nature of everyday hand use in many individuals. For example, Beukelaar and Kroonenberg (1983) state: That the environment is not a negligible factor is also borne out by the item bottle-fop. The anti-clockwise movement necessary to unscrew a bottle-top makes it the only item in our questionnaire favouring the left hand and, indeed, more right-handers prefer to use their left hand for this item than for any other one...if it is assumed that handedness is a natural dichotomy, the deviations from the non-preferred hand do not occur randomly. (p. 42) An ambilateral view of hand preference is not new (see Downey, 1927), but opinions, such as the one above, foster a re-emergence of the notions that, first, unimanual activities can be environmentally as well as neurologically driven and, second, that ambihanded activities commonly exist, even in individuals who are strongly lateralized for other hand preference behaviours. Thus, studies of handedness classification schemes have generated paradoxical conclusions and two groups of researchers. One group pursues the handedness dichotomy and the best method to achieve it, and the other has abandoned dichotomies in favour of classifying the diverse nature of hand use behaviours. The notion that the environment, training, and life-style factors influence handedness is a recurrent theme in the research literature. For over 100 years,

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Table 1: Incidence of reported hand change attempts

Study

Year

Location

Age Group

N

Incidence Rate

N

Percent

2518

80

3.2

109

6

5.5

L e i b e r & Axelrod

1981

USA

Adults

Shettel-Neuber & O'Reilly

1983

USA

M=47.1 yrs.

Salmaso & Longoni

1985

Italy

14-62 y r s .

1694

117

6.9

Porac e t a1 .

1986

Canada

M=19 y r s .

650

73

11.2

L indesay

1987

England

M=31.1 y r s .

94

6

6.4

Payne

1987

Nigeria

Adults

201

9'

4.5

Lansky e t a1 .

1988

USA

18-80

yrs.

2083

15

0.7

Porac e t a l .

Study 1

Canada

M=25.3 y r s .

518

125

24.1

Porac e t a l .

Study 2

Canada

M=45.7 y r s .

703

126

17.9

'Author's estimate

researchers have acknowledged societal pressure to switch, at least, the behaviour of left-handed writing to the right side (see Corballis, 1983; Harris, 1980; Porac & Coren, 1981 for reviews). Recent studies, summarized in Table 1, verify that overt pressures to change handedness patterns exist even in samples studied over the past ten years. Researchers have also reported for many years that the incidence of righthandedness varies substantially in different cultural and racial groups (Porac & Coren, 1981). Intense cultural pressure to foster right-handed writing and/or religious biases against left hand use are two reasons cited for this diversity of manifest right-handedness (Payne, 1981,1987;Teng, Lee, Yang, & Chang, 1976). Dawson (1972,1977) argues that the fluctuating rates reflect the degree to which a given society tolerates the use of the left hand. In a "right-handed world," lefthanders experience varying degrees of overt and covert pressure to shift towards right hand use. Societies displaying a higher incidence of left-handedness are those that apply few overt pressures to conform to the majority pattern. Table 2 presents a summary of these cross-cultural trends from a set of studies published over the last 30 years. It is based on similar tables prepared

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263

by Porac and Coren (1981) and Salmaso and Longoni (1985), with the addition of recent studies not included in their reviews. Table 2 is arranged chronologically, within cultural and racial sub-headings, and a consistent criterion to determine left-handedness was applied to each report (ix. on a laterality scale ranging from -100, all left, to t 100, all right, left-handers had scores
264

Porac et al.

Table 2: Incidence of reported left-handedness as a function of country and racial group Caucasian: United States & Canada Study

Year

Location

Method

N

Age

~~

Oawson B r i g g s & Nebes Hardyck e t a l . Thompson & Marsh Peterson L e i b e r & Axelrod Porac & Coren

1972 1975 1975 1976 1979 1981 1981

Bryden Sanders e t a l . I n g l i s & Lawson Plato e t a l . Schachter e t a l . Searleman & Fugagl i Chapman & Chapman Lansky e t a l . Sc hwar t z

1982 1982 1984 1984 1987 1987 1987 1988 1988

USA

USA USA USA USA USA USA & Canada Canada USA USA USA USA USA USA USA Canada

%Left ~~

na 1599 3820 722 1045 2257 5147 4846 879 1880 705 1117 277 5825 1741 290

~

Questions Adult 5.6 Questions Adult 9.1 Performance C h i l d 10.2 Questions Adult 4.0 Questions Adult 9.4 Questions Adult 8.6 Questions L i f e span 11.8 Questions Questions Quest. & P e r f . Performance Questions Questions Questions Questions Performance

Adult Adult Adult Adult Adult Adult Adult Adult Child

10.2 4.9 9.0 7.1 12.0 12.6 6.6 6.7 19.5

Age

%Left

Child Adult Adult Adult Adult Child Adult Adult Child Child Child Adult Child Adult

8.5 6.1 3.8 4.3 19.8 7.0 3.2 8.8 13.2 8.0 11.5 15.5 11.8 7.9

Age

%Left

Adult Child Adult Adult Child Adult Adult

5.4 10.3 6.2 15.4 1.0 4.1 6.2

Caucasian: U n i t e d Kingdom Study

Year

Location

Clark Sutton Annett Annett Oldf i e l d Dawson Newcombe & R a t c l i f f Fleminger e t a l . Brackenridge Whittingham e t a l . McManus e t a l . Harvey

1957 1963 1967 1970 1971 1972 1973 1977 1981 1987 1988 1988

Ellis et al.

1988

Scotland 330 Australia 772 England 1003 England 2322 Scotland 1128 Scotland 6000 England 823 England 800 Australia na England 11032 England 314 England 400 1838 England 6097

N

Met hod Performance Performance Questions Questions Questions Questions Questions Questions Questions Quest. & P e r f . Performance Questions Questions Questions

Caucasian: Western Europe Location

N

Met hod

Study

Year

Beckman e t a1 . Pelecanos Salmaso & Longoni Beukelaar & Kroonenberg

1962 Sweden 981 1969 Greece 2254 1985 I t a l y 1694 1986 N e t h e r l ' d s 1996

Questions Performance Questions Questions

Levander & S c h a l l i n g Cosi e t a l .

1988 Sweden 1988 I t a l y

Questions Questions

921 178

265

Switching Hands Table 2: Continued Caucasian: Other Study

Year

Location

N

Method

Age

%Left

Silverberg

1979

Israel

1171

Questions

Child

4.0

Age

%Left

Negro: Various Study

Year

Locat i o n

N

Verhaegen & Ntumba Dawson

1964 Congo 1047 95 1972 Aust r a 1i a Congo 83 7 S i e r r a Leone 204 Hardyck e t a l . 1975 USA 3178 1976 USA Thompson & Marsh 575 Payne 1981 N i g e r i a 56779 Saunders & Campbell 1985 USA & 379 Caribbean 389 Payne 1987 N i g e r i a Nachshon & Denno 1987 USA 987 Lansky e t a l . 1988 USA 342

Method Performance Performance Performance Performance Perf ormance Questions Questions Questions

Chi I d Adu 1t Adu 1t Adu 1t Chi I d Adult Chi I d Adu 1t

2.1 10.5 0.6 3.4 9.5 4.9 4.5 11.9

Quest ions Performance Quest ions

Adult Chi I d Adu 1t

10.0 10.9 9.0

O r i e n t a l , I n d i a n and o t h e r s : Various l o c a t i o n s Year

Sutton Dawson

1963 Polynesia 257 1972 Alaska-Eskimo 53 Hong Kong (Boat) 64 Hong Kong (Hakka) 68 1973 Solomon I s l a n d s 1438 1975 USA ( O r i e n t a l ) 538 USA (Mexican) 148 1976 Japan 1199 1979 Taiwan 4143 1983 Japan 4282 1984 Japan 725 1984 B r a z i l 689 1986 Canada ( K w a k i u t l ) 120 60

Rhodes & Damon Hardyck e t a l . Hatta e t a l . Teng e t a l . Shimizu & Endo Rymar e t a1 . Brito et al. Marr i o n

Location

N

Study

Met hod

Age

%Left

Performance A d u l t 7.4 Performance A d u l t 11.3 Performance Adu 1t 9.4 Performance Adu 1t 1.5 Performance Adu 1t 2.8 Performance C h i l d 6.5 Performance C h i l d 8.8 Questions Adult 3.1 Questions L i f e span 5.0 Questions Child 3.2 Performance Chi I d 3.7 Questions Adult 3.6 17.5 Quest. & P e r f . C h i l d Quest. & P e r f . A d u l t 15.0

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Porac et al.

these investigations that switching hand use leads either to an increased use of both hands or, at least, to a tolerance for the use of the contralateral limb.

Switching Hands: Who, How, When and Why Study One General Description. We collected these data in 1986 and 1987 at the University of Victoria in British Columbia, Canada, with 518 university students, staff and retired staff participating as volunteer subjects. Each participant completed two questionnaires in the context of taking part in a number of research projects. The first inventory contained sixteen items that assessed the four types of lateral preference, hand, foot, eye and ear. The lateral preference inventory was similar to that described by Porac and Coren (1981, p. 34). Each preference category was measured with four items. Respondents answered either "right," "left"or "both," and their score on each preference dimension was the sum of the four items. "Left" responses were scored 1, "both," 2, and "right," 3, giving a graded score for each preference type, ranging from 4 (all left) to 12 (all right). These items have a demonstrated 90% concordance with behaviourial testing of lateral preference (Coren & Porac, 1978; Coren, Porac, & Duncan, 1979). The second questionnaire was devised by Porac et al. (1986), and it is fully reprinted in that paper. It contains eight items to assess the nature of hand change attempts. Individuals described, either by multiple choice or freeresponse, the persons responsible for changing their handedness, the timing, methods used, and the success of the attempt. Respondents chose one of six descriptions of their change experience. They were: 1.) A switch from right hand use to left hand use; 2.) A switch from left hand use to right hand use; 3.) A switch from complete left hand use to ambidexterity (the use of both hands); 4.) A switch from complete right hand use to ambidexterity (the use of both hands); 5.) A switch from ambidexterity to the complete use of the right hand; 6.) A switch from ambidexterity to the complete use of the left hand. Choices of items 1,4, or 6 were scored as Lefi Shifts, while choices of items 2, 3, or 5 were called Right Shifrs. We divided the sample of 518 into a No Shijl group (N=393), a Riglit Shift group (N=31) and a Left Shift group (N=94). These group N's are maximum values and they vary for certain comparisons where individual respondents did

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267

not answer all questions. When we dichotomized hand preference groups, individuals scoring from four to eight on the handedness items were called lefthanders and individuals scoring above eight were classified as right-handers. We analyzed the data using chi-square and analysis of variance procedures; the chisquare analyses followed the recommendations of Glass and Hopkins (1984) for cases of small expected values. Comparisons of the No Shift and Shift Groups. Table 3 compares the three groups on age, sex and handedness characteristics. First, the groups differed significantly in age, F(2,489) =5.82, p < .01. Internal comparisons with the HSD test indicated that the mean age of the Right Shift group was older than that of the Left Shift, pc.05; neither differed significantly from the No Shijt group. Second, there was a trend toward a higher percentage of males in the two Shift groups, but this difference was only significant when the Left Shift group was compared to the No Shift, d (1)=5.75, p < .05. The hand preference composition of the No Shift and Left Shift groups did not differ significantly; however, the Right Shift group contained significantly more left-handers than the other two groups, 2 (1)=67.79, p<.Ol (No Shift versus Right Shift) and d (1) = 52.76, p < .01 (Left Shift versus Right Shift). Ten percent of the No Shift and 3.6% of the Left Shift group scored as left-handed; however, 65.5% of the Right Shift group scored in the left hand range. Despite attempts to shift in the rightward direction, the Right Shift individuals remained left-handed. The same trend appeared in the Left Shift group; it remained predominantly right-handed. Analyses of the graded handedness scores (a continuum from 4 to 12) repeated this pattern. The three mean hand preference scores were No Shift, 11.15, Right Shift, 7.10 and Left Shift, 11.57, and they differed significantly from each other, F(2,440)=63.34, p<.O1. The HSD test indicated further that the Right Shift group differed significantly from the other two groups, p< .05, but they, in turn, did not differ from each other. Thus, the Right Shift group remained left-handed (with an average score below eight), while the other two groups showed the more typical strong right-handed pattern. We obtained information on familial handedness for a subset of the total sample (N=229). Individuals in the Right Shifi group reported a significantly higher percentage of left-handed parents than individuals in the other two groups, d = 16.27, p < .01. The No Shift group reported an incidence of parental left-handedness of 8.8%. It was 3.3% in the LeJ Shift group but 36.8% in the Right Shift. This result is similar to that of Falek (1959), who also found an

268

Porac et al.

Table 3: Characteristics of shift and no shift groups (Study One)'

No S h i f t

Maximum N

Shift

Total

Right

Left

393

31

94

518

132 261

15 16

44 50

191 327

316 35

10 19

81 3

407 57

25.6

28.2

22.8

25.3

Sex'

Male Female Hand Preference' Right Left Age4 Mean (Years)

'Table e n t r i e s a r e t h e N i n each group. N i s s m a l l e r f o r t h e handedness comparison, s i n c e not a1 1 respondents answered these q u e s t i o n s . 'Left S h i f t group has s i g n i f i c a n t l y more males than t h e No S h i f t group. 'Right Shift group has s i g n i f i c a n t l y more l e f t - h a n d e r s than t h e o t h e r two roups. $ i g h t S h i f t group i s s i g n i f i c a n t l y o l d e r than t h e L e f t S h i f t group.

over-representation of left-handed parents among a group of offspring reporting pressures to change to the right side. There were no significant differences related to sibling handedness patterns.

Comparisons of Shift Attempts and Success Rates. Table 4 shows the response patterns of the Riglif and Left Shifr groups when queried about the characteristics of the hand change attempt. The same response categories were used by Porac el al. (1986). Three of the four characteristics showed significant differences in response proportions, 2 (3) = 24.97, p< .O1 for the "who changed" categories, 2 (4)=15.83, pc.01 for the "method used" comparisons and 2 (2) = 36.74, p < .O1 for the "when changed" comparisons. Only the "behaviour changed" categories showed similar response patterns in the two shift groups. The pattern of handedness change for right shifts indicated that a parent or

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Table 4: Comparisons of agents, behaviours, methods, and timing of right and left shifts (Study One)

Right S h i f t

Left Shift

Total

N

N

N

Who changed' Parent Teacher Self Other Tota 1

4 12 6 6 28

6 9 65 11 91

10 21 71 17 119

Behaviour changed Writing Eating Sports Other Tota 1

12 3 5 6 26

37 8 16 24 85

49 11 21 30 111

Method used' Switch Punish Verbal Persuasion Personal experimentation Other Total

11 4 6 3 3 27

21 2 7 33 15 78

32 6 13 36 18 105

When changed' Before grade 3 A f t e r grade 3 Throughout grade school Tota 1

16 9 3 28

6 72 13 91

22 81 16 119

'xz p r o b a b i l i t y c.01

teacher (57.2% of the respondents) initiated the attempt before the early grade school years (57.1%), using the method of switching an implement from one hand to the other (40.7%). Left shifts showed a different set of characteristics. Most individuals initiated the attempts themselves (71.4%) out of a spirit of personal experimentation (42.3%). Frequent responses from this group were, "I just wanted to see if I could do it," "I was interested to see if I could use my left hand" or "I was worried that I was too dependent on writing with my right hand, so I wanted to train myself to use my left hand." As a result, the change attempts occurred later

270

Porac et al.

in life, usually during the elementary or early high school years (93.4%), when the curiosity about left-handed abilities seemed strongest among young righthanders. The Left Sh@ individuals represent contemporary proponents of a popular movement of the late 19th and early 20th centuries in Great Britain and the United States. The Ambidextral Culture movement advocated the benefits of using both hands, especially left hand use among natural right-handers. The illustrious members of the Ambidextral Culture Society (including Lord BadenPowell, founder of the Boy Scouts) claimed many advantages to ambihandedness, in various professions, such as medicine, and in increased proficiency in sport and military training (see Harris, 1980 for a review). The responses of the Left Shfl group remind us that, even today, some individuals perceive advantages to the use of both hands. Most left shifters indicated that they sought ambihandedness rather than a complete switch to the left hand. We assessed the success of the hand preference change in two ways. First, we categorized individuals as successful or unsuccessful by their self-rating of success using a four-point scale ("very successful,'' "moderately successful," "moderately unsuccessful," "very unsuccessful"). We classified responses in the first two categories as successful, while those in the other two categories were unsuccessful. Second, we compared an individual's current handedness score to his or her shift category. In other words, we looked at the incidence of lefthzndedness among left shifters and right-handedness among right shifters. Comparisons of males and females showed no differences in success rates; however, there were success differences related to the direction of the shift. Self-rated success in the Riglit Sliiji group was 33.3%; the success rate based on current handedness classification was 34.5%. The self-rated success in the Left Sliift group was 34.2%, but the current handedness classification revealed only a 3.6% success rate. Left shifters had a comparatively lower success rate, averaged across the two assessment methods (18.9% versus 33.9%). In addition, they showed a significant discrepancy between perceived and actual success not shown by right shifters. A comparison of the frequency of success as measured by the two methods in the Right versus Left Shift groups revealed a significant effect, ?(1) = 10.01, p< .05. The two shift attempts are different in kind as well as in direction and seem to be assessed with different criteria of success by the participants. Right-handers attempting to use the left hand may rate this shift attempt as successful when a transitory or unstable use of the left hand is achieved. However, left-handers may only rate a shift as successful if the change in right hand use becomes a permanent part of everyday behaviours.

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Since the success rates across the two methods of assessment were the same for the Right Shift group, we analyzed differences in hand preference scores as a function of self-rated success, and a small effect emerged. The scores of both the successful and the unsuccessful right-shifters fell toward the left-handed side of the handedness continuum (scores below 9) with mean scores of 8.22 and 6.41; however, the scores of the successful group were more right-sided (8.22) than those of the unsuccessful group (6.41). A t-test of this mean difference revealed a trend toward stronger right-handedness in the group that perceived itself as successfully shifted. We assessed the behaviourial implications of handedness shifts using current hand preference scores rather than self-ratings as the criterion of success. We looked at only the individuals dichotomously classified as right-handed (scores between 9 and 12) in the three groups (No Shift, Right Shft and Left Shift). These persons represent the successful shifts in the Right group and the unsuccessful shifts in the Left group, with the No Shifi right-handers acting as a control. The mean handedness scores for the three groups, No, Right, and Left were 11.77, 10.9 and 11.68, respectively. The Right Shift group showed a significantly weaker right-handed pattern than that of the other two groups, F (2,387)=9.79, p c .01. These individuals tended toward ambihandedness, with average scores displaced toward the middle of the lateral preference continuum. Thus, overt right shift attempts were related to an ambihanded pattern, with some of the measured handedness behaviours remaining with the left hand or becoming ambihanded. A similar analysis of the left-handers in each group produced no significant differences; however, the small number of left-handers in the Left Shift group ( 3 ) made valid comparisons difficult. This study was our first investigation of individuals who attempted to shift hand preference to the left side. We found that the characteristics of left shifts differed significantly from those of right shifts. Left shifts occurred during late childhood and early adolescence and were initiated by the individual involved out of personal interest. They were not as systematic in application as right shift attempts and were typically of a shorter duration. The only characteristic shared by right and left shifts was the focus on handwriting as the major behaviour to change. In addition, left-shjfters appeared to use a loose criterion to judge the success of the shift attempt, leading to a large discrepancy between the perceived success of left hand use and its behaviourial incidence in the Left Shft group. These differences support the contention that individuals who experience overt attempts to shift handedness from the left to the right side are not identical in experience nor, perhaps, in the strength of original hand preference, to those

272

Porac et al.

who shift from the right to the left side. We conducted a second study of hand change attempts to explore further the qualities of both right- and left-shifters.

Study Two General Description. The second data collection took place from 1987 lo 1989. The primary purpose of this separate study was lo examine the consequences of hand injury on everyday hand use behaviours. Therefore, all respondents were individuals who had suffered a hand injury or disability. Most were contacted through the cooperation of two physiotherapy clinics in Victoria, British Columbia, Canada, and mailed a questionnaire. We also advertised in newspapers throughout British Columbia for individuals who had suffered a hand injury. We mailed a handedness survey to interested individuals contacted in this way. We collected some data at shopping malls in Victoria, British Columbia, where individuals completed questionnaires in the presence of the researchers. In addition to questions about handedness and hand injury, we asked respondents if they had experienced any attempt to change their hand use prior to the injury or disability. If the response was "Yes,"they were asked to describe the nature of the event in their own words. In this way, we acquired information about hand change attempts that was analogous, but not identical, to that of Study One. We received responses from 703 adults residing throughout British Columbia. Thirty individuals did not answer the query about hand change, leaving a maximum total for this investigation of 673. Of these, 126 indicated that they had experienced a handedness change prior to their injury or disability, 45 to the right and 81 to the left. The remaining 547 respondents constituted the No Shift group. Once again, these are maximum values because individuals varied both in the type of information they offered about their experiences and in their response patterns to the relevant handedness questions. We coded the free responses of the shift groups to approximate the directed questions of the survey in Study One. We obtained handedness information by asking respondents to describe typical hand use before any injury or disability. The relevant question stated: "Before your injury or disability, when you had full use of both hands, how would you have described yourself when doing things that used only one hand (such as writing, eating with a fork, brushing your teeth and so forth)?" Individuals used a five-point scale to answer this question: 1, indicated "right hand always"; 2, "usually used my right hand but sometimes used my left"; 3, "no hand preference, used either right or left hand; 4, "usually used my left hand but sometimes used my right" and; 5, "left hand always." This

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Table 5: Characteristics of shift and no shift groups (Study Two)' No S h i f t Maximum N

Shift

Total

Right

Left

547

45

81

673

2 53 284

26 18

50 31

329 333

506 41

21 24

71 10

598 75

42.2

38.3

Sex' Male Female Hand Preference' Right Left Age4 Mean (Years)

47.1

45.7

'Table e n t r i e s a r e t h e N i n each group. T o t a l N i s s m a l l e r f o r t h e sex comparison, because not a1 1 respondents answered t h e q u e s t i o n . 'Left S h i f t group has s i g n i f i c a n t l y more males than t h e No S h i f t group. 'Right S h i f t group has s i g n i f i c a n t l y more l e f t - h a n d e r s than t h e o t h e r two groups. 4 L e f t S h i f t group i s s i g n i f i c a n t l y younger than t h e No S h i f t group.

response scale was modelled after that used by Healy et al. (1986). In the dichotomous classification of handedness, responses of 1and 2 were called "righthanded and responses 3 , 4 and 5 were "left-handed." Comparison of No Shift and Shift Groups. Table 5 compares the characteristics of the no shift and shift groups. The LeftShift respondents were significantly younger than those in the No Shift group, F(2, 655)=9.48, p<.O1. The two Shift groups did not differ from each other, nor did the mean age of the Right Shift group differ from that of the No Shift. There was a trend toward a higher percentage of males in the Left! Shift as compared to the No Shqi group, 2 (1) = 6.02, p < .05. Comparisons between the two Shift groups and the Right versus No Shgt groups showed no significant differences in gender composition. As in Study One, we found significant differences in the handedness composition of the three groups. Fifty-three percent of the Righf Shift group scored in the left hand range (responses of 3 to 5 on the handedne.ss scale); only 7.5% of the No Shift and 12.4% of the LeftShiff achieved this left-handed score. The Righf Shijf group differed significantly in handedness composition from the No Shift, 2 (1) = 89.38, p< .01, and from the Left Shift, 2 (1) = 24.67, p < .01, while

274

Porac et al.

the Left Shift and the No Shift groups did not differ. The Right Shift group remained predominantly left-handed, repeating the result of Study One. This pattern reemerged when we analyzed the handedness responses as a continuous measure. The three groups differed significantly from one another, F(2,670) = 58.19, p< -01, and HSD, p < .05 for all possible comparisons. The No Shift group showed the strongest right-handed behaviours with a mean score of 1.53. The Left Shift group was displaced toward a more ambihanded pattern with a mean score of 1.86, while the Right Shvt group showed the most extreme ambihanded response with a mean of 2.98. Both shift groups, in this case, were significantly weaker in the lateralization of their handedness responses than was the No Shift group. Comparisons of Shift Attempts and Success Rates. Table 6 shows the characteristics of the shift attempts reported in this sample using the categorization scheme of Study One. The totals for each comparison varied because we could not derive a complete picture of each shift attempt from every written description. The response proportions showed significant differences across all the categories describing the hand change attempts, d (3) =58.90, p< .01 for the "who changed comparison, ? (3)= 19.29, p < .01 for the "behaviour changed responses, 9 (4) = 31.95, p < .01 for the "method used" categories and d (2) = 35.91, p< .01 fot the "time of change" comparison. Change attempts described by the Right Shift respondents indicated that a teacher (63.6%) instituted a shift of the writing hand (65.8%) during primary school (87.5%) by using the method of switching implements from the left to the right hand (58.8%). This result is analogous to that found for right shifts in Study One. Descriptions of left shifts were also similar to that of Study One. They were self-generated (82.3%) out of personal interest or experimentation (36.5%). The predominant description of the timing of the change suggested that it occurred in the teenage or young adult years, leading to a high percentage of individuals in the "Other" category (46.0%). Left shifters described attempts to use the left hand for a variety of reasons often related to their work; this tendency led, again, to a high percentage of descriptions categorized as "Other" (54.4%). The following survey response from a left shifter is typical of this group.

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Table 6 Comparisons of agents, behaviours, methods, and timing of right and left shifts (Study Two)

Right S h i f t

Left Shift

Total

N

N

N

Who changed’ Parent Teacher Self Other Total

8 28 7 1 44

4 8 65 2 79

12 36 72 3 123

Behaviour changed’ Writing Eating Sports Other Tota 1

25 2 0 11 38

19 1 11 37 68

44 3 11 48 106

Method used’ Switch Punish Verbal Persuasion Personal experimentat i o n Other Tota 1

20 8 2 1 3 34

20 2 3 27 22 74

40 10 5 28 25 108

When changed’ Before grade 3 A f t e r grade 3 Other Total

28 1 3 32

10 17 23 50

38 18 26 82

~

~~~

’xz p r o b a b i l i t y c . 0 1

I’ve always tried to use both hands (not always successfully of course) because I’ve always felt you could train yourself to do things equally...As an example of using and teaching myself to use both hands is when I learned how to weld. I decided it would be advantagious (sp) to use both hands while welding (separately) because of the tough positions you find yourself in while working. When I damaged my hand I could still work as a high quality welder with my left hand.

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We had only one method available to estimate the success of the hand change in this study, and that was to compare shift category membership to current adult handedness classification. This concordance analysis revealed that right shifts in this sample were more frequently successful than left shifts, 2 (1) = 19.14, p c .01; 47.7% of the Right Shifi group was classified as right-handed as compared to a 12.4% incidence of left-handedness in the Left Shif group. There were no sex differences either in overall success rate irrespective of Shift group or when each Shift group was analyzed separately. As in Study One, we explored the behaviourial implications of shift attempts by computing mean handedness scores for right-handers in each of the three shift categories. We compared the handedness scores of the successful shifts in the Right Sh@ group to those of the unsuccessful shifts in the Left Shif group; both were compared to the right-handers in the No Shif group, who provided the baseline preference score. The mean handedness scores of the three groups were 1.34 (No Shift), 1.76 (Right Shif) and 1.62 (Left Shif). Although the mean scores were all in the right hand range (scores of 1 and 2), the two shift groups showed a weaker right-handed tendency than the No Shifi group, F (2, 595) = 17.57, p c .01. However, the two shift groups did not differ significantly from each other. The successful right shifters resembled the unsuccessful left shifters; both showed a more ambihanded response than those who reported no shift attempts. Apparently, regardless of the direction of the shift, the switch attempts coincide with the use of both hands or either hand for some unimanual activities. A similar analysis of left-handers in the three groups showed no significant findings.

Summary and Conclusions Comparisons to Previous Research First, we will compare these data to those of other independent investigators; second, we will relate the present results to those of Porac et al. (1986). This latter comparison is, perhaps, the most meaningful. The Porac et al. (1986) report is the only systematic exploration of right shift attempts and, although the sample is independent of the two reported here, the data were collected using similar procedures. Studies One and Two and those listed in Table 1 show a large difference in the reported incidence rate of hand change attempts. The mean rate across the

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nine papers in Table 1 is 8.9% of the samples studied; however, if one deletes the two studies in this chapter, the average rate drops to 5.5%. However, we encouraged individuals to describe any shift situation, while the studies in Table 1 (other than that of Leiber and Axelrod, 1981 and Porac et al., 1986) collected these data in an incidental fashion. Also, the sample in Study Two consisted of individuals who had injured a hand. Although our analyses are based on preinjury hand use, it is possible that the hand injury increased their sensitivity to their hand use patterns and history to an extent not observed in uninjured persons. However, it is important to note that reports of overt hand change attempts are found commonly in contemporary samples of adults throughout the world, as the summary in Table 1 indicates. Table 7 compares the incidence and success rates of the change attempts reported by Porac et al. (1986), Study One and Study Two. The total maximum N in the three studies is 1841; of these individuals, 17.7% reported some hand change attempt. Surprisingly, left shifts appeared to be more prevalent than right shifts, with incidence rates of 10.7% versus 7%, respectively. All three studies indicated that the majority of shift attempts were unsuccessful, when success was established by the classification agreement between the direction of the reported shift (right or left) and current handedness category. The overall success rate was only 20.4%. Right shifts showed a success rate of 40.6% but left shifts were rarely successful (7.1% agreement). Unlike the report of Porac et al. (1986), Studies One and Two did not find a sex difference favouring females in the rate of success of right shifts. Table 8 is a summary of the major characteristics of right shifts as reported in the three studies and left shifts as reported in this chapter. The principal differences between the two reside in the major agent of change and the timing of the switch event. Right shifts were most likely to occur at the prompting of a parent or teacher during the early childhood years. They were described as long-term systematic attempts to discourage the use of the left hand. Left shifts were primarily self-generated and occurred in late childhood or young adulthood. Left shifters frequently indicated that their goal was ambihandedness, rather than complete left-handedness, and the switch attempt was often casual and of short duration. These qualitative differences in the two change attempts, along with possible differences in original handedness strength, probably account for the variation in success rates.

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Porac et al.

Table 7: Comparison of reported incidence and success rates of right and left shifts'

Porac e t a l .

Study One

Study Two

Total

(1986) Maximum N

650

518

673

Incidence Rates Right S h i f t Left Shift Total

52 21 73

31 94 125

45 81 126

128 (7.0%) 196 (10.7%) 324 (17.7%)

Success Rates Right S h i f t Left Shift Total

21 l2 22

10

21 10 31

52 (40.6%) 14 (7.1%) 66 (20.4%)

3

13

1841

'Table e n t r i e s are the N ' s i n each category. 2Estirnate based on the reported success rates o f the two studies i n t h i s chapter. Porac e t a l . (1986) d i d not report the data o f t h e i r L e f t S h i f t group.

Implications of Shift Attempts Shift Attempts and Handedness Classification. Individuals in the shift groups displayed a more ambihanded response pattern than no shift controls. In Study One, right-handers in the Right Shift group showed a significantly weaker right-handed mean score than the right-handers in the other two groups (Noand Left Shift). A shift experience may change handedness category, but some activities remain on the left side or became ambihanded. Cross-cultural reports on attempts to switch the writing hand from left to right often describe similar patterns; switch attempts are incomplete and some unimanual behaviours remain on the left side (Dean, Rattan, & Hua, 1987: Teng et al., 1976). Both right and left shift individuals in Study Two responded with a significantly more ambihanded pattern than shown by the no shift group; comparisons of all respondents and right-handers alone revealed this pattern. Thus, switch attempts coincide with an increased use of either hand for a given activity or an increased use of both hands across activities. Table 9 uses the incidence data in Tables 3 and 5 to estimate misclassification probabilities when samples are dichotomously classified into

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Table 8: Most frequently reported characteristics of right and left shifts'

Porac e t a l . (1986)

Study One

Study Two

Total

128

Major c h a r a c t e r i s t i c s o f r i g h t s h i f t s Maximum N

52

31

45

Agent Parent/teacher

35

16

36

87 (68.0%)

Behav i o u r Writing

22

12

25

59 (46.1%)

Method S w i t c h i n g hands

20

11

20

51 (39.8%)

31

16

28

75 (58.6%)

Time B e f o r e grade 3

Major c h a r a c t e r i s t i c s o f l e f t s h i f t s Maximum N

__

94

81

175

Agent Self

--

65

65

130 (74.3%)

Behav iour Writing/sports

--

53

30

83 (47.4%)

Method Experiment/switch --

54

47

101 (57.7%)

72

40

112 (64.0%)

Time A f t e r grade 3

--

'Table e n t r i e s a r e t h e N i n each category

right- and left-handed groups. As mentioned previously, there is considerable current debate about handedness misclassification rates when only one

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unimanual behaviour (usually the writing hand) is used to form handedness groups (see Beukelaar & Kroonenberg, 1983 and Salmaso and Longoni, 1985). The analysis in Table 9 assumes that individuals who have experienced a hand change attempt are more likely to be ambihanded in their unimanual activities than those without this experience. Therefore, they are more likely to be misclassified with one item because it is not an accurate representation of their everyday hand use. The two studies in this chapter suggest that this is a likely pattern in shifted persons. Let us assume that individuals classified as right-handed on one item, who also report a right shift, could be left-handed for other, if not all other, unimanual activities. Since right shifts are typically incomplete, these individuals are likely to be ambihanded. Additionally, individuals classified as right-handed on one item, who report a left shift, are primarily right-handed with a tendency to use the left hand for some activities. The first two rows of Table 9 give the probabilities of the occurrence of these "mixed or ambihanded right-handers derived from their incidence in Studies One and Two (Tables 3 and 5). As shown, the probability of occurrence in a group of dichotomously-classified right-handers is approximately 18%. Thus, the likelihood of categorizing a person as a right-hander, who may be left-handed for all or a number of other activities, could approach 18%. The incidence of shift attempts in the three studies conducted by Porac and colleagues in British Columbia is higher than that reported by other investigators. Therefore, estimates of right-hander misclassification based on these data provide an upper limit value. Usually, handedness categories are formed using either a strict (all behaviours must be performed by the right hand) or a lax (handedness scores fall in the right-handed range on a preference continuum) criterion; the first criterion typically results in a lower incidence rate of both right- and left-handers. To illustrate, Porac et al. (1986) and Studies One and Two all report an incidence of right-handedness, based on a lax dichotomy criterion, of approximately 88%. Porac and Coren (1981) reported a 72% incidence of right-handedness using analogous questionnaire procedures and a strict classification criterion. This 16% criterion-dependent variation represents the occurrence of ambihanded right-handers. Interestingly, it is a value close to the estimate of ambihandedness in right-handers based on reports of attempts to switch hand preference. The third and fourth rows of Table 9 contain a similar analysis for individuals dichotomously classified as left-handers. Since dichotomous classifications of left-handedness frequently represent "nonright-handedness" (includes everyone

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Table 9: Handedness category misclassification estimate based on incidence of shift attempts in right- and left-handers

Study One

Study Two

Tota 1

p( r t . hand/rt . s h i f t )

10/407 0.0246

21/598 0.0351

31/1005 0.0308

p ( r t . hand/lt.shift)

81/407 0.1990

711598 0.1187

152/1005 0.1512

p ( m a n i f e s t r t . hand, 1 item, but I t . hand f o r o t h e r s )

0.1802

p( It. hand/rt. s h i f t )

19/57 0.3333

24/75 0.320

43/132 0.3258

p( It. hand/lt. s h i f t )

3/57 0.0526

10/75 0.1333

13/13? 0.0985

p(manifest I t . hand, 1 item, b u t r t . hand f o r o t h e r s )

0.4243

falling outside of the right hand range of scores), it is not surprising that the one item misclassification estimate for left-handedness is approximately 42%. Thus, one has a 42% likelihood of categorizing an individual as left-handed with one item, who displays a number of right-handed activities. This value is consistent with misclassification figures for left-handers reported by Beukelaar and Kroonenberg (1983). As our research reveals, a lax criterion to establish left-handedness results in an approximate 12% incidence rate, while Porac and Coren (1981) report a 5.3% incidence rate using a strict criterion of total left hand use. In other words, in a sample of 100 adults, twelve will fall in the lefthanded range and five will show total left hand use. The remaining seven (58.3%) will be ambihanded and, perhaps, a substantial portion of these can be accounted for by postulating a past history of hand change attempts. We have identified individuals who have experienced shifts in the expected direction, rightward because of majority pressure, and individuals who actively try to increase their proficiency with the left hand. Their presence in young and middle-aged adult samples presents a misclassification problem when only one item is available to categorize handedness, such as is often the case in archival research. We suggest, therefore, that researchers attempt to correct for misclassification probabilities, especially if the categories of right- and left-

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handedness are used as independent variables to study group differences in other behaviours. The Effect Size of Shift Attempts. How much of the variability in unimanual hand use is explained by these tangible environmental effects? Researchers often resort to environmental explanations for ambihandedness or for variations in the incidence of right- and left-handedness but specific estimates of the effect of these influences are not available. As Beukelaar and Kroonenberg (1983) state, some actions, like writing and eating, are assumed to come under cultural or environmental pressure, How much variability can be attributed to these sources? In Studies One and Two, we analjzed graded handedness scores as a function of group membership (No Shift, Right Shift and Left Shift). This method permits the assessment of the relationship between group membership and the handedness variable using an explained variance approach. How much variability in handedness scores is explained by knowledge of an individual's membership in one of the three groups? The Pearson r that we computed for this effect from Study One was 0.17; it was 0.38 from Study Two. The mean r from the two samples was 0.28, leading to an average explained variance estimate of 7.8%. Thus, approximately 8% of the variability in handedness scores in an adult sample is explained by knowledge of the history of hand change attempts. This leaves the majority of the variance, approximately 92%, arising from other sources, which could include subtle environmental pressures and neurological, biological and genetic factors. The above Pearson r's estimate the variability explained by overt change pressure within one culture. However, other information can be used to assess broader environmental effects. The wide variation in the incidence of lefthandedness demonstrated in the literature summary of Table 2 (0.6% to 19.8% left-handedness across 47 studies and 63 samples) typically is explained by crosscultural differences in the tolerance for the use of the left hand and/or crosscultural differences in training pressures applied to hand use activities (Dawson, 1972; Marrion, 1986; Payne, 1987). These data can also estimate the effect size of direct pressures to switch handedness or to use both hands. We used meta-analytic techniques described by Wolf (1986) and Mullen and Rosenthal (1985) and coded a number of variables associated with each of the 63 samples listed in Table 2. These variables included the racial group, the country of origin of the study, the year of publication, the age group of the sample and the percentage of left-handedness reported. We arbitrarily coded

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Caucasian samples and countries with Caucasian majorities with low numbers and Oriental samples and countries with high numbers. We performed a multiple regression analysis using country of origin and racial group as the predictor variables and the reported percentage of left-handedness as the dependent measure. The two predictors were chosen to explore whether sources of cross-cultural variation in left-handedness were related to differences in cultural orientation to handedness (an environmental variable) or differences in the racial composition of the sample (a biological variable). The relationship between country of origin of the study and the percentage of reported left-handedness explained 23.5% of the variance (bivariate r= -0.485 where the higher country codes of non-Caucasians were associated with lower rates of left-handedness). The addition of the race variable to the equation accounted for only another 1.9% of the variability (bivariate r of -0.10 with the larger number codes of non-Caucasians associated with lower rates of lefthandedness). The country of origin accounted for a higher percentage of the variability in cross-cultural rates of left-handedness than the biological variable of racial group. Generally, studies done in Oriental and Negro countries reported lower rates of left-handedness; this effect was ameliorated when these racial groups were studied outside of their native cultural context. Table 10 shows the mean percentage of left-handedness and the number of samples for the cultural and racial groups coded from the data of Table 2. There is a clear trend toward lower rates of left-handedness in studies conducted in African and Oriental countries. This trend is not as apparent when the same data is viewed as a function of race, irrespective of the country of origin of the study. Thus, a higher proportion of the approximate 20% variation in the incidence of left-handedness in studies conducted throughout the world is explained by environmental rather than biological variables.

Final Remarks We have stressed a number of points in this chapter. First, adults with a past history of overt pressures to alter "natural" handedness exist in contemporary research samples with an estimated incidence of 8.9%. Second, there are two distinct types of shift attempts with differing characteristics, Right shifts occur in the expected direction toward the dextral majority. They are instituted by parents and teachers during an early stage of life. Left shifters are contemporary remnants of the Ambidextral Cultural

284

Porac et al.

Table 10 Incidence of left-handedness as a function of location of the study and racial group'

Location o f Study Country

Number o f Samples

Mean % Left-handedness

USA & Canada

26

9.7

United Kingdom Western Europe Africa China Japan Other

15 7

9.3 6.9 4.1 5.3 3.3 4.5

5 3 3 4

Racial Group Group

Caucasian Negro Or ienta 1 Indian or Mixed

Number of Samples 3a 11

7 7

Mean % Left-handedness

a. 7 7.0 4.6 9.5

'Based on the studies listed in Table 2.

movement of the early 20th century. These are individuals who attempt to increase the proficiency of left hand use through personal experimentation and effort. Left shifts usually occur during the late childhood or early adult years. Third, the average success rate of these hand change attempts is approximately 20% (40.6% for right and 7.1% for left shifts); therefore, the majority are not successful in producing a change in handedness classification in the direction of the shift. Individuals who have a past history of shifts, instead, display more ambihanded behaviour patterns when compared to no shift controls. Fourth, the ambihanded pattern displayed by the shift groups can be explained in two ways. Either the shift attempt is only partially successful and only some behaviours move in the direction of the attempted switch, or these individuals are naturally ambihanded and this tendency produces an interest in switching hands. The descriptions offered by the shifted individuals seem to favour the partial success explanation.

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Fifth, the presence of substantial numbers of individuals who have experienced overt environmental pressures on their handedness, along with the ambihanded nature of their hand use, compromises the accuracy of forming dichotomous handedness categories. Based on our data, approximately 18% of individuals categorized as right-handers can be expected to use their left hand for some activities, while as many as 42% of left-handers may also use their right hand. These estimates of classification accuracy should be taken into account when handedness categories are formed using only one behaviourial item. Sixth, approximately 8% of the within-cultural variability in adult handedness scores can be explained by knowledge of overt environmental pressures. This figure rises to 23.5% when one examines cross-cultural variations in handedness patterns. Variability in incidence rates reported in studies conducted throughout the world shows stronger correlations to cultural rather than racial (biological) differences. However, the largest percentage of variability in handedness scores must still be explained by other sources and they are most likely of physiological (neurological, genetic and so forth) origin. Finally, handedness is a behaviour with variability arising from a number of sources. Only a few contemporary investigators emphasize this multidimensional approach. Rather, most search for pure handedness types and non-overlapping dichotomies of right- versus left-handers to form unambiguous independent groups for experimental purposes. The presence of ambihandedness and studies of its possible causes are a nuisance under such a scheme. Our research shows that there is an interest in and some success at switching hand use. Shift attempts seem to be most frequently associated with varying types of bimanual handedness patterns. Whether it is the partial switch of a left-hander to the right side or the partial success of a right-hander shifting to the left side, a significant number of adults find a place for the left hand in a right hand world and actively seek and continue its use. Our studies confirm the plasticity, within limits, of human handedness and offer an explanation of a non-pathological origin for the existence of mixed and ambihanded types.

Acknowledgements This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada, the University of Victoria Committee on Faculty Research and Travel and the Government of Canada Challenge '86, '87, '88 and '89 Student Employment Programs.

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We gratefully acknowledge the assistance of the following individuals in the collection and analyses of these data: Kevin Bisschop, Michael Harnadek, Deborah Krieg, Eileen McKie, Mary Petersen and David Wesley. Also, we gratefully acknowledge the assistance of the following individuals and organizations in the data collection of Study Two: Carmie Verdon and Irene Lavers, Island Hand Therapy Clinic, Victoria, British Columbia; Frances Williams and Adele Hern, Gorge Road Hospital, Victoria, British Columbia; the administrative staff of Harbour Square and Tillicum Malls, Victoria, British Columbia. Address reprint requests to C. Porac, Department of Psychology, University of Victoria, Victoria, British Columbia V8W 2Y2 Canada.

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Perelle, I. B., Ehrman, L., & Manowitz, J. W. (1981). Human handedness: The influence of learning. Perceptual and Motor Skills, 53, 967-977. Peters, M. (1986). Incidence of left-handed writers and the inverted writing position in a sample of 2194 German elementary school children. Neuropsychologia, 24(3), 429-433 Peterson, J. M. (1979). Left-handedness: Differences between student artists and scientists. Perceptual and Motor Skills, 48, 461-462. Plato, C. C., Fox, K. M., & Garruto, R. M. (1984). Measures of lateral functional dominance: Hand dominance. Human Biology, 56(2), 259-275. Porac, C., & Coren, S. (1981). Lateral preferences and human behavior. New York: Springer-Verlag. Porac, C., Coren, S., & Searleman, A. (1986). Environmental factors in hand preference formation: Evidence from attempts to switch the preferred hand. Behaviour Genetics, 16(2), 251-261. Rhoades, J. G., & Damon, A. (1973). Some genetic traits in Solomon Island populations: I1 Hand clasping, arm folding and handedness. American Journal of Physical Anthropology, 39, 179-184. Roszkowski, M. J., Sacks, R., & Snelbecker, G. E. (1982). Young children’s subjective reports of manual preference: Internal consistency and the selection of the most representative activity on the basis of item-total correlations. Journal of Clinical Neuropsychology, 4, 35-37. Rymar, K., Kameyama, T., Niwa, S., Hiramatsu, K., & Saitoh, 0. (1984). Hand and eye preference patterns in elementary and junior high school students. cortex, 20,441-446. Salmaso, D., & Longoni, A. M. (1985). Problems in the assessment of hand preference. COIW, 21, 533-549. Sanders, B., Wilson, J. R., & Vandenberg, S. G. (1982). Handedness and spatial ability. Cortex, 18, 79-90. Saunders, D. A., & Campbell, A. L. (1985). Handedness incidence in a population of Black University students. Perceptual and Motor Skills, 60,355360. Schachter, S. C., Ransil, B. J., & Geschwind, N. (1987). Associations of handedness with hair colour and learning disabilities. Neuropsychologia, 25( lB), 269-276. Schwartz, M. (1988). Handedness, prenatal stress and pregnancy complications. Neuropsychologia, 26(6), 925-929. Searleman, A., & Fugagli, A. K. (1987). Suspected autoimmune disorders and left-handedness: Evidence from individuals with Diabetes, Crohn’s Disease and Ulcerative Colitis. Neuropsychologia, 25(2), 367-374. Shaw, H. J. (1902). Right-handedness and left-brainedness. The Lancet, p. 1486. Shettel-Neuber, J., & O’Rielly, J. (1983). Handedness and career choice: Another look at supposed left/right differences. Perceptual and Motor Skills, 57, 391-397. Shimizu, A., & Endo, M. (1983). Handedness and familial sinistrality in a Japanese student population. Cortex, 19, 265-272.

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Silverberg, R., Obler, L. K., & Gordon, H. W. (1979). Handedness in Israel. Neuropsychologia, 17, 83-87. Sutton, P. R. (1963). Handedness and facial asymmetry: Lateral position of the nose in two racial groups. Nature, 198, 909. Teng, E. L., Lee, P. H., Yang, K., & Chang, P. C. (1976). Handedness in a Chinese population: Biological, social and pathological factors. Science, 193, 1148-1150. Teng, E. L.,Lee, P. H., Yang, K.,& Chang, P. C. (1979). Lateral preferences for hand, foot and eye and their lack of association with scholastic achievement in 4143 Chinese. Neuropsychologia, 17, 41-48. Thompson, A. L., & Marsh, J. F. (1976). Probability sampling of manual asymmetry. Neuropsychologia, 14(l), 217-223. Verhaegen, P., Ntumba, A. (1964). Note on the frequency of left-handedness in African children. Journal of Educational Psychology, 55(2), 89-90. White, K., & Ashton, R. (1976). Handedness assessment inventory. Neuropsychologia, 14, 26 1-264. Whittington, J. E., & Richards, P. N. (1987). The stability of children’s laterality prevalence and their relationship to measures of performance. British Journal of Educational Psychology, 57, 45-55. Wolf, F. M.(1986). Meta-analysis: Quantitative Methods for Research Synthesis. Beverly Hills: Sage.

SECTION n7: COGNITIVE, SPATIAL AND LANGUAGE ABILITY IMPLICATIONS

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LEFT-HANDEDNESS Behavioral Implications and Anomalies, S . Coren (Editor) 0 Elscvier Scicncc Publishers B.V. (North-Holland), 1990

293

Chapter 10

Mental Retardation and Left-Handedness: Evidence and Theories Margaret-Ellen Pipe University of Otago

For most of this century there has been a persistent interest in whether lefthandedness is associated with cognitive deficit. Numerous studies have examined hand and other lateral preferences of groups characterized by specific deficits such as reading disability, stuttering, perceptual problems or mild intellectual deficits (for reviews see Hardyck & Petrinovich, 1977; Naylor, 1980). Generally the evidence has not been compelling (Hardyck & Petrinovich, 1976, 1977; see also Benbow, this volume). But in contrast to data from groups characterized by specific deficits, it is argued in the present chapter that there is strong evidence from groups with more severe and generalized deficits, linking left-handedness to mental retardation (Bishop, 1983; Burt, 1937; Gordon, 1921; Pipe, 1987, 1988; Porac, Coren & Duncan, 1980). The first section of the’present chapter examines this evidence and shows that left-handers are clearly over-represented in mentally-retarded populations. The association between left-handedness and mental retardation has significant implications not only for the general question of the relation between left-handedness and cognitive deficit, but more importantly for theoretical accounts of lateral preferences. Theories of lateral preferences must, of course, be able to account for both the occurrence of lefthandedness in the general population and for its special incidence among certain groups. In the accounts evaluated in the second section of this chapter, the raised incidence of left-handedness among the mentally retarded has been attributed to factors in learning (Hildreth, 1950; Wile, 1934), development

294

Pipe

(Delacato, 1959; Pickersgill & Pank, 1970) or pathology (Gordon, 1921; Satz, 1972,1973) factors. The most popular idea is that left-handers are over-represented among mentally retarded people because of pathological influences, such as brain injury at birth. The second section of this chapter argues that although there is support for the general notion of pathological lefthandedness in retarded groups, recent data relating to familial influences (Pipe, 1987; Searleman, Cunningham & Goodwin, 1988), aetiology of retardation (Batheja & McManus, 1985; Pipe, 1987) and location of brain lesions (Silva & Satz, 1979) challenge current theories and models. It is concluded that pathological left-handedness is unlikely to be a unitary concept and that the specific mechanisms which contribute to the excess of left-handers in retarded groups are at present poorly understood.

Left-Handedness in Retarded Groups The evidence indicating that left-handers are generally over-represented among groups characterized by mental retardation is now unequivocal. Table 1 summarizes studies of hand preferences of mentally retarded children and adults. A two-fold or greater increase in the frequency of left and mixed-handers (nonright-handers, NRHs) among people with mental retardation is typical. When left and mixed-handers have been distinguished, the most usual finding is an increase in both for the retarded group (Table 1). In studies not including a non-retarded comparison sample, estimates have been higher than the accepted incidence of left-handedness for the general population at the time of the study. Only two studies have failed to find NRHs to be more common among retarded subjects (Barry & James, 1978; Wile, 1934). However, Wile (1934) noted that the subjects for his study had been selected for another (unspecified) purpose and were therefore not necessarily representative. Indeed, perhaps the most unusual aspect of Wile’s data is the very high incidence of NRHs reported for the non-retarded group (Table 1). Similarly, Barry and James (1978) selected their retarded subjects to match a small group (n = 16) of autistic subjects; in particular, the small subject sample means their estimates are likely to be unreliable (Annett & Turner, 1974). Table 1 shows that although there is general agreement across studies that the incidence of NRH is raised for retarded groups, there are very marked differences in the actual estimates reported. For mentally retarded groups estimates range from 6% (Wilson & Dolan, 1931) to 60% (Porac, Coren &

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295

Duncan, 1980; Wile, 1934) while for non-retarded groups the range is from 4% (Ballard, 1912; Wilson & Dolan, 1931) to 63% (Wile, 1934). This variation almost certainly reflects the influence of factors relating to the measurement of hand preferences, the selection of subject samples and changes in the incidence of left-handedness over time. There is little doubt different procedures used to assess handedness and different criteria for claiming left-handedness contribute to the discrepant estimates across studies (Hardyck & Petrinovich, 1977). The use of multi- item measures in some studies (in accord with the belief that handedness is not a unitary phenomena) increases the probability of a subject being classified as nonright-handed (Pickersgill & Pank, 1970). Several studies indicate neither the basis nor the criterion for their classifications. Further, early studies, particularly those prior to the 19503, tend to report lower incidences of left-handedness for both retarded and non-retarded samples (Table 1). Changes over time may reflect different measures and criteria for assessing left-handedness. Additionally, however, they probably reflect a changing tolerance of the manifestation of leftward preferences (Brackenridge, 1981; Hildreth, 1950). Differences in subject samples may also contribute to discrepant estimates of the incidence of NRH across studies. In particular, several studies have shown that left-handedness is more common among more severely retarded groups (Berman, 1971; Blau, 1946; Bradshaw-McAnulty, Hicks & Kinsbourne, 1984; Burt, 1937; Gordon, 1921; Hicks & Barton, 1975; Hildreth, 1950; McManus, 1983; but see Lonton, 1976; Lucas, Rosenstein & Bigler, 1989, for counter-examples). For example, in what remains the most extensive study of left-handedness and retardation to date, Gordon (1921) noted that the percentage of left-handers increased in relation to degree of retardation, beginning at 7 percent among elementary school children, rising to 18 percent among the feeble minded and again to between 16 and 30 percent for "idiots" and epileptics. Burt (1937) similarly found a lower incidence of left-handedness for backward children (8%) than for the more severely handicapped sample of "mentally defective" children (12%), and Hicks and Barton (1975) have reported more than twice as many lefthanders among severely and profoundly handicapped children as among mildly and moderately retarded groups. Lucas et al. (1989) reported a higher incidence of NRHs for a group of severely and profoundly retarded (34%) than for mildly and moderately retarded (27%) subjects, although this difference was not significant, Lonton (1976), however, found the reverse pattern, that is fewer NRHs among more disabled children, for a group of for children with spina

Table 1. Summary of sludies examining the hand preferences of mentally retarded groupsshowing the percentageof left handers (LH) and mixed handers (MH) in retarded and non- retarded samples. Percent of Samnle

non-reludtd. 4-14y (13.189) non-rcmlcd. 8-14y.F(5,758) ~-nIdd 8-14y.M . (6.181)

Smith (1917)

QuesliOnMirc

fecblcminded. (200) F febblcminCled,(200) M

11.0 8.5

Gordon (1921)

4 FcrIarinancc Task

mentally clclcciivc (4.620) subgroup F. (355) subgroup M. (374) menlally defective with speech defects (239) n~n-~ludtd. 4-14y (3298)

18.2 20.7 16.6 32.6 21.4

.

Wilsonand Dolan (1931) NS

Doll (1933)

NS

subnormal (151) school children (1.297) rclarded. motor signs (29) retarded. no motor signs (10)

institution sample (33) Wilc (1934)

NS

~rarded(IQ 50-89) (72) non-rcmded (IQ 90-109) (81) non-rclarclcd (IQ 1lo+) (33)

4.0 5.0

I .o

3.0

.5 .4

3.O

4.5 5.5

-

1.4

-

7.3 6.4 41.0 10.0 12.0

3.5

-

25.0 18.0

38.0 42.0

9.0 0 8.0

14.0 46.0

Table 1ctd...

Percent of Samplc A u h Burl (1937)

AsscssmenlP

d m

quwc;onnrireand

Rrformance'Ibslrs

Subject Sample (N) backward.(NS) F backward M mentally defectiive.(NS) F , menially defective.(NS) M school childrcn (>S.OaO) F school children (>S.ooO) M

Retarded LH 6.0 9.6 10.3 13.5

MH

-

-

Mink (1947)

3 Paformance Tasks

feebleminded. M (97)

15.5

1.2

Murphy (1962)

NS

Down's syndrome (32)

13.0 31.0 25.0

6.0 13.0 13.0

9.8

42.6

familial rclardcd (32) brain injured (32) Lenncberg. Niehols. and Roscnbaga (1964)

NS

Clausen (1966)

Hanis Twu (shortcncd) W d d . 8-lOy (68)

Down's syndrome. 3-22y (61)

rcrarded, 12-15y (105) ntardcd 20-24y (103)

21.0 28.0 12.0 10.0 18.0 10.0

non-rttarded. 8-1Oy (112) PicJcersgill and wnk (1970) 6 Pufonnancc Tasks

Down's syndrome (16) r(la) w-rtlarded (32)

18.8 31.0

Non-rclarded LH MH

3.7 5.8

-

21 .o

3.O

15.6

-

-

-

N

'Ihble 1 cld...

Perccnt or Sainplc Rc~ardccl Non-ranrdcd LII MH LII MI1

Aulhor

Asscssmcn Pmcdurc

Subjcct Samplc (N)

Hicks and Barton (1975)

Carctnkas Report

rclardcd (550) remdation = mild 10modualt recardah = scvcm to proround

20.2

Spirw-bifida and hydrocephalus ( m a n 1Q = 85) (203) IQ < 70 lQ<50 non-rctnrdd(200)

22-

11 -

16-

32-

2-

9-

Lonion (1976)

2 Pcrforrmncc Twiks

13.0 28.0

Barry a d l a m (1978)

14 Pcrformencc TDslu

rcrankd (34) non-re&udcd(M)

11.8

Silva and Salt (1979)

Cer~dtu Rcport

rc~ardcd.6-66~ (1409)

15.5

bilateral EEG abnormality (439) lcft s i d d EEG abrmality (21) rigk sidcd EEG abnormality (1 16)

16.9

normal EEG (96) Poroc. Corcn and Duncan (1980)

McManus (1983)

4 PcrIormancc 78sks

Molhu's Reportl Doclor's Exmination

33.0 6.3 9.4

rc(;lrdccl(138) non-rclydcd. 3-Sy (384) non-rwrdcd. high school (171)

15.9

rclardcd,no handicap (68) sliglil handksp (101)

13.2

.

18.11

-

19-

3-

8.8

17.6

6.2 5.8

25.5

14.7 12.7

-

44.2

-

-

13.5

Table 1 ctd...

Author

Asscumat Rocedurc

Subject Sample 0 " moderate handxap (62)

" severe handicap (16) non-retarded (1 1538)

Bathcja and McManus (1985)

Pipe (1987)

10 Mormance Tasks

6 RrfamMce Tasks

Percent of Sample Retarded Non-relarded

F 21.0 50.0

-

11.2

Down's syndrome (85)

28.9

rttardcd (45) non-rctardcd. 7-12y (47)

26.7

Down's syndrome (85) rcliudcd (233) non-relarded (239)

9.4 13.5

-

-

10.6

-

5.7

12.1

12.7

3.3

25.9 22.9

Searlcnun.Cunningham and Goodwin (1987)

4 PcrfonnanceTa?,lcd Qucslionnairt

rclarded. 16-22y (90) m-relarded(212)

17.8

5.6

Sopa. SaQ. Onii. VanGorp and Gncn (1987)

8 Mmence Tasks

rclardcd. 19-60~(73)

9.6

45.2

Lucas.Rosenain and B i g k (1989)

4 hfonnance Tasks

retarded. 2 1 - 8 2 ~(238) mild/moderatehandicap (115) sevaciprOfoundhandicap (1 17) language deficit (89) no deficit (143)

21.5 17.4 25.6 34.8 13.3

9.1 9.6 8.6 16.9 4.2

z a E CD

w

B fE;' a

300

Pipe

bifida and hydrocephalus (Table 1). Unfortunately Lonton does not specify the number of children in the IQ groups on which this conclusion is based, and the reliability of his estimates is therefore unknown. Nonright-handedness is also more likely when retardation is accompanied by speech defects (Ballard, 1912; Gordon, 1921; Lucas et al., 1989) or motor impairments (Doll, 1933). Gordon (1921) reported that some 50% of a subgroup of retarded children with speech deficits were left-handed compared to 18% for the general retarded sample. Ballard (1912) similarly reported an over-representation of what he referred to as "dextro-sinistrals,""congenitally lefthanded children who wrote with their right hand in accordance with convention, among a group of retarded children who stammered. More recently, Lucas et al. (1989) reported a high incidence of NRHs among retarded children with language deficits. They suggested that the incidence of NRH for children without deficits is the same as in the nonretarded population. Unfortunately the latter conclusion is difficult to evaluate in the absence of a nonretarded comparison sample and in the light of the considerable variation in handedness estimates noted above. Doll (1933) reported that retarded children with motor impairments were much more likely to be left-handed than those without although this finding must be treated with caution in view of the very small subject samples (Table 1). Few studies have compared the lateral preferences of groups differing with respect to aetiology of retardation. In most studies aetiology of retardation is simply not reported but is likely to be mixed and unknown for many individuals. The only homogeneous aetiological groups for whom handedness data is available are people with autism and Down's syndrome. People with autism are frequently mentally retarded in addition to having the specific language and communication deficits associated with autism. However, the data relating autism and left-handedness is examined elsewhere (Bryson, this volume) and will not be examined here. For people with Down's syndrome, the incidence of NRH appears to be of a similar order to that for other retarded people. Batheja and McManus (1985) and Pipe (1987) found no significant difference between hand preferences of Downs syndrome and of other retarded children, and that both groups contained approximately twice as many NRHs compared to groups of non-retarded children (Table 1). Further, although Lenneberg, Nichols & Rosenberger (1964) did not include a comparison group, they found a very high incidence of left and mixed-handedness among a group of young (preschool) Down's syndrome children (Table 1). In contrast, Murphy (1962) and Pickersgill & Pank (1970) failed to find a raised incidence of NRH for Down's syndrome

Mental Retardation

301

children. However, the small sample sizes of Murphy (1962) and Pickersgill and Pank (1970), in particular fewer than 50 subjects (Annett & Turner, 1974), mean that their estimates may be unreliable. Additional evidence that Down’s syndrome is associated with atypical lateral asymmetries comes from dichotic-listening studies. Dichotic studies have found either smaller ear asymmetries (Sommers & Starkey, 1977; Tannock, Kershner & Oliver, 1984) or atypical left ear advantages suggesting right-hemisphere language for many Down’s syndrome children (Hartley, 1981; Pipe, 1983; Zekulin-Hartley, 1981, 1982). In particular, the high incidence of right-hemisphere language appears to be associated with right-handedness in Down’s syndrome subjects (Pipe, 1988), a pattern of lateral asymmetries which is particularly rare in the general population (Annett, 1975). In contrast, other retarded groups (of mixed or unknown aetiology of retardation) typically show the more usual dichotic-listening right ear advantage suggesting left-hemisphere language representation (Hartley, 1981; Hornstein & Mosley, 1986; Pipe & Beale, 1983; Pipe, 1983); Zekulin-Hartley, 1982). No studies to date have examined the cerebral asymmetries of retarded NRHs. It is, however, clearly of interest to know whether retarded NRHs typically have left, right or even bilateral language representation and, further, whether the atypical asymmetries observed for Down’s syndrome right-handers are also observed among NRHs. Recently there has been interest in whether NRH in retarded samples is associated with a positive or negative history of familial sinistrality. Evidence of a familial influence on handedness in the general population is that two lefthanded parents are more likely to have left-handed offspring than are two right-handed parents. When only one parent is left-handed the probability of left-handed offspring is intermediate (Annett, 1974; Chamberlain, 1928; Rife, 1940). On the basis of such data, several genetic models of handedness have been proposed (Annett, 1972,1975; Corballis & Morgan, 1978; Levy & Nagylaki, 1972). Three recent studies suggest that left-handedness in retarded groups has a familial component (Bradshaw-McAnulty et al., 1984; Pipe, 1987; Searleman et al., 1988). Bradshaw-McAnulty et al. (1984) examined hand preferences of children ranging from mildly to severely retarded, on a number of tasks. Group mean laterality quotients (LQs), based on differences in left and right hand preference, showed an increasing trend towards the left hand as severity of retardation increased. This result is consistent with studies in which individuals have been classified as left- or right-handed and incidences compared across groups (Table 1). Bradshaw-McAnulty et al. (1984) found that parental LQs

302

Pipe

paralleled those of the children implicating a familial association of lefthandedness. Pipe (1987) found a higher incidence of left-handers with a history of familial sinistrality for groups of developmentally retarded (14%) and Down’s syndrome (19%) people than for a non-retarded sample (6%). Forty one percent of the developmentally retarded NRHs, 52% of the Down’s syndrome NRHs and 36% of the non-retarded NRHs had at least one NRH parent or sibling (Pipe, 1987). Searleman et al. (1988) similarly found a higher incidence of familial sinistrality for left-handed retarded subjects (88%) than for right-handed retarded subjects (50%) or non-retarded left or right-handers (44% and 42% respectively). In summary, there can be little doubt that left-handers, including familial left-handers, are over-represented in mentally retarded populations including individuals with Down’s syndrome. An even greater excess of NRHs has been reported for more severely retarded children and retarded children with particular difficulties in speech or with motor impairments. Dichotic-listening studies suggest that the majority of right-handed retarded individuals have language represented primarily in the left hemisphere. However, Down’s syndrome groups are atypical in that there appears to be a significant increase in the incidence of right-hemisphere language among Down’s syndrome righthanders.

Theoretical Accounts There have been three main accounts of the association between a high incidence of left-handedness and mental retardation, namely that it is due to maturational factors (Delacato, 1959, 1966, 1974; Pickersgill & Pank, 1970), learning factors (Blau, 1946; Burt, 1937; Hildreth, 1950) or pathological factors (Gordon, 1921; Satz, 1972; Geschwind & Galaburda, 1985). These different accounts make different assumptions not only about handedness in retarded samples, but also about the determinants of handedness generally. Maturation Accounts One explanation of the greater sinistrality of retarded than normal children is that it reflects a developmental delay in neural maturation presumed to underlie the establishment of lateral preferences (Clausen, 1966; Delacato, 1959,1966; Lenneberg, 1966; Pickersgill & Pank, 1970). The general idea here is

Mental Retardation

303

that hand preferences develop as the result of the gradual establishment of cerebral lateralization. According to this view at birth neither cerebral hemisphere (or hand) is dominant and in normally developing children dominance is established over the first several years of life (Lenneberg, 1966, 1967; Lenneberg et al., 1%4). In retarded children the whole developmental process is slowed down and arrested at an immature stage (Lenneberg, 1966). Hence, retarded children are characterized by reduced cerebral lateralization of function and will be less biased towards right-handedness. Failure to establish cerebral lateralization may, in turn, contribute to the learning and performance deficits of the child, particularly those relating to language (Berman, 1971; Delacato, 1959, 1966; Lenneberg 1966). Support for the developmental delay account of left-handedness has been limited. Porac et al. (1980) reasoned that if the expression of lateral preferences of retarded individuals is delayed, then their lateral preferences should be more like those of younger children than of their non-retarded peers (Porac, et al., 1980). They compared the hand, foot, eye and ear preferences of retarded and non- retarded children and found that retarded children showed reduced congruency across hand, foot, eye and ear preferences, as well as reduced rightward bias, more like younger children than their chronological age peers. Also consistent with a developmental delay in the establishment of handedness, Clausen (1966) found a much higher incidence of mixed-handedness for a group of young retarded children compared to two older groups (Table 1). However, in both of these studies the retarded groups were characterized not only by reduced lateral asymmetries but also by more sinistrality than expected. A developmental delay account is consistent with reduced rightward bias for retarded children, but does not predict the observed increased incidence of leff preferences also (Table 1) and is therefore inadequate to account for the atypical asymmetries of retarded children. Consistent with this conclusion, dichotic-listening studies do not support the prediction of reduced lateralization for retarded children (Pipe, 1988). Moreover, the evidence indicating that in normally developing children lateral asymmetries are present at birth (Best, Hoffman & Glanville, 1982; Coryell, 1985; Wada, Clarke & Hamm, 1975) is inconsistent with the general assumptions of a developmental delay account. Learning Accounts A second group of explanations attribute the excess of NRHs in retarded

groups to learning factors. According to learning-based theories, retarded

304

Pipe

children are less likely LO become right-handed compared to normal children, because of a general learning difficulty, or because of an environment less biased in favour of the right (Blau, 1946; Hildreth 1950; Porac & Coren, 1981; Wile, 1934). Blau (1946) and Hildreth (1950) both argued that the excess of lefthanders in retarded group is due to retarded children being less trainable. In particular, although she did not discount possible pathological influences, Hildreth claimed that retarded children were less responsive to environmental cues and parental training that would encourage right-handedness. Wile stated this view as follows: "The high percentage of left-handedness so frequently reported among mentally defective children, arises in my estimation from the fact that it is more difficult to train mentally defective children, and that there is a greater likelihood that their original biologic habit will be maintained despite efforts at social and educational coercion" (Wile, 1934, p. 67). Consistent with the implicit assumptions of Hildreth, Wile and others, Collins (1970, 1975) proposed that hand preferences in the general population can be accounted for in terms of social and cultural factors. That is, hand preferences develop from bilaterality at birth to dextrality (usually) in response to subtle social and environmental influences. For instance Collins (1975) demonstrated the influence of a biased environment in producing consistent dextral paw preferences in mice who are otherwise typically inconsistent (across individuals) in their preferences. According to Collins, decreased ability to adapt to the right-handed world among certain groups would result in a raised incidence of NRH (Barnes, 1975). There is no doubt that there is some environmental influence on hand preference. This would account, for instance, for different incidences in lefthandedness across cultures (Hardyck & Petrinovich, 1977; Porac & Coren, 1981) and over time (Brackenridge, 1981). Nonetheless, variations in handedness over time and across culture are restricted; as Corballis and Beak (1976) have pointed out, right-handedness appears to be a universal human phenomenon and its origins are therefore likely to be biological rather than cultural. Also consistent with a biological basis for hand preferences, lateral asymmetries which may be related to later hand preferences are present from within the first few weeks of life (Corballis, 1983; Coryell, 1985; Coryell & Michel, 1978; Turkewitz, 1977; Viviani, Turkewitz & Karp, 1978). It remains likely, however, that environmental factors contribute to the reduced dextrality of retarded children. As Porac and Coren (1981) point out, children raised in institutions may be exposed to an environment less biased in favour of the right which could contribute to their reduced ddxtrality. In

Mental Retardation

305

addition, the retarded child's lack of skill with tasks used to assess hand preferences is likely to result in more mixed preferences for retarded children (Bishop, 1983). The influence of such environmental influences would be to produce a greater incidence of mixed preferences, particularly among young retarded children. As noted above, Clausen's (1966) data are consistent with this. An account of lefi' hand preferences in terms of learning factors is less plausible, however. Although a familial association of left-handedness could explain the retarded child's left-handedness, any general association between left-handedness and retardation would remain to be explained. Pathological Factors The most persistent view in the literature on hand preferences is that the excess of left-handers among the mentally retarded is due to early insult or injury to the left cerebral hemisphere (Brain, 1945; Gordon, 1921; Satz, 1972, 1973). This idea dates at least from Gordon (1921) who suggested that many retarded lefthanders may have been natural right-handers driven to the use of the left hand by early lesion to, or defective development in, the left hemisphere. Gordon also distinguished such instances of left-handedness from natural left-handedness which he believed resulted from right-hemisphere dominance. A distinction between pathological and natural or genetic influences on hand preferences has strong support in current theories of the origins of hand preferences (Annett, 1972, 1975; Corballis & Beak, 1976; Corballis & Morgan, 1978). Annett's (1972, 1975) account of left-handedness in the general population has probably been the most influential. According to Annett (1972, 1975) left-handedness is determined by a combination of genetic and non-genetic factors. The majority of individuals inherit the so-called "right shift" factor (RS + ) and are therefore genetically biased towards right-handedness and left-hemisphere speech dominance. For a minority, however, the right shift factor is missing, and for these individuals (who are RS-) hand preference and hemisphere dominance are determined by random influences. Additionally, Annett recognized that pathological influences may result in left-handedness, resulting in a reversal of the preference of some RS+ individuals from right to left-handedness (Annett, 1974; Corballis & Morgan, 1978). According to this view, a distinction between pathological and natural or familial left-handedness is therefore not exclusive, since some left-handers will be RS- and lefthandedness will result from chance factors, including pathology. Others, however, will be RS + individuals for whom pathological influences have caused

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a switch in hand preference (Corballis & Beale, 1976; Corballis & Morgan, 1978). The general idea of a pathological basis for left-handedness is supported insofar as left-handedness in retarded groups is more likely in association with language related deficits (Gordon, 1921; Lucas et al., 1989) and following unilateral left hemisphere abnormalities than following right hemisphere abnormalities. Further, there is evidence of atypical cerebral asymmetries for language in left-handers with left-hemisphere abnormality (Orsini & Satz, 1986). Orsini and Satz (1986) found that nine out of ten presumed pathological left-handers showed dichotic-listening left ear advantages indicating right hemisphere involvement in language. This finding is consistent with the idea that early left hemisphere abnormality results in a switch in (the potential for) both right-handedness and left hemisphere language representation (Corballis & Morgan, 1978; Satz, 1972). In order to examine some of the implications of a distinction between pathological and non-pathological left-handedness, Satz and colleagues outlined a model based on the raised incidence of left-handedness among mentally retarded and epileptic groups (Satz, 1972; 1973; Silva & Satz, 1979). Satz pointed out that a greater incidence of pathological left than of pathological righthandedness can be expected if it is assumed that there are different base rates of left and right-handedness in normal populations together with random distributions of lesions to left and right hemispheres in brain-injured groups. Based on his model, Satz (1972) estimated, for example, that the probability of a switch in hand preference following damage to the hemisphere contralateral to the preferred hand was .21, and that the probability of a primary lesion in the left hemisphere of a manifest left-handed retarded individual .8. Support for the prediction from Satz’s model that hand preference switches from right to left following damage to primarily the left hemisphere comes from studies by Silva and Satz (1979) and Satz, Baymur and Van der Vlugt (1979). Silva and Satz (1979) examined the relation between EEG abnormalities and hand preferences in a retarded sample and found that the incidence of lefthandedness was greater for subjects with EEG abnormalities than for those without abnormality. Further, asymmetrical left-hemisphere abnormalities were more strongly associated with left-handedness than were abnormalities of the right hemisphere. But inconsistent with Satz’s (1972) model, Silva and Satz (1979) also reported a raised incidence of left-handedness in association with bilateral abnormalities, that is, comparable abnormalities in EEG recordings from the left and right

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hemispheres. According to Satz’s model, transfer of hand preference occurs following unilateral rather than bilateral lesions. Silva and Satz (1979) concluded, therefore, that there may be two accounts of the raised incidence of lefthandedness in the retarded populations. First, pathological factors account for instances where there is asymmetric brain injury. Second, decreasing probability of right-handedness with poorer levels of cortical functioning accounts for lefthandedness in association with bilateral damage. This latter explanation is however, problematic in the absence of data indicating that bilateral EEG abnormalities are associated with more severe retardation than either unilateral or no abnormalities. Silva and Satz (1979) cited evidence indicating lefthandedness was more common among more severely retarded groups, but to date none of these studies has reported EEG data. Satz et al. (1979) summarized the data from Silva and Satz (1979) and those from two unpublished studies examining the relation between EEG abnormalities and left-handedness in epileptic groups. They concluded that the incidence of left-handedness in association with unilateral left hemisphere EEG abnormality was approximately twice that for subjects with bilateral abnormality, in all three studies. They now interpret the data as consistent with the assumption of Satz’s model that a switch in hand preference results from unilateral left hemisphere damage, in contrast to Silva and Satz’s (1979) earlier conclusion. Nonetheless, Silva and Satz’s (1979) data show that the incidence of left-handedness in association with bilateral abnormalities is approximately twice that observed for retarded subjects without EEG abnormalities (Table 1). Further, as Batheja and McManus (1985) note, Satz’s model has trouble accounting for the raised incidence of NRH among Down’s syndrome groups. A comparable incidence of NRH is reported for Down’s syndrome as for other retarded groups, although Down’s syndrome individuals typically have ”diffuse and non-specific neuropathology” (Batheja & McManus, 1985). Therefore, while the incidence of NRH associated with unilateral left EEG abnormality is even higher, it appears that bilateral abnormality is also associated with a high incidence of NRH in retarded subjects as Silva and Satz (1979) had concluded. Whether this is also the case for epileptic individuals cannot be determined from Satz et al’s data since they do not report the incidence of NRH for subjects with normal EEG recordings. Soper and Satz (1984) extended the model of pathological left-handedness to account for the raised incidence of mixed (or ambiguous) hand preferences of certain groups. They assumed that mixed hand preferences are rare in the normal population but, on the basis of data from autistic subjects, are

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significantly more likely among groups with severe bilateral early brain lesions (e.g., individuals with mental retardation or infantile autism). Assuming no mixed-handedness in the normal population they predict that all instances are pathological. The assumption that mixed-handedness is rare is, however, puzzling to say the least. As Soper et al. (1987) remarked of mixed-handers, "this subtype is commonly observed in nonretarded samples whenever a multiple-item questionnaire or demonstration task is administered" (p 95). Table 1 shows that in studies using at least four performance measures to distinguish between left, mixed and right-handedness, mixed-handers are indeed to be found in nonretarded samples. Further, contrary to Soper et al.3 (1987) suggestion that mixed-handers have not been identified in retarded groups, Table 1 shows that several studies have found an increase in mixed-handedness for retarded compared to non-retarded groups of a similar order to that for left-handedness. It is therefore not clear what Soper & Satz's (1984) extension of the model adds to the original model. In particular, there is no compelling evidence to date to support Soper and Satz's (1984; Soper et al., 1987) claim that mixed-handedness is the result of early brain damage of such severity that the establishment of consistent hand preferences is prevented. This would imply that mixed-handers are more common among more severely retarded groups. However, Lucas et al. (1989) found a very similar incidence of mixed-handedness for groups of mildly-to-moderately retarded subjects and severely-to-profoundly retarded subjects. Although an association between left-handedness and bilateral brain damage is inconsistent with Satz's model, it is not necessarily inconsistent with the general notion of pathological left-handedness. If, for instance, right-handedness in the general population results from a developmental or maturational advantage (Corballis & Beale, 1976; Corballis & Morgan, 1978) left-hemisphere damage could conceivably deprive the left hemisphere of its developmental lead and open the way for chance factors in the determination of hand preferences as would be expected for non-retarded left-handers. Alternatively, Batheja and McManus (1985), following McManus (1984), proposed that biological insults during a critical period may cause a reversion to a more atavistic state of fluctuating asymmetry, the ultimate form of which will result in 50% of individuals being left-handed and 50% right-handed. More of a problem for Satz's model of pathological left-handedness, and in particular for the general distinction between pathological and genetically-based or familial left-handedness, are data indicating a familial association of lefthandedness in retarded groups (Bradshaw-McAnulty, 1982; Pipe,' 1987;

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Searleman et al., 1988). The general assumption has been that left-handedness in retarded groups is not familial, at least not to the same extent as in nonretarded populations (e.g., Corballis & Morgan, 1978; Satz, 1972). Satz (1972) for instance, hypothesized that left-handers from groups such as mentally retarded children where pathological left-handedness may be due to early cerebral injury should have a lower incidence of familial sinistrality than lefthanders from the general population where familial sinistrality is likely. It would appear, however, that left-handedness in retarded groups has a familial component at least as strong as that for non-retarded groups (Bradshaw-McAnulty et al., 1984; Pipe, 1987; Searleman et al., 1988). Bradshaw-McAnulty et al. (1984) noted a familial association of left-handedness for their retarded subjects, but also concluded that their results were consistent with the notion that pathological left-handedness accounted for the raised incidence of left-handedness in the retarded sample. The laterality quotients (LQs) based on differences in right and left hand preference, were more in favour of the left hand for the retarded children than were the LQs of parents or siblings. Bradshaw-McAnulty et al. claimed that left-handedness of the retarded children could not therefore be the simple result of membership of families with a sinistral tendency. However, LQs of parents and retarded individuals may not be comparable because LQs of parents were based on self categorizations, whereas LQs of retarded individuals were based on several performance tasks. Moreover, interestingly parental IQ’s showed the same increasing trend towards the left hand as did those of the children, with increasing severity of retardation. Further, the finding of a similar incidence of familial sinistrality for retarded and non-retarded samples (Pipe, 1987) does not support the prediction of a higher incidence of sinistrality for left-handers from the general population compared to those from a retarded sample where according to the models of Satz and others pathological left-handedness is likely. The data of Pipe (1987) and Searleman et al. (1988) indicate that, in fact, there is an excess of familial rather than non-familial left-handers in retarded groups. Further, data from retarded groups appear to be consistent with those from non-retarded groups. Orsini and Satz (1986), for example, failed to find support for their prediction of a lower incidence of familial sinistrality for a group of individuals identified as likely to be pathological left-handers. Searleman (1985) cautioned against equating pathological left-handedness with absence of a family history of sinistrality, citing Bakan, Dibb and Reed’s (1973) data linking birth stress and familial sinistrality (but see Bradshaw & Taylor, 1979).

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At least two recent accounts predict that pathological left-handedness is also familial (Bakan, 1971, 1975; Geschwind & Galaburda, 1985). Bakan (1971, 1975) has suggested that all left-handedness, including in the general population, is due to early left cerebral trauma, albeit mild in many instances. Hence, righthandedness and left hemisphere language representation are considered to be the normal state of affairs and deviations from them, pathological. In Bakan’s account the familial association of left-handedness is the product of a familial association of birth complications. The familial association of left-handedness for most retarded groups is, at least superficially, consistent with Bakan’s account. However, Bakan’s theory has been somewhat controversial when applied to non-clinical groups (Annett & Ockwell, 1981; McManus, 1981, 1983; Searleman, Porac & Coren, 1989). Perhaps particularly problematic for it are inconsistent data relating measures of birth stress to subsequent left-handedness. Some studies have found evidence in support (Bakan, 1971, 1977; Bakan, Dibb & Reed, 1973; Bishop, 1980), but others have not (Annett & Ockwell, 1981; McManus, 1981). Searleman et al. (1989) recently conducted a series of metaanalyses of studies examining the relation between birth stress indicators and lateral preferences in non-clinical populations. They concluded that although there was a statistically significant relation between specific birth stressors and left-handedness the relationships were very weak and accounted for less than 1% of the variance. Interestingly, they predicted a stronger relation in clinical populations, such as the mentally retarded, but this awaits examination. Additionally, the data from Down’s syndrome studies pose a problem for Bakan’s theory. The raised incidence of NRH could conceivably be accounted for in terms of birth stress, but the raised incidence of right hemisphere language in conjunction with right-handedness does not easily fit with the model. In particular, it is generally acknowledged that left hemisphere injury is more likely to be accompanied by a switch in hand preference than in hemisphere representation of language (Corballis & Beak, 1976; Corballis & Morgan, 1978; Satz, 1972). The theory would therefore have to be expanded to account for the Down’s syndrome data, if only to treat them as a special case. Recently, Geschwind and colleagues (Geschwind & Behan, 1982; Geschwind & Galaburda, 1985a,b,c) have outlined a theory in which, as in Bakan’s theory, left-handedness and disability are both products of the same process. Instead of birth trauma, however, they propose that hormones and testosterone in particular may play a major role in determining hand preferences. Like Bakan, Geschwind assumes that right-handedness is the normal state of affairs and results from an anatomical advantage to the left hemisphere. However, development of the left

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hemisphere is subject to hormonal influences. An excess of testosterone or a particular sensitivity to it can delay left-hemisphere development resulting in mixed or left-handedness. Geschwind and colleagues implicate testosterone in the control of autoimmunity also and hence predict an association between lefthandedness and autoimmune disorders (Geschwind & Behan, 1982; Geschwind & Galaburda, 1985a). Genetic influences play a role in determining lefthandedness by controlling levels of hormones and sensitivity to them (Geschwind & Galaburda, 1985a,b; see also Galaburda, this volume). When testosterone effects are marked the result is greater left-hemisphere abnormality resulting in learning disabilities also, and in extreme cases “a distinctly inferior overall level of functioning“ may result (Geschwind & Galaburda, 1985b, p.522). Geschwind and Galaburda (1985b) cite the raised incidence of NRH in retarded groups in support of the latter hypothesis. The theory predicts a familial association for left-handedness in retarded groups, since many instances will be due to hormonal anomalies which, in turn, have a genetic basis. Additionally, they acknowledge the influence of genetic and non-genetic factors on hand preference, including birth stress and early lefthemisphere injury. In general, however, birth stress is not considered to be a major determinant of NRH but rather a concomitant of the anomalies which result in NRH (Geschwind & Galaburda, 1985a,b). Geschwind’s theory offers a mechanism which could account for the lateral asymmetries of people with Down’s syndrome in that the timing of hormonal effects may influence hand preference and language dominance separately. Geschwind and Galaburda (1985a), for instance, speculate that neuronal migration relating to hand preference may occur earlier and more quickly than that relating to language functions. Thus, differences in the timing of hormonal influences could account for the atypical lateral asymmetries of Down’s syndrome individuals. In this context it is interesting to note that Geschwind and Behan (1982) cite evidence of an association between immune disorder and both autism and Down’s syndrome. These are the two subgroups within retarded samples associated with a raised incidence of right hemisphere specialization in association with right-handedness (Pipe, 1988). A further implication of Geschwind’s theory in relation to retarded groups is that there may be two main types of pathological left-handedness, namely lefthandedness due to early mechanical injury to the left hemisphere, as at birth, and left-handedness resulting from abnormal hormonal or developmental influences. Left-handedness resulting from hormonal anomalies might be expected to have a stronger familial association than left-handedness resulting from mechanical

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injury which, in turn, may be associated with specific cvents such as serious perinatal complications. As Searleman et al. (1989) noted, the relation between birth stressors and left-handedness has not been examined in retarded groups although perinatal complications are common in association with retardation (Lott, 1983). Only one study has examined familial sinistrality in different aetiological groups (Pipe, 1987).As noted earlier, Down’s syndrome left-handers had the highest incidence of familial sinistralty compared to other retarded and nonretarded groups. Further, whereas the incidence of NRHs with a familial history was raised in both the developmentally- retarded and Down’s syndrome groups, NRH without a familial history was significantly raised only in the developmentally-retarded group (Pipe, 1987). While there are a number of appealing features of Geschwind’s theory it may be premature to embrace it wholeheartedly. Several studies with non-retarded groups support an association between left-handedness and immune disorders (Geschwind & Behan, 1982; Searleman & Fugagli, 1987; Smith, 1987; but see Satz & Soper, 1986 for a criticism). But many aspects of the theory remain speculative, including a different timetable for the development of the neural basis of hand preference and language. In relation to data from retarded groups, Geschwind’s theory predicts that reduced cerebral asymmetry for language should be more common among retarded individuals. But as already noted, there is little evidence for this position (Pipe, 1988). In summary, maturational and environmental accounts of the raised incidence of left-handedness in retarded groups predict a greater incidence of mixed but not of left-handedness in retarded groups and have therefore not been widely accepted. Nonetheless, the role of environmental influences, including familiarity with tasks used to assess preferences, have probably been underestimated. The idea that pathological factors account for the excess of left-handedness in retarded groups is generally supported but the specific theories and models require modification to account for all of the data. In particular, the familial handedness data for retarded groups together with those indicating an association between left-handedness and bilateral abnormalities are inconsistent with Satz’s (1972,1973) model. A familial association for left-handedness in retarded groups is consistent with Bakan’s (1971,1975) theory which attributes all left-handedness to pathological factors. However, Bakan’s theory does not easily accommodate the data from Down’s syndrome groups and has only limited support in relation to non-clinical groups. Geschwind’s proposal that left-handers are prone to certain conditions, some of which result in or are associated with mental retardation (cf. Geschwind & Galaburda, 1985a,b; see also McManus, 1983)

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predicts both a familial association of left-handedness and can accommodate the Down’s syndrome data. But as currently formulated Geschwind’s theory predicts reduced lateral asymmetries for retarded groups for which there has been little direct evidence to date (Pipe, 1988).

Conclusion Some 45 years ago Blau commented that “More important than the general incidence of sinistrality is its peculiar special frequency among certain groups” (Blau, 1946, p.86). Among individuals with mental retardation there is clearly a significant increase in the incidence of left-handedness. The data reviewed here show that the incidence of left and mixed-handedness among mentally retarded groups is approximately double that in non-retarded comparison groups although exact incidences are related to severity of retardation and whether language deficits accompany retardation. The popular explanation that lefthanders are over-represented in retarded groups as the result of pathological influences is generally supported. In contrast, however, there is currently no evidence to support the also widely accepted idea that pathological and natural or familial influences contribute different proportions of left-handers to retarded and general populations respectively. It appears that a significant proportion of pathological left-handers in retarded groups are also familial left-handers. It seems likely therefore that conditions which result in left-handedness may also predispose an individual to retarded development as Geschwind and Galaburda (1985a,b) suggest. It also seems likely that in retarded groups pathological left-handedness is not a unitary phenomenon although it has generally been treated as such. Evidence of right hemisphere speech in association with right-handedness for Down’s syndrome but not other retarded groups (Pipe, 1983) suggests that the atypical asymmetries in Down’s syndrome and other retarded groups may have a different pathological basis consistent with Geschwinds theory. Pathological left-handedness has yet to be understood. Although many studies have examined hand preferences of retarded groups, there is a paucity of research examining specific questions such as the lateral preferences of different aetiological groups, the relation between perinatal complications and later hand preferences (Searleman et al., 1980) and the relation between left-handedness and atypical cerebral asymmetries. The role of birth stress factors and its possible familial association is clearly of theoretical interest (cf., Searleman et al., 1989). Also of

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interest is the relation between left-handednessin retarded groups and left, right or bilateral language representation. No studies have examined either of these questions to date. Such studies may well help illuminate the specific mechanisms resulting in pathological left-handedness. Finally, future studies with retarded groups may confirm the "special case" explanations of their hand preferences (Satz, 1972). Alternatively they may support the view that left-handednessin retarded groups is on a continuum with that in the general population (Geschwind & Galaburda, 1985b). In either case studies with retarded groups will contribute to our knowledge about the conditions under which left-handedness occurs.

Acknowledgements Preparation of the manuscript was supported in part by a Claude McCarthy Postdoctoral Fellowship to the author. The author wishes to thank K. Geoffrey White for helpful comments on the manuscript. Requests for reprints should be sent to M-E. Pipe, Department of Psychology, University of Otago, Box 56, Dunedin, New Zealand.

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Satz, P., & Soper, H.V. (1986). Left-handedness, dyslexia and autoimmune disorder: A critique. Journal of Clinical and Experimental Neurology, 8, 453-458. Searleman, A. (1985). Immunoreactive theory and pathological left-handedness. The Behavioural and Brain Sciences, 8, 458-459. Searleman, A., & Fugagli, A.K. (1987). Suspected autoimmune disorders and left-handedness: Evidence from individualswith diabetes, Crohn’s disease and ulcerative colitis. Neuropsychologia, 25, 367-374. Searleman, A., Cunningham, T.F., & Goodwin, W. (1988). Association between familial sinistrality and pathological left-handedness: A comparison of mentally retarded and nonretarded subjects. Journal of Clinical and Experimental Neuropsychology, 10, 132-138. Searleman, A., Porac, C., & Coren, S. (1989). Relationship between birth order, birth stress, and lateral preferences: A critical review. Psychological Bulletin, 105, 397-408. Silva, D.A., & Satz, P. (1979). Pathological left-handedness: Evaluation of a model. Brain and Language, 7, 8-16. Smith, J. (1987). Left-handedness: Its association with allergic disease. Neuropsychologia, 25, 665-674. Smith, L.G. (1917). A brief survey of right- and left- handedness. Pedagogical Seminag 24, 19-35. Sommers, R.K., & Starkey, K.L. (1977). Dichotic verbal processing in Down’s syndrome children having qualitatively different speech and language skills. American Journal of Mental Deficiency, 82, 44-53. Soper, H.V., & Satz, P. (1984). Pathological left-handedness and ambiguous handedness: A new explanatory model. Neuropsychologia, 22, 511-515. Soper, H.V., Satz, P., Orsini, D.L., Van Gorp, W.G., & Green, M.F. (1987). Handedness distribution in a residential population with severe or profound mental retardation. American Journal of Mental Deficiency, 92, 94-102. Tannock, R., Kershner, J.R., & Oliver, J. (1984). Do individuals with Down’s syndrome possess right hemisphere language dominance? Cortex, 20, 221231. Turkewitz, G. (1977). The development of lateral differentiation in the human infant. Annals of the New York Academy of Sciences, 299, 309-318. Viviani, J., Turkewitz, G., & Karp, E. (1978). A relationship between laterality of functioning at 2 days and at 7 years of age. Bulletin of the Psychonomic Society, 12, 189-192. Wada, J.A., Clarke, R., & Hamm, A. (1975). Cerebral hemispheric asymmetry in humans. Archives of Neurology, 32, 239-246. Wile, 1.S (1934). Handedness: Right and Ie@. Boston: Lothrop, Lee & Shepard Co. Wilson, M.,& Dolan, L. (1931). Handedness and ability. American Journal of Psychology, 43, 261-268. Zekulin-Hartley, X.Y. (1981). Hemispheric asymmetry in Down’s syndrome children. Canadian Journal of Behavioral Science, 13, 210-217 Zekulin-Hartley, X.Y. (1982). Selective attention to dichotic input of retarded children. Cortex, 18, 311-316.

LEFT-HANDEDNESS Behavioral Implications and Anomalies, S. Coren (Editor) 0 Elsevier Science Publishers B.V. (North-Holland), 1990

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Chapter 11

Handedness, Sex, and Spatial Ability Richard S. Lewis Pomona College and Lauren Julius Harris Michigan State University

After more than a century of neuropsychological research, there is abundant documentation for what is now regarded as a basic principle of human biology - the asymmetric functional organization of the cerebral hemispheres. The left hemisphere is specialized for speech production and verbal and temporalsequential processing of information, the right hemisphere for spatial-holistic processing (Bradshaw & Nettleton, 1981). In light of this evidence, the question has been raised whether and how lateral specialization itself contributes to cognition, more particularly, to the efficient performance of those cognitive tasks that are presumed to be lateralized more to one hemisphere than the other. Because individuals vary from one another in their ability to perform these tasks, a way to test the "lateralization-cognition"hypothesis has been to ask whether differences in ability are related to differences in the nature and degree of lateral organization of the component cognitive processes. The basic strategy would be to identify individuals either known or presumed to vary from one another in lateral cerebral organization, and then to compare these individuals on appropriate cognitive tasks. The most common approach to this question has been to treat handedness as an index of cerebral lateralization and to compare left- and right-handers on

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cognitive tests. The rationale is straightforward. Left-handers show a more variable pattern of cerebral lateralization than right-handers, at least with respect to language representatjon. On average, left-handers as a group show a more symmetrical pattern, with estimates of left hemisphere language dominance typically ranging between 50 and 70 percent, well below the 95 percent-andabove estimates for right-handers (e.g. Goodglass & Quadfasel, 1954; Rasmussen & Milner, 1977; Segalowitz & Bryden, 1983). With greater variability of cerebral lateralization in left-handers, a direct comparison of left-handers with righthanders thus offers a convenient way to assess the cognitive consequences of atypical lateralization or, more generally, of the relationship between cerebral lateralization and cognitive ability.

Left-Handedness and Spatial Skill Perhaps the first to test the lateralization-cognition hypothesis in this way was Levy (1969). Her subjects were fifteen left-handed and ten right-handed male post-baccalaureate science and engineering students at the California Institute of Technology. On the Wechsler Adult Intelligence Scale (WAIS), the lefthanders had a mean Verbal I.Q. of 142, and a mean Performance I.Q. of 117, whereas the Verbal and Performance scores for the right-handers were 138 and 130, respectively. The left-handers thus showed a much larger Verbal I.Q. Performance I.Q. discrepancy than the right-handers. In an attempt to use purer measures of verbal and perceptual ability, Levy (1974) re-analyzed the data using factor analyzed subtest scores from the WAIS. Now, the left-handers not only did significantly worse than the right-handers on the perceptual (presumably right hemisphere) items, they were significantly better on the verbal (presumably left hemisphere) items. Based on these findings, Levy suggested that the bilateral organization of language functions in left-handers leads to aboveaverage verbal ability but, because of incomplete specialization of the right hemisphere for spatial functions, at some cost to spatial ability. A similar hypothesis has been advanced by Lansdell(1969), based on a study of left hemisphere epileptics. He found that patients with neurological symptoms prior to age five showed a smaller deficit on Wechsler-Bellewe verbal scores than on non-verbal scores. When sodium amytal testing indicated that all of these patients had right hemisphere speech, Lansdell proposed an explanation of the pattern of cognitive performance, namely, that the relative sparing of language function was due to the right hemisphere’s development of language

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representation but that this had come at the expense of non-verbal functions, which had been displaced by language representation. Although the two hypotheses are similar, it is important to keep in mind that Lansdell was postulating the outcome of a pathological process in brain-injured patients, that is, that the compromising of spatial processing in the right hemisphere was caused by a pathologically-induced shift in language functions to the right hemisphere following early damage to left hemisphere language zones. When this shift occurs in genotypic right-handers, a related possible consequence is pathological left-handedness (PLH) (see Harris & Carlson, 1988, for a review). Satz, Orsini, Saslow, and Henry (1985) reported supporting evidence of a non-verbal deficit in patients with early left hemisphere damage. Out of 12 patients with predominantly left hemisphere damage, 10 patients had a Verbal I.Q. superiority at least 15 points over their Performance I.Q. Satz and his colleagues therefore suggested that a visual-spatial deficit is one of the distinguishing features of PLH. By contrast, Levy was postulating the outcome of a specific cerebral-organizational pattern in "natural" left-handers. Both models, however, whether applied to natural or pathological left-handers, predict the same result -- a relative deficit of spatial functions. Both models also assume that cognitive ability is related to the extent of the cortical neural networks serving a given function. A relative deficit in spatial ability in the left-hander, then, is seen as resulting from an under-representation of these neural regions. This explanation of spatial deficit, whether its origins are pathological or not, has been dubbed the "cognitive crowding" hypothesis, the term implying that spatial ability has been compromised (partly "crowded out") by the relatively greater degree of commitment of right hemisphere neural networks to language (Milner, 1974; Sperry, 1974; Teuber, 1974). Further Studies In the case of the normal left-hander, the evidence for the cognitive crowding hypothesis has been mixed. For example, on the positive side, Miller (1971) found that left-handers were worse than right-handers on a measure of visual manipulation of shapes in two and three dimensions but were no different on a verbal intelligence test. Nebes (1971) found that left-handers did worse than right-handers (most of the subjects were California Institute of Technology students) on his Arc-Circle test (on this task, subjects use their index fingers to explore an arc that is out of sight and then, after looking at three different size circles, must find the one that corresponds to the same angle as the palpated

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arc.) Right-handers also have been reported to excel left-handers on copying and maze tasks (Flick, 1966); on identification of the sidedness of pictured body parts, mirror-tracing, and localization of tactile stimulations (Silverman, Adavai & McGough, 1966); and on recalling the positions of letters shown in a matrix (Nagae, 1985a). Other positive evidence includes reports that left-handers performed more poorly than right-handers on the Performance section of the WAIS (Bradshaw, Nettleton, & Taylor, 1981); that right-handed children outperformed left-handers on the Block Design and Object Assembly subtest from the Wechsler Intelligence Scale for Children - Revised (Eme, Stone & Izral, 1978); that right-handers performed better than left-handers on the Thurstone and Jeffrey Flags Test (Johnson & Harley (1980); that right-handed undergraduates outperformed left-handed undergraduates on the figure combination subtest of the Kyoto University NX Intelligence Scale (Kashihara, 1979); that right-handed undergraduates performed better than left-handed undergraduates with an inverted writing posture on the DAT Space Relations Test (Gregory, Alley & Morris, 1980); that llth-grade right-handed children tended to score better (p c .07) than left-handers on the Space Relations Test of the Differential Aptitude Test, although this difference was not found for 9thor 10th-graders (Sherman, 1979); and that left-handers showing a left-ear advantage (indicating a relatively greater right hemisphere contribution to la-iguage processing) on a verbal dichotic listening test performed more poorly than right-handers on the PMA Space Relations Test and the Block Design Subtest of the WAIS (McGlone & Davidson, 1973). On the negative side, a number of reports have uncovered no evidence of weaker perceptual-spatial scores in left-handers. In one of the earlier studies, Wittenborn (1946) did not find a handedness difference on the Spatial Visualization subtest of the Yale Freshman Aptitude Battery. Fagen-Dubin (1974), using the Wechsler Intelligence Scale for Children, was unable to find a Verbal-Performance I.Q. difference in 32 kindergarten children, and two other studies found no handedness differences on Nebes’ Arc-Circle task (Hardyck, 1977; Kutas, McCarthy, & Donchin, 1975). There also have been negative reports for tests on a figure copying and non-verbal intelligence in children (Hardyck, Petrinovich & Goldman, 1976), an abbreviated version of the Performance scale of the WAIS (Newcombe & Ratcliff, 1973), the Vandenberg Mental Rotalions Test (Casey, Brabeck & Ludlow, 1986; McGee, 1976), the visual-spatial subtest of the Primary Mental Abilities Test (Fennell, Satz, VanDenAbell, Bowers, & Thomas, 1978), the WAIS (Briggs, Nebes & Kinsbourne, 1976); the Block Design and Object Assembly subtest of the WAIS

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(Gilbert, 1977); the Block Design subtest of the WAIS (Berry, Hughes & Jackson, 1980; Fennel1 et al., 1978) and WISC (Sheehan & Smith, 1986), the Block Design and Picture Arrangement subtests of the WAIS (Johnson & Harley, 1980), the position of tachistoscopicallypresented stimuli (Nagae, 1985b), the rate of ambiguous figure reversals in architectural students (Simpson, Lansky, Senter, & Peterson, 1983), the Spatial Visualization section of the GuilfordZimmerman Aptitude Survey (Burnett, Lane & Dratt, 1982); the Performance I.Q. scales from the WAIS (Gibson, 1973); the Punching a Folded Paper subtest of the Kyoto University NX Intelligence scale (Kashihara, 1979); and the PMA Space Test, Progressive Matrices and the Embedded Figures Test (Sheehan & Smith, 1986). Based on so many negative reports, we would seem to have all the more reason to agree with Hardyck and Petrinovich's (1977) conclusion more than a decade ago that the data linking left-handedness to perceptual-spatial deficits are "far from compelling." Crystallized vs. Fluid Intelligence When Hardyck and Petrinovich (1977) published their review, Hicks and Beveridge (1978) questioned their negative verdict by pointing out that studies that failed to find handedness differences more often used cognitive tasks that are measures of what Cattell (1971) has called "crystallized intelligence," that is, tests of knowledge or automatized skills. Hicks and Beveridge suggested that the more sensitive measure might be tests of "fluid intelligence," which call for active problem-solving. In support of this idea, they presented their own data showing right-handers to perform better than left-handers on the Culture Fair Intelligence Test (a purported measure of fluid intelligence) while showing no handedness difference on the Cooperative Vocabulary Test (a purported measure of crystallized intelligence). They also cited their own re-analysis of a study by Orme (1970), which revealed a deficit for left-handers on Raven's Progressive Matrices, another purported measure of fluid intelligence. Today, the crystallized-fluid distinction continues to be of potentially great value. Not all of the negative results, however, have used tests of the "crystallized intelligence" kind. To complicate matters still more, at least one study has found left-handers to be superior to right-handers on a spatial task. The task was to make a same or different judgment of two patterns when displayed in identical or non-identical orientations (Hermann & van Dyke, 1978). Left-handers also have been reported to be over-represented among children

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with extremely high ability in mathematical reasoning (Benbow, 1986; Benbow & Benbow, 1984); and to be over-represented among architects (Peterson & Lansky, 1974, 1978, 1980, 1983; see also Shettel-Neuber & O’Reilly, 1983, and Lansky & Peterson, 1985), advanced art students (Mebert & Michel, 1980), and mathematicians (Annett & Kilshaw, 1982; Peterson, 1979). The task used in the first-named report (Hermann & van Dyke, 1978) has obvious spatial components. In the remaining reports, the assumption is that the intellectual and/or aesthetic skills required for success in these domains of achievement involve at least average or even above-average spatial ability. The models proposed by Levy and Lansdell are designed to explain spatial deficits in left-handers (at least relative to right-handers), not spatial superiority. In this connection, a new study by Kimura and D’Amico (1989) is of potentially great significance. They identified two subgroups of left-handed college students on the basis of presumptive and actual level of spatial ability: science students and non-science students. The left-handed science students exhibited as great a degree of left hemisphere language lateralization, as indexed by dichotic listening, as did right-handed science students. They also had comparably high scores on a battery of spatial tests, and at least as good verbal ability. In contrast, the left-handed non-science students were less language-lateralized than right-handed non-science students and generally had poorer scores on spatial tests than their right-handed counterparts in similar academic programs. Presumably, in this sample, only the non-science left-handers have a more symmetrical pattern of language (and poorer spatial ability). The results therefore suggest that the “crowding”hypothesis may apply to only a subgroup of left-handers (namely, those with a lesser degree of left hemisphere language dominance). Sex Differences

In addition to the distinction between crystallized and fluid intelligence, and the degree of language lateralization, another potentially confounding factor in the literature is the sex of the subject. The studies cited above have used different proportions of males and females (ranging from all of one or the other, with the numbers of each not always reported). Especially, however, where the lateralization-cognition hypothesis is tested with spatial tasks, it would be crucial to control for the sex of the subject because males typically excel females on tests of spatial ability (e.g., Ben-Chaim, Lappon & Houang, 1986; Halpern, 1986; Harris, 1978, 1981; Harris, Hanky, & Best, 1977; Johnson & Meade, D87; Just

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& Carpenter, 1985; Linn & Petersen, 1985; Maccoby & Jacklin, 1974; Sanders, Soares, & D'Aquila, 1982). Not only might sex of subject have been an uncontrolled source of variation in prior studies, some research suggests that the effect itself might depend on the handedness of the subject. Here again, though, the evidence is inconsistent. In three studies with large samples (879-2477 subjects), three different patterns have been found: Yen (1975) found that left-handedness was associated with low spatial scores in males (consistent with the cognitive crowding hypothesis) but was unrelated to spatial scores in females; Sanders, Wilson, and Vandenberg (1982) found that left-handedness was associated with high spatial scores in males and with low scores in females; and Inglis and Lawson (1984) failed to find either a handedness effect or a sex by handedness interaction on WAIS-R performance, although the males overall did score significantly higher than females, although by a very small margin.

Reasoning Ability as a Moderator Variable Harshman, Hampson, and Berenbaum (1983) suggested that "reasoning ability" serves to moderate the relationship between sex and handedness as these variables, in turn, are related to spatial ability. If differences in reasoning ability exist among the three large scale studies cited above then we may be able to account for the discrepancy among these studies. In a retrospective analysis of data from three different data sets, they categorized subjects as being either high or low in reasoning ability. To make this determination, they had to use a different measure of reasoning ability for each sample -- the Raven Progressive Matrices (Raven, 1960), the Inference Test (French, Ekstrom & Price, 1963), and Nonsense Syllogisms (French et al., 1963). The results suggested that the direction of the interaction between handedness and sex depended on the level of reasoning ability of the subjects. Among "high reasoners," right-handed males performed better than left-handed males on every one of 15 different tests of spatial ability, whereas right-handed females performed worse than left-handed females on 12 of the same 15 spatial measures. Among "low reasoners," the general superiority of males was maintained, but the direction of the handedness by sex interaction was reversed. Now, right-handed males tended to perform worse than left-handed males, and right-handed females tended to perform better than left-handed females. The use of reasoning ability as a moderator variable not only helps to explain the data sets in the Harshman et al. (1983) study, it can be used to explain the

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results in the three large-sample studies cited above. Harshman et al. (1983) suggested that Yen's (1975) subjects were more representative of high reasoners because their PMA Space scores were three-fourths of a standard deviation above the mean. The superior performance of the right-handed males on the PMA Space test is consistent with the Harshman et al. study. In contrast, Sanders et al.'s (1982) subjects appear to have been more representative of the "low reasoning" group in Harshman et al.3 (1983) study (Harshman et al.'s "low reasoners" and Sanders et al.'s subjects had similar Raven Progressive Matrices scores). The superiority of left-handed males and the inferiority of left-handed females found in Sanders et al.'s study is a pattern similar to that found among "low reasoners" in the Harshman et al. study. Inglis and Lawson (1984) used data gathered for the revision of the WAIS-R, which consisted of subjects stratified for all levels of intelligence. Having an equal number of subjects of high and low intelligence may have masked any sex by handedness interactions existing for specific intelligence levels. Note also that the pattern found by Harshman et al. for "high reasoners" is like that found by Levy (1969) and Nebes (1971), where right-handed males significantly excelled left-handed males on WAIS Performance I.Q. and Nebes' Arc-Circle test. Inasmuch as the subjects in both studies were post-baccalaureate science students whose I.Q.3 (both verbal and performance) were well above average, both groups can be said to have been "high reasoners." The explanation of a sex difference in spatial ability by itself has been controversial. Some have argued that the usual male advantage is the result of sex-differentiated training and socialization, whereas others have proposed that it reflects, at least in part, underlying sex differences in cerebral lateralization for higher cognitive functions, namely, greater bilaterality in females than in males (see review in Harris, 1978, 1981; McGlone, 1980). The neuropsychological explanation has been challenged on the grounds that independent evidence of sex differences in cerebral lateralization is either weak or inconsistent (e.g. Fairweather, 1982; but see Kimura, 1983; Kimura & Harshman, 1984, for a contrary view). As Harshman et al. (1983) have pointed out, what logically strengthens the case for a neuropsychological explanation is the possibility that sex and handedness interact, not only with each other but with reasoning ability as well. This poses problems for a pure environmental, or socialization, explanation of sex differences in spatial ability, if only because it is hard to imagine how such a model could predict so complex a relationship. That is, if socialization were the primary factor, why would the direction of the effect depend on the handedness of the individual, much less on the combination of

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handedness and reasoning ability? The interaction (at least the simple sex by handedness interaction), however, is consistent with a particular neuropsychological model of the relationship between cerebral lateralization and cognitive ability. Starting with the assumption that competence in a particular cognitive functions is related to the extent of its cortical representation, Levy and Gur (1980) proposed that bilateral representation of spatial functions will be associated with greater spatial ability, but at some cost to verbal ability, whereas bilateral representation of verbal functions will be associated with greater verbal ability, this time at a cost to spatial ability. They further proposed that cerebral lateralization will be related to the interaction of sex with handedness. When the "main language hemisphere" is on the right (as is the case for a greater percentage of left-handers than right-handers), then verbal functions will be bilaterally represented in males and spatial functions bilaterally represented in females. Their model, therefore, predicts that on spatial tasks, right-handed males will outperform left-handed males, whereas left-handed females will outperform right-handed females. The Nature of the Spatial Task We have referred to Hicks and Beveridge's (1978) proposal that tests of fluid intelligence are more sensitive than tests of crystallized intelligence in tests of the lateralization-cognition hypothesis. It is also possible that certain tests of "fluid intelligence" are more sensitive indicators than others. If so, the inconsistencies in the aforementioned studies also might reflect uncontrolled differences in the nature of the spatial task. This may well be the case where sex differences are concerned, since new evidence indicates that the sex difference is stronger in certain kinds of tasks and weaker, perhaps even absent, in others. In a meta-analytic study, Linn and Petersen (1985) concluded that the male advantage was largest on tests of mental rotation (requiring rapid and accurate mental rotation of two- and three-dimensional figures, for example, the Primary Mental Abilities Spatial Relations Test), smaller on tests of spatial perception (requiring determination of spatial relationships with respect to orientation of one's own body, for example, the Rod and Frame test), and weakest (essentially absent) on tests of spatial visualization (requiring carrying out complicated, multi-step manipulations of spatially-presented information, for example, the Block Design Test from the WAIS-R). The demonstration of sex differences in spatial skill may depend, therefore, on the extent to which the spatial task

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incorporates the critical (that is, the mental rotational) element. If so, a similar effect might operate for handedness differences. Why are certain cognitive components likely to be more important than others for demonstrating these individual differences? The supposition behind the cognitive crowding hypothesis would be that the essential component of the task draws predominantly on the specialization of the right hemisphere. For mental rotation tasks, evidence from several sources suggests that this is the case. For example, a study of regional cerebral blood flow in the two cerebral hemispheres while subjects performed a variety of visuospatial tasks revealed that the most marked asymmetry, favouring the right hemisphere, occurred during performance of a mental-rotation task (Deutsch, Bourbon, Papinicolaou, & Eisenberg (1988). Converging evidence has come from studies of brain-injured patients showing that mental rotation is more likely to be impaired following posterior lesions of the right, but not the left, cerebral hemisphere (e.g., Ratcliff, 1979); and from studies of commissurotomized patients that indicate a left hemifield advantage for mental rotation of patterns (Corballis & Sergent, 1988). The evidence is complex (see reviews in Corballis & Sergent, 1989; Fischer, & Pellegrino, 1988), but, on balance, we agree with the conclusion that there is a truly "mental manipulative" aspect to the right hemisphere advantage in visuospatial performance (Deutsch et al., 1988, p. 445).

Spatial Ability in Lett- and Right-Handed "High Reasoners" Before speculating further about neuropsychological mechanisms of individual differences in spatial ability, we decided to find out whether the particular sex by handedness interaction reported by Harshman et al. (1983) for persons with "high reasoning" ability could be corroborated in an independent sample of persons whose intellectual ability was more directly measured, and also whether the effect would appear for some categories of spatial test but not for others, as Linn and Petersen's (1985) meta-analysis seems to imply. We therefore administered three different kinds of spatial tests -- mental rotation, spatial perception, and spatial visualization -- to a sample of left- and righthanded male and female college students, all with high reasoning ability as defined by exceptionally high scores on a standard test of academic achievement. The subjects were 56 18-and 19-year-old Michigan State University(M.S.U.) freshman. They were recruited from letters written to on-campus freshman

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residents having American College Test composite scores (derived from performance on four subtests: a) English Usage; b) Social Studies Reading; c) Natural Sciences Reading; and d) Mathematics Usage) of 27 or above (the upper 15th percentile of entering M.S.U. students), corresponding to the 93rd percentile of national performance. The initial pool of eligible subjects was 820 in number, of whom 337 responded to the recruitment letter, with 301 agreeing to participate. After administration of a test of handedness to this group, the final sample of 56 subjects was randomly selected, with the constraint that each sex X handedness cell contained an equal number of subjects. Handedness was assessed with B r i g s and Nebes' (1975) modification of Annett's (1967) hand preference questionnaire. The questionnaire asks about the strength of hand preference for 12 actions, where an "always" response receives a score of 2, a "usually" response is scored 1, and a "no preference" response is scored 0. Right-hand preference is scored as a positive value, lefthand preference as a negative value. The total score equals the sum of scores for all 12 items. Subjects with a positive score of 20 or above were classified as right-handers. Subjects who wrote with their left hand and had a negative score were classified as left-handers. Because hand preference is commonly more variable among left-handers than right-handers such that proportionately more left-handers have weaker preference than right-handers (Oldfield, 1971), a less strict criterion for handedness was permitted for left-handers than for righthanders. Four spatial tests were given, each representing one of the categories of spatial ability according to the typology proposed by Linn and Petersen (1985): a) the Vandenberg and Kuse (1978) paper and pencil version of the Shepard and Metzler (1971) Mental Rotation test, a measure of three-dimensional mental rotation requiring the subject to determine which two of four three-dimensional figures can be mentally rotated to match a target figure (the two distracters are mirror images of the targets); b) four items from a paper and pencil version (Harris, Hanky, & Best, 1977) of the Water Level test (Piaget & Inhelder, 1956), a measure of spatial perception requiring the subject to judge the correct orientation of fluid in a tilted container with respect to the horizon; c) the Block Design subtest of the WAIS-R (Wechsler, 1981), a measure of spatial visualization requiring the subject to manipulate red and white blocks to match a target pattern; and d) the first six items of the Embedded-Figures Test (Witkin, 1971), a measure of spatial visualization requiring the subject to find a geometrical figure embedded within a complex design.

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Although the subjects all came from the extreme upper range (93rd percentile and above) of the distribution of ACT scores, this did not preclude the possibility of differences in composite ACT scores among the sex by handedness groups. Because any such differences could confound the comparisons of the subgroups on the spatial tests, we tested for these differences with a two-way ANOVA. Although the results showed no main effects of sex or handedness, and no sex by handedness interaction (allps = .133 to .936), they did suggest a trend for males to have higher ACT scores. We therefore used composite ACT scores as a covariate in the analysis of performance on the spatial tests. The direction of the means is completely consistent with the hypothesis for women, since, for women, left-handers outperformed right-handers on all four tasks. For men, however, the direction of the means was only partly consistent with the hypothesis: right-handers outperformed left-handers on two tasks (Embedded figures and Mental rotation) but not on two others (Block Design and Water Level) (see Figures 1-4).

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A multivariate ANOVA was used to test for significant differences between the sex and handedness groups, with sex and handedness as between-group factors, and composite ACT as a covariate. Because performance on EmbeddedFigures was measured in time rather than number of correct items, as with the other tasks, this task was reverse-scored for the following statistical analyses. The results revealed a significant main effect of sex, F(4,48) = 2 . 7 2 , ~= .041. Univariate ANOVA’s revealed that the men had significantly higher scores than the women on Mental Rotation, F(1,55) = 7.01, p = .011. The next largest difference was on the Water Level task, although the difference was not significant, F(1,55) = 2.56, p = ,116. The main effect of handedness, however, was only marginally significant, F(4,48) = 2 . 1 9 , ~= .084. Univariate ANOVA’s showed that left-handers significantly outscored right-handers on Block Design, F(1,55) = 5.00, p = .OW, and by a marginally significant degree, F(1,55) = 3.48, p = .068, on Water Level.

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Standard Discriminant Coefficients

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The key analysis was of the handedness by sex interaction, but this proved to be non-significant. Because the univariate ANOVA’s, however, found significant sex and handedness differences on only some of the tasks, we performed a discriminant analysis (Bray & Maxwell, 1982; McCall, 1970). This test is more sensitive to the presence of the interaction because it provides a simultaneous estimate of the relationship between the four spatial measures and the sex by handedness factors. By addressing whether some multivariate weighing of the tasks was able to produce a sex by handedness interaction, it is possible to determine, from the standard discriminant function coefficients, which of the spatial measures contribute to the interaction and which do not. If significant, the means then can be examined to interpret the direction of the interaction. The results of this analysis disclosed a significant discriminant function for the sex by handedness interaction 0, < .05). Inspection of the standardized discriminant coefficients indicates that Embedded-Figures made the largest positive contribution to the discriminant function, Mental Rotation made the next largest positive contribution, and Water Level made a negligible contribution. Block Design, however, acted as a suppressor variable (i.e. Block Design

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correlated with a non-discriminating component of the positive contribution), making a small negative contribution (see Figure 5 ) . In summary, the men, as expected, did significantly better than the women on the Mental Rotation task. They also made higher scores on the Water Level task, consistent with earlier findings (Harris, Hanky, & Best, 1977), although the sex difference was not significant. By contrast, overall sex differences were absent on the Embedded-Figures and Block Design test. These results are consistent with Linn and Petersen's (1985) meta-analytic study, which found that males outperformed females on mental rotation and spatial perceptual tasks (such as Water Level) but not on spatial visualization tasks (such as EmbeddedFigures and Block Design). The results also would seem to be consistent with Harshman et al.'s (1983) analysis of the relationship of sex, handedness, and reasoning level to spatial ability, at least for that part of the relationship pertaining to persons with high reasoning ability. That is, in our own study, where only high academic achievers were considered, right-handed men performed better than left-handed men (at least on two of the tasks), and left-handed women performed better than righthanded women. Although the pattern of results was similar to that found by Harshman et al. (1983), the results of the discriminant analysis suggest that the sex by handedness interaction may be limited to particular spatial functions. Mental Rotation and Embedded-Figures, both of which contributed positively to the sex by handedness discriminant function, were the same type of measures included in the Harshman et al. study. They used two mental rotation tests (the Primary Mental Abilities test of Mental Rotation and the Vandenberg and Kuse (1978) adaptation of the Shepard-Metzler Mental Rotation test) and two disembedding tests (the Educational Testing Service's Hidden Patterns and Copying). They did not, however, include measures comparable to Water Level and Block Design, measures that in our study had a negligible effect and suppressor effect, respectively. The sex by handedness interaction among "high academic achievers," therefore, seems to be present for one spatial visualization measure and for a mental rotation measure but not for another spatial visualization measure and with only a very modest contribution from a spatial perception measure. This pattern therefore is inconsistent with Linn and Peterson's (1985) conclusion that Embedded Figures and Block Design represent the same category of spatial ability (i.e., spatial visualization). As we suggested earlier, the sex by handedness interaction presents a problem for a strictly socialization model of individual differences in cognitive

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ability. For this reason, it would seem that consideration should be given to neuropsychological models. We cited the Levy and Gur (1980) model as one that predicts a sex by handedness interaction in the same direction as that found in our own study and in the study by Harshman et al. (1983), at least for subjects with high reasoning ability. Their model, however, assumes bilateral spatial representation for males, a finding with, as yet, little support. Recently, however, Harshman and Hampson (1987) reported dichotic listening data for subjects in their study of reasoning ability. The results disclosed a sex by handedness interaction among high reasoners that is consistent with Levy and Gur’s (1980) model. The results also revealed that left-handed females, in comparison to right-handed females, had a more symmetrical pattern of performance for dichotically presented melodies, to go along with their better spatial scores. For males, the reverse was found: now it was the right-handers with the more symmetrical pattern of performance for the dichotically presented melodies, to go along with their better spatial scores. For the low reasoners the pattern of dichotic listening scores were reversed. Future explanations of individual differences in cognitive ability will have to address the fact of the interaction of handedness and sex, as well as the possible moderating influence of reasoning ability. They also must deal with the question why a sex by handedness interaction is present for some types of spatial measures and not others, and why, as Harshman et al. (1983) concluded, the interaction holds for individuals with high reasoning ability but not with low reasoning ability, and indeed, may even change direction in the latter group.

Conclusions The relationship between handedness and spatial ability turns out to be far more complex than it first appeared. The relationship evidently is moderated not only by the type of spatial task involved but by the subject’s sex and intellectual level as well. Our review, furthermore, has focused on manifest handedness and has not addressed such further variables as strength of handedness (e.g., Searleman, Hermann, & Coventry, 1984) and familial sinistrality (e.g., McKeever, 1989), both of which, as potential indexes of membership in one or another subgroup of left-handers, have been proposed to be related to spatial ability. Finally, most studies of the relationship between handedness and spatial ability have used handedness alone as an index of cerebral lateralization. Handedness, however, is at best only a rough index. More direct measures are

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needed to assess the relationship between cerebral lateralization and cognitive ability (see Lewis & Harris, 1988). The few new studies that incorporate this feature (e.g., Harshman & Hampson, 1987; Kimura & D’Amico, 1989) have already demonstrated the value of this methodology. Nor is the lateral axis of brain organization the only relevant dimension of individual differences. The anterior-posterior dimension also shows variability of cognitive representation that is related to relevant subject variables (see Kimura, 1983, 1987; Leris & Christiansen, 1989; Mateer, Polen & Ojemann, 1982). Variation in the organization of subcortical structures, may, as well, contribute to individual differences in cognitive ability, though this has not been adequately investigated yet. Finally, the possibility that there may be differences between left- and righthanders in callosal mechanisms should be considered (Kertesz, Polk, Howell, & Black, 1987; Witelson, 1985). All of these variables will have to be incorporated into any comprehensive account of the relationship between cognitive ability and cerebral organization.

References Annett, M. (1967). The binomial distribution of right, mixed, and lefthandedness. Quarterly Journal of Expeninental Psychology, 19, 327-333. Annett, M., & Kilshaw, D. (1982). Mathematical ability and lateral asymmetry. Cortex, 18, 541-568. Ben-Chaim, D., Lappon, G., & Houang, R.T. (1986). Development and analysis of a spatial visualization test for middle school boys and girls. Perceptual and Motor Skills, 63, 659-669. Benbow, C.P. (1986). Physiological correlates of extreme intellectual precocity. Neuropsyckologia, 24, 719-725. Benbow, C.P., & Benbow, R.M. (1984). Biological correlates of high mathematical reasoning ability. In G.J. DeVries, J.P.C. DeBruin, H.B.M. Uylings, & M A . Corner (Eds.), Progress iit Brain Research, 61, 469-470. Berry, GA., Hughes, R.L., & Jackson, L.D. (1980). Sex and handedness in simple and integrated task performance. Percephial and Motor Skills, 51,807812. Bradshaw, J.L. & Nettleton, N.C. (1981). The nature of hemispheric specialization in man. The Behavioral and Brain Sciences, 4, 51-91. Bradshaw, J.L., Nettleton, N.C., & Taylor, M.J. (1981). Right hemisphere language and cognitive deficits in sinistrals? Neuropsyclologia, 19, 113-132. Bray, J.H. and Maxwell, S.E. (1982). Analyzing and interpreting significant MANOVAs. Review of Educational Research, 52, 340-367. Briggs, G.G. & Nebes, R.D. (1975). Patterns of hand preference in a student population. Cortw 11, 230-238.

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Brigs, G.G., Nebes, R.D., & Kinsbourne, M. (1976). Intellectual differences in relation to personal and family handedness. QuarterlyJournal ofExperimental PvcholoD, 28, 591-601. Burnett, S.A., Lane, D.M. & Dratt, L.M. (1982). Spatial ability and handedness. Intelligence, 6, 57-68. Casey, M.B., Brabeck, M.M., & Ludlow, L.H. (1986). Familial handedness and its relation to spatial ability following strategy instructions. Intelligence, 10, 389-406. Cattell, R.B. (1971). Abilities: Their sttucture, growth, and action. Boston: Houghton-Mifflin. Corballis, M.C., & Sergent, J. (1989). Hemispheric specialization for mental rotation. Cortex, 25, 15-25. Corballis, M.C., & Sergent, J. (1988). Imagery in a commissurotomized subject. Neuropsyclologia, 26, 13-26. Deutsch, G., Bourbon, W.T., Papinicolaou, A.C., & Eisenberg, H.M. (1988). Visuospatial tasks compared via activation of regional cerebral blood flow. Neuropsychologia, 26, 445-452. Eme, R., Stone, S., Izral, R. (1978). Spatial deficit in familial left-handed children. Perceptual and Motor Skills, 47, 919-922. Fagen-Dubin, L. (1974). Lateral dominance and development of cerebral specialization. Cotter, 10, 69-74. Fairweather, H. (1982). Sex differences: little reason for females to play midfield. In J.G. Beaumont (Ed.), Divided visual field studies of cerebral organizafion (pp. 147-192). London: Academic Press. Fennell, E., Satz, P., VanDenAbell, T., Bowers, D., & Thomas, R. (1978). Visuospatial competency, handedness, and cerebral dominance. Brain and Language, 2, 206-214. Fischer, S.C., & Pellegrini, J.W. (1988). Hemispheric differences for components of mental rotation. Brain and Cognition, 7, 1-15. Flick, G.L. (1966). Sinistrality revisited: A perceptual-motor approach. Child Development, 37, 613-622. French, J.W. Ekstrom, R.B. & Price, LA. (1963). Kit of reference tests for cognitive factors. Princeton, NJ: Educational Testing Service. Gibson, J.B. (1973). Intelligence and handedness. Nature, 243, 482. Gilbert, C. (1977). Nonverbal perceptual abilities in relation to left-handedness and cerebral lateralization. Neuropsycltologia, 15, 779-791. Goodglass, H., & Quadfasel, F.A. (1954). Language laterality in left-handed aphasics. Brain, 77, 521-548. Gregory, R.J., Alley, P., & Morris, L. (1980). Left-handedness and spatial reasoning abilities: The deficit hypothesis revisited. Intelligence, 4, 151-159. Halpern, D.F. (1986). Sexdifferences in cognitive abilities. Hillsdale, NJ: Erlbaum. Hardyck, C. (1977). Handedness and part-whole relationships: A replication. Cortex, 13, 177-183. Hardyck, C. & Petrinovich, L.F. (1977). Left-handedness. Psychological Bulletin, 84, 385-404.

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Hardyck, C., Petrinovich, L. 8i Goldman, R. (1976). Left-handedness and cognitive deficit. Corta 12, 266-279. Harris, L.J. (1978). Sex differences in spatial ability: possible environmental, genetic, and neurological factors. In M. Kinsbourne (Ed.) Asyntmetrical function of the brain (pp. 405-522). Cambridge: Cambridge University Press. Harris, L.J. (1981). Sex-related variations in spatial skill. In L.S. Liben, A.H. Patterson, & N. Newcombe (Eds), Spatial representation and behavior across the life span: Theory arid application (pp 83-125). New York: Academic Press. Harris, L. J., & Carlson, D. F. (1988). Pathological left-handedness: An analysis of theories and evidence. In D.L. Molfese & S.J. Segalowitz (Eds.), Brain lateralization in children (pp. 289-372). New York: Guilford Press. Harris, L.J., Hanky, C., & Best, C.T. (1977). Conservation of horizontality: Sex differences in sixth-graders and college students. In R.C. Smart & M.S. Smart (Eds.), Readings in child development and relationship (2nd ed.) (pp. 375-387). New York: Macmillan. Harshman, R.A. & Hampson, E. (1987). Normal variation in human brain organization: relation to handedness, sex and cognitive abilities. In D. Ottoson (Ed.), Weruler-GrenCenter International Symposium Series: Vol. 47. Duality and Utiity of the Bruin (pp. 83-99). Houndmills, England: MacMillan. Harshman, RA., Hampson, R., & Berenbaum, SA. (1983). Individual differences in cognitive abilities and brain organization: Part I. Sex and handedness differences in ability. Canadian Journal of Psychology, 37, 144-192. Hermann, D.J. & Van Dyke, K. (1978). Handedness and the mental rotation of perceived patterns. Corta 14, 521-529. Hicks, R.E., & Beveridge, R. (1978). Handedness and intelligence. Cortex, 14, 304-307. Inglis, J., & Lawson, J.S. (1984). Handedness, sex, and intelligence. Cortex, 20, 447-451. Johnson, E.S., & Meade, A.C. (1987). Developmental patterns of spatial ability: An early sex difference. Child Development, 58, 725-740. Johnson, 0. & Harley, C. (1980). Handedness and sex differences in cognitive tests of brain laterality. Cotter, 16, 73-82. Just, M.A., & Carpenter, P.A. (1985). Cognitive coordinate systems: Accounts of mental rotation and individual differences in spatial ability. Psychological Review, 92, 137-172. Kashihara, E. (1979). Lateral preference and style of cognition. Perceptual and Motor Skills, 48, 1167-1172. Kertesz, A., Polk, M., Howell, J., & Black, S.E. (1987). Cerebral dominance, sex, and callosal size in MRI. Neurology, 37, 1385-1388. Kimura, D. (1983). Sex differences in cerebral organization for speech and praxic function. Canadian Journal of Psychology, 37, 326-332. Kimura, D., & D’Amico, C. (1989). Evidence for subgroups of adextrals based on speech lateralization and cognitive patterns. Neriropsychologia, 27, 977986. Kimura, D. & Harshman, R.A. (1984). Sex differences in brain organization for verbal and non-verbal functions. In G.J. DeVries, J.P.C. DeBruin, H.B.M.

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Uylings, & MA. Corner (Eds.), Sex differences in the brain: The relation between structure and function: Vol. 61. Progress in brain research, (pp. 423-441). Amsterdam: Elsevier Science. Kutas, M., McCarthy, G., & Donchin, E. (1975). Differences between sinistrals’ and dextrals’ ability to infer a whole from its parts: A failure to replicate. Neuropsychologia, 13, 455-464. Lansdell, H. (1969). Verbal and nonverbal factors in right-hemisphere speech: Relations to early neurological history. Journal of Comparative and Physiological Psychology, 69, 734-738. Lansky, L.M., & Peterson, J.M. (1985). Some comments on Shettel-Neuber and O’Reilly’s “Handedness and career choice: another look at supposed left/right differences” (1983). Perceptual and Motor Skills, 60,141-142. Levy, J. (1969). Possible basis for the evolution of lateral specialization of the human brain. Nature (London), 224,614-615. Levy, J. (1974). Psychobiological implications of bilateral asymmetry. In S.J. Dimond & J.G. Beaumont (Eds.), Hemisphericfunction in the human brain (pp. 121-183). New York: John Wiley & Sons. Levy, J., & Gur, R.C. (1980). Individual differences in psychoneurological organization. In J. Herron (Ed.), Neuropsychology of left-handedness (pp. 199-210). New York: Academic Press. Lewis, R.S., & Christiansen, (1989). Intrahemispheric sex differences in the functional representation of language and praxic functions in normal individuals. Brain and Cognition, 9, 238-243. Lewis, R.S., & Harris, L.J. (1988). The relationship between cerebral lateralization and cognitive ability: Suggested criteria for empirical tests. Brain and Cognition, 8, 275-290. Linn , M.C., & Petersen, A.C. (1985). Emergence and characterization of sex differences in spatial ability: A meta-analysis. Child Development, 56, 14791498. Maccoby, E.E., & Jacklin, C.N. (1974). The psychology of sex diflerences. Stanford, Calif.: Stanford University Press. Mateer, C.A., Polen, S.B. & Ojemann, GA. (1982). Sexual variation in cortical localization of naming as determined by stimulation mapping. The Behavioral and Brain Sciences, 5, 310-311. McCall, R.B. (1970). The use of multivariate procedures in developmental psychology. In D.H. Mussen (Ed.), Cannichael’sntanual of child psychology (3rd Ed.) New York: Wiley. McGee, M. (1976). Laterality, hand preference, and human spatial ability. Perceptual and Motor Skills, 42, 781-782. McGlone, J. (1980). Sex differences in human brain asymmetry: a critical survey. Behavioral and Brain Sciences, 3, 215-227. McGlone, J. & Davidson, W. (1973). The relation between cerebral speech laterality and spatial ability with special reference to sex and hand preference. Neuropsychologia, 11, 105-113.

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McKeever, W.F. (1989). Handedness, language laterality, and spatial ability. In Cerebral lateralily: Theory and research: 77te Toledo Symposium. New York: Erlbaum. Mebert, C.J., & Michel, G.F. (1980). Handedness in artists. In J. Herron (Ed.), Neuropsychology ofleft-handedness (pp 273-278).New York: Academic Press. Miller, D. (1971). Handedness and the pattern of human ability. British Journal Of P ~ c h o l o 62, ~ , 111-112. Milner, (1974). Hemispheric specialization: Scope and limits. In F.O. Schmitt and F.G. Worden (Eds.) 77ie Neurosciences Third Study Progrant. Cambridge, MA: MIT Press. Nagae, S. (1985a). Handedness and sex differences in the processing manner of verbal and spatial information. American Journal of Psychology, 98, 409-420. Nagae, S. (1985b). Handedness and sex differences in selective interference of verbal and spatial information. Journal of Experintental Psychology: Human Perception and Performance, 11, 346-354. Nebes, R.D.(1971). Handedness and the perception of part-whole relationships. Cortex, 7, 350-356. Newcombe, F., & Ratcliff, G . (1973). Handedness, speech lateralization, and ability. Neuropsychologia, 11, 399-407. Oldfield, R.C. (1971). The assessment and analysis of handedness: the Edinburgh Inventory. Neuropsychologia, 9, 97-114. Orme, J.E. (1970). Left-handedness, ability and emotional instability. British Jountal of Social and Clinical Psychology, 9, 87-88. Peterson, J.M. (1979). Left-handedness: differences between student artists and student scientists. Perceptual and Motor Skills, 48, 961-962. Peterson, J.M., & Lansky, L.M. (1974). Left-handedness among architects: Some facts and speculations. Perceptual and Motor Skills, 38, 547-550. Peterson, J.M., & Lansky, L.M. (1978). Left-handedness among architects: partial replications and some new data. Perceptual and Motor Skills, 45, 1216-1218. Peterson, J.M., & Lansky, L.M. (1980). Success in architecture: Handedness and visual thinking. Percephial and Motor Skills, 50, 1139-1143. Peterson, J.M., & Lansky, L.M. (1983). Success in architecture: a research note. Percephtal and Motor Skills, 57, 222. Piaget, J., & Inhelder, B. (1956). 7he child’s conception of space. New York: Humanities Press. Rasmussen, T., & Milner, B. (1977). The role of early left-brain injury in determining lateralization of cerebral speech functions. Annals of the New York Academy of Sciences, 299, 355-369. Ratcliff, G. (1979). Spatial thought, mental rotation, and the right cerebral hemisphere. Neuropsyc)~ ologia, I 7, 49-54. Raven, J.C. (1960). Guide to the standard progressive matrices. London: H.K. Lewis. Sanders, B., Soares, M. D., & D’Aquila, J.M. (1982). The sex difference on one test of spatial visualization: A nontrivial difference. Child Developntent, 53, 1106-1110.

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Sanders, B., Wilson, J.R., & Vandenberg, S.G. (1982). Handedness and spatial ability. Cortex, 18, 79-90. Satz, P., Orsini, D.L., Saslow, E., & Henry, R. (1985). The pathological lefthandedness syndrome. Brain and Cognition, 4, 27-46. Searleman, A., Hermann, D.J., & Coventry, A.K. (1984). Cognitive abilities and left-handedness: An interaction between familial sinistrality and strength of handedness. Intelligence, 8, 295-304. Segalowitz, S.J. & Bryden, M.P. (1983). Individual differences in hemispheric representation of language. In S.J. Segalowitz (Ed.) Language Functions and Brain Organization (pp. 341-372). New York: Academic Press. Sheehan, E.P. & Smith, H.V. (1986). Cerebral lateralization and handedness and their effects on verbal and spatial reasoning. Neuropsychologia, 24, 531-540. Shepard, R.N., & Metzler, J. (1971). Mental rotation of three-dimensional objects. Science, 171, 701-703. Sherman, J. (1979). Cognitive performance as a function of sex and handedness: An evaluation of the Levy hypothesis. Psychology of Women Quarterly, 3, 378-390. Shettel-Neuber, J., & O’Reilly, J. (1983). Handedness and career choices: another look at supposed left/right differences. Perceptual and Motor Skills, 57, 391-397. Silverman, A.J., Adavai, G. & McGough, W.E. (1966). Some relationships between handedness and perception. Journal of Psychosomatic Research, 10, 151-158. Simpson, M.T., Lansky, L.M., Senter, R.J. & Peterson, J.M. (1983). Figure reversals and handedness: a research note. Perceptual and Motor Skills, 57, 326. Sperry, R.W. (1974). Lateral specialization in the surgically separated hemispheres. In F.O. Schmitt & F.G. Worden (Eds.), The neurosciences: 77iird study program (pp. 5-19), Cambridge, Mass.: MIT Press. Teuber, H.L. (1974). Why two brains? In F.O. Schmitt and F.G. Worden (Eds.) 77ie Neurosciences Third Study Program. Cambridge, MA: MIT Press. Vandenberg, S.G., & Kuse, A.R. (1978). Mental rotation: A group of tests of three-dimensional spatial visualization. Perceptual and Motor Skills, 47,599604. Wechsler, D. (1981). WecltslerAdult Intelligence Scale -- Revised. New York: Psychological Corporation. Witelson, S.F. (1985). The brain connection: the corpus callosum is larger in left-handers. Science, 229, 665-668. Witkin, H. A. (1971). The emheddedfigures test. New York: The Psychological Corporation. Wittenborn, J.R. (1976). Correlates of handedness among college freshmen. Journal of Educational Psychology, 37, 161-170. Yen, W.M. (1975). Independence of hand preference and sex-linked genetic effects on spatial performance. Perceptual and Motor Skills, 41, 311-318.

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LEFT-HANDEDNESS Behavioral Implications and Anomalies, S. Coren (Editor) 0 Elsevier Science Publishers B.V. (North-Holland), 1990

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Handedness and Its Relationship to Ability and Talent Michael W. O’Boyle and Camilla Persson Benbow Iowa State University Handedness, as recently defined by Kee (in press), is ”the preference shown by clddren and adults for left versus right hand use in unimanual tasks.” A variety of questionnaires have been developed to measure such preference, with the Edinburgh Handedness Inventory (Oldfield, 1971) being the most famous and widely employed. At the population level, it has been estimated that approximately 85% - 90% of individuals are right-handed, while the remaining 10% - 15% are either left-handed or without hand preference (Porac & Coren, 1981; Springer & Deutch, 1989). For present purposes, handedness is important in that it serves as a potential neuropsychological marker of underlying brain development and may also reflect the manner in which cognitive functions are localized to the left or right cerebral hemispheres. This proposed link between brain function and hand dominance raises a question of particular interest: Does hand preference systematically relate to individual differences in intellectual abilities and/or talents?

The Left Hand Deficit Hypothesis As early as 1920, investigators had suggested that left-handedness was associated with minor brain damage (MBD) (Gordon, 1920). Support for this position emanated from a number of studies indicating that the incidence of lefthandedness is unusually high in the mentally retarded (Hildreth, 1949), the epileptic (Bingley, 1958), and the learning disabled (Bishop, 1983; Neils Rr Aram,

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1986; Orsini, 1986; Satz & Fletcher, 1987) These clinical populations often exhibit some form of cortical damage, and, as in the case of dyslexia, may have brains characterized by abnormal neural cytoarchitecture (Best, 1985; Duffy, Denkla, Bartels & Sandini, 1980; Geschwind & Galaburda, 1984; Harris & Carlson, 1988). In light of the perceived association between left hand dominance and MBD, Paul Bakan (Bakan, 1971) and his colleagues (Bakan, Dibb & Reed, 1973) hypothesized that left-handedness was the by-product of traumatic pregnancy and/or complication during the birthing process. Perinatal hypoxia and low oxygen supply were implicated as its cause. From their perspective, all lefthandedness is seen as "pathological" in origin, occurring as a by-product of early injury to the left cerebral hemisphere (LH) (cf. Satz, 1973; Orsini, 1986). This selective impairment is thought to induce a shift in hand preference, away from the natural predominance of the right hand and more towards the nondominant left hand. Empirical support for Bakan's pathological left-handedness hypothesis is mixed at best (Harris and Carlson, 1988). Rather recently, it has been challenged by a more moderate position, which suggests that two different types of left-handers develop from two independent causal factors: 1) those who are left-handed as a result of birth trauma, and 2) those who are "natural" lefthanders as dictated by genetic predisposition (Satz, 1973; Orsini, Saslow & Henry, 1985; For a discussion of genetic bases of handedness see Annett, 1964, 1974, 1981; Levy & Nagylaki, 1972). A lengthy discussion of each of these positions concerning origins of lefthandedness is beyond the scope of the present paper (but see Harris and Carlson, 1988; 1 9 this ~ volume). An issue of particular interest, however, is whether or not those left-handers not classified as clinically disabled show subtle deficits in higher-order intellectual functioning as compared to right-handers. The notion here is that determination of hand preference may be related to brain development and underlying patterns of cerebral organization. Specifically,Levy (1969) noted that a large percentage of left-handers have linguistic functions represented in both cerebral hemispheres, as opposed the prototypical left hemisphere (LH) localization of such functions. Hence, she theorized that in left-handed individuals, the neural space allotted for the spatial capacities in the right hemisphere (RH) may be encroached upon as a result of the bilateral development of the language faculties. This uncharacteristic "crowding" might impair neural development of the RH, and manifest itself as a decrement in

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spatial ability, along with a corresponding enhancement of the verbal capacities. Several studies have indeed demonstrated a reliable deficit/enhancement pattern of this sort in the left-handed. Levy (1969) reported that left-handers do less well than right-handers on the performance component (e.g. block design, picture completion, object assembly, etc.) of the Wechsler Adult Intelligence Scale (WAIS), and slightly better than right-handers on the verbal component (e.g. vocabulary, simple abstraction, etc.). Her subjects, however, were graduate students in science and engineering who had well above-average scores on both components. Hence, it may be that this finding is not generalizable to the public at large. Similarly, Johnson and Harley (1980), using college undergraduates as subjects, found that strongly left-handed students performed more poorly than mixed-handed and strongly right-handed students on the Space Thinking (Flags) Test (a measure of spatial ability), but exhibited superior performance on the Mill Hill Vocabulary Scale. Miller (1971) reported that left-handed college students were equivalent to right-handers in verbal 10, but inferior in spatial IQ. Freedom and Rovegno (1981) found lower spatial ability scores for left-handers on the Vandenburg Mental Rotation Test, while Nebes (1971) identified a deficit in left-handers' ability to solve spatial "part-whole" relationships. Also, Hicks & Beveridge (1978), using a college student sample, found lower scores for left- as compared to right-handers on the nonverbal Cattell Culture Fair Intelligence Test. More recently, decrements in spatial ability for left-handed individuals have been reaffirmed in a series of experiments conducted by McKeever, Rich, Deyo & Conner, 1987). In deference to the investigations cited above, numerous studies have failed to verify the left-handed deficit hypothesis. Hardwyck, Petrinovich & Goldman (1976) conducted a large scale cross-sectional study (N = 7,688) in which no difference in either the pattern or overall level of IQ performance between rightand left-handers could be discerned. Using smaller sample sizes, neither Ledlow, Swanson & Carter (1972) nor Fagin-Dubin (1974) could find a reliable difference in IQ between the two handedness groups. Neither Roberts & Engle (1974) who studied children ages six to eleven, nor Newcombe & Ratcliffe (1973) who used adult subjects, were able to detect differences in WISC vocabulary or block design scores of the two handedness groups. In an earlier review paper, Hardyck & Petrinovich (1977) report that of the eight studies they identified that measured intelligence, seven found n o difference in IQ between left- and righthanders and of the fourteen studies involving reading ability, thirteen reported no difference between the two handedness groups. Additional features to verify the left-handed deficit hypothesis include, but are not limited to, Ashton (1982);

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Birkett (1980); Brigs, Nebes & Kinsbourne (1976); Fennel, Satz, VanDenAbell, Bowers & Thomas (1978); Gibson (1973); Gilbert (1977); Hardyck (1977a,b); Inglis & Lawson (1984); Kashihara (1979); McKeever & VanDeventer (1977); Satz & Fletcher (1987); Sheehan Lyr Smith (1986); and Sunseri (1982). In summary, the data in support of the left-handed deficit hypothesis are seemingly outweighed by the number of studies that fail to find a difference between left- and right-handers in either general intellectual ability or in any specialized verbal or spatial capacities.

Factors Moderating Relations of Handedness with Ability In the previous section we highlighted the conflicting nature of the published reports on the cognitive deficits in left-handers. Although a majority of those studies revealed no difference between right- and left-handed individuals, when such differences did emerge they tended to favour right-handers in tests of spatial ability and left-handers in measures of verbal ability. It may be that some of the contradictory findings are due to various procedural variations and/or methodological shortcomings, including small sample sizes (especially of left- or mixed-handers), the differing nature of the cognitive tests utilized, the lack of adequate handedness measures, the tendency to view handedness as a dichotomous or trichotomous variable, and, finally, underestimating the effects of familial sinistrality (Burnett, Lane & Dratt, 1982). Thus, some sizeable differences in cognitive abilities between various sub-groups of right- and lefthanders may yet be found. Below we consider several factors that may serve to moderate the relationship between handedness and intellectual ability.

Strength of Handedness, Familial Sinistrality and Sex Among those variables thought to moderate the relationship between hand dominance and ability, strength of handedness and familial sinistrality have probably received the most attention. (McKeever, in press).

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Johnson and Harley (1980), though finding no difference between handedness groups on the verbal and performance subcomponents of the WAIS, did find that strong left-handers of both sexes obtained higher verbal scores (Mill Hill Vocabulary Scale) but lower spatial scores (Flags Test) than did right- or mixed-handers. In partial support of this pattern, Carter (1977), using a variety of tasks, found a tendency for strongly left-handed (as well as right-handed) individuals to have higher verbal scores, while mixed-handers tended to have significantly higher spatial scores. Klintenberg, Levander & Schalling (1987) report that strongly left-handed girls showed lower performance on a perceptual maze task than any other handedness by gender group. In contrast to the above findings, Tan (1988), using Cattel's Culture Fair Intelligence test (primarily a measure of nonverbal reasoning ability), observed moderate left-handedness to be associated with the highest scores. Also, Sanders, Wilson & Vandenberg (1982) found that in three ethnic groups, strongly lefthanded males had liigller spatial scores than the strongly right-handed males on the Sheppard-Metzler Rotation task and the ETS Card Rotation Test, while the opposite pattern was found for females. In addition, among those individuals of Chinese or Japanese (but not European) ancestry, strongly left-handed subjects had higher spatial scores that both right-handed and ambidextrous subjects. Again, the direction of the finding was opposite for males and females. No difference between handedness, sex or ethnic group membership were found for measures of verbal ability. Thus, when strength of hand preference is considered, results from three of the five studies identified conform reasonable well to the pattern of cognitive strengths and weaknesses hypothesized by the left hand deficit hypothesis. None of the studies measured familial sinistrality, however, and this may be problematic. Familial Sinistntlity

Familial sinistrality (FS), in the most general sense, refers to individuals who have left-handed or ambidextrous relatives. Briggs, et al. (1976) were among the first to report on both the strength of handedness and FS within the context of the same study. They found that moderately strong left- and right-handed subjects with a positive history of familial sinistrality (FS +) had slightly lower WAIS IQ scores than left- and right-handers with no evidence of familial

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sinistrality (FS-). Yet, on a series of specific ability tests, which included the Scholastic Aptitude Test - Mathematics (SAT-M) and Verbal (SAT-V), FS did not relate to test performance. Also, using the WAIS, Bradshaw, Nettleton & Taylor (1981) found that among strongly left- and right-handed subjects, only FS t left-handers scored lower on both the verbal and performance subscales. In particular, left-handed FSt males had performance IQ scores that were significantly lower than the other groups. Yeo & Cohen (1983) found that even among right-handers, the presence of FS was related to performance decrements in spatial ability. Searle, Herrmann & Coventry (1984) examined SAT scores of left-handed college students and found an interaction between FS and strength of handedness for aptitude test performance. Specifically, FS did not relate to aptitude for weakly to moderately left-handed students. However, strong left-handers with FS t scored much lower on the composite SAT than did strongly left-handed FSindividuals. Yet, interestingly, the strongly left-handed FS- subjects had the highest scores on the SAT-V. Also, Murray (1988), using a tactile motor task with blind-folded subjects, reported that FS t left-handers were primarily responsible for the lower performance of the left-handed group as compared to right-handers. Hence, the results of these five studies seem to indicate that if cognitive deficits are found, it is typically FS t left-handers who exhibit a deficit in spatial ability. None of these studies, however, controlled for the potential effects of sex.

Sex McChee (1976) reports that right-handed females outperform left-handed females in the Shepard-Metzler Mental Rotations task, although no difference between the handedness groups was observed for males. Yen (1975) had high school students perform a variety of spatial tests and found evidence of higher spatial ability in right-handed as compared to left-handed males but no difference between left- and right-handed females. In a somewhat more complex study, Burnett, et al. (1982) examined sex, hand dominance, strength of handedness and FS in the context of a single analysis. Spatial ability was found to be systematically related to each of these variables individually and in combination. Using the Vandenberg Test of Spatial Ability, the lowest scores were obtained by strongly left-handed or strongly right-

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handed individuals, with the highest scores predominantly earned by right-handed F S t individuals. By way of summary, it appears in this study that F S t is negatively related to spatial ability in right-handers with the pattern somewhat more pronounced in males than females. Casey, Brabeck & Ludlow (1986) compared the performance of familial nonright-handers ( i s . left-handers, mixed-handers and FS t right-handers) with that of FS- right-handers on a mental rotation task. Each group was instructed to use one of two different spatial strategies. Their results suggested that "familial nonright-handers may be stronger in the ability to use one spatial strategy, transformation of mental images, and weaker on a second, reorientation in relation to left-right cues" (p. 389). Familial nonright-handers apparently do not use their enhanced rotation ability spontaneously. Rather, their superiority emerged only after prompting and instruction. Moreover, Casey and Brabeck (1989) reported that in tests of spatial ability, right-handed females who have nonright-handed relatives, along with extensive spatial experiences (i.e. via toys) score higher than other females and do not score substantially lower than males. The studies conducted by Casey and colleagues, when taken in composite, imply that, given the genetic potential for high spatial ability (i.e. in their view, nonright-handedness), such potential will only be realized if individuals are given the opportunity to develop specific cognitive strategies (Casey & Brabeck, 1989).' Healey, Rosen Gerstman, Steiner & Mattis (1982) found a different pattern than that described above. In their study, thepresence (rather than the absence) of FS corresponded to enhanced spatial abilities (e.g. mental rotation) in females and decreased performance in males. Generally speaking, the effects were more pronounced among left- than right-handers. This pattern is consistent with the report by Schwartz (1983) who found that left-handed, left-eyed females (who may have a higher incidence of FS+ in general, see Bayless, 1981) exhibited better performance at solving mazes than right-handed, left-eyed females. Kocel (1977) found FS t to be related to higher spatial scores in females and lower

'

The findings of Casey and colleagues may relate to McKeever's observation (1986) that language laterality (as measured by dichotic listening techniques) separates handedness groups more distinctly of only responses from the second half of the trial sequence are analyzed. We t o o have found (see O'Boyle & Benbow, submitted) that responses on the second set of a dichotic listening task more clearly distinguish patterns of laterality in gifted as compared to control subjects. Such results suggest that strategies are developed with practice and that failure to find differences in laterality patterns among various handedness groups may be related to this factor. Thus investigators need to provide ample opportunity for subjects to develop such optimal strategies.

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spatial scores in males. Further, Searleman, et al. (1984) detected that FS + in left-handers was associated with poorer performance on the SAT-M and SATV for males, while for females, a decrement was found only on the SATV. McKeever, Seitz, Hoff, Marino & Diehl (1983) also studied spatial ability among right-handers differing in FS and sex. They too found an interaction between sex and FS on spatial ability that was small but reliable. Specifically, FS-females and FS t males exhibited a higher level of spatial ability than the other two groups. FS-females demonstrated spatial ability that was equal to that of FS- males but lower than F S t males. McKeever (1986) attempted to replicate these findings but could do so only for right-handed females. For the males in this replication, F S t and FS- individuals scored similarly to one another, with only a slight advantage for the FS- group. In fact, McKeever (1986) reports that FS- related favourably to spatial ability in all but the lefthanded FS- females. In an additional study, Marino & McKeever (1989) corroborate most of the original McKeever, et. a1 (1983) results. Moreover, Marino (1983), studying left-handers, found that an interaction between FS and sex related positively to spatial ability but varied considerably as a function of the degree of handedness.

Familial Sinistrality and Brain Lateralization The findings on FS are particularly interesting as they may relate to underlying brain organization. Several studies have been conducted in an effort to identify a relationship between FS, hand preference and patterns of cerebral lateralization (e.g. Hannay & Malone, 1976; Hardyck & Petrinovich, 1977; Hecaen & Sauget, 1971; Hicks, 1975; Hines & Satz, 1973; Kee, Bathurst & Hellige 1983; McKeever & Gill, 1972; Newcombe & Ratcliffe, 1973; O’Boyle & Hoff, 1987; Satz, Achenbach & Fennel, 1967; Semmes, 1969; Varney & Benton, 1975; Warrington & Pratt, 1973; Zurif & Bryden, 1969). By way of summary, these studies tend to suggest that FS is associated with a diffuse (perhaps bilateral) pattern of lateralization for cognitive functions, particularly with regard to language. An interesting relationship between language lateralization and handwriting posture has been proposed by Levy & Reid (1976). They hypothesized that an inverted handwriting posture (is. the hand positioned above the line of writing) was indicative of ipsilateral (or, perhaps bilateral) hemispheric control of linguistic functions while the non-inverted posture (is. the hand positioned below

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the line of writing) suggested that the language faculties were under strict contralateral hemispheric control. Gregory, Alley & Morris (1980) have demonstrated that left-handers who write with an inverted handwriting posture score lower than right-handers on the DAT Space Relations Test. It should be noted, however, that the work of Searleman, Tweedy & Springer (1979) among others, suggests that the sex of subjects rather than handwriting posture may play a more prominent role in the determination of underlying patterns of lateralization and their subsequent relationship to cognitive abilities. With regard to the possible combinatorial effects of sex and FS on hemispheric organization and ability, McKeever and Hoff (1981) and McKeever, et al. (1983) studied language lateralization in right-handers using the Object Naming Latency task. A sex by FS by visual field interaction was found with FSfemales and F S t males showing a tendency towards bilateral or diffuse localization of verbal functions. Moreover, Hecaen, DeAgostini & MonzonMontes (1981) report a substantially lower rate of aphasia following LH lesions in FS- females and FS t males. The findings of each of these studies suggest less strict LH dominance for language in FS- females and FS t males. In keeping with these findings, Kilshaw & Annett (1983) have hypothesized that strong LH lateralization of language is achieved by handicapping the development of the RH. Thus left-handers who exhibit weak patterns of lateralization may have escaped the impact of this inhibitory consequence, thereby having their spatial abilities preserved. It might then follow that individuals with less strict lateralization of language to the LH would show the highest spatial ability. In fact, McKeever et al. (1983) report just this pattern, with FS- females and F S t males performing better than FS+ females and FSmales on a spatial visualization task. A similar conclusion is reached by Burnett, et al. (1982). In direct contrast, however, Levy (1969; 1974), Berenbaum (1977) and Harshman, Hampson & Berenbaum (1986) suggest that strict lateralization of the verbal capacities to the LH may lead to enhanced spatial ability. Presumably, this increased spatial capacity is the by-product of two factors: 1) R H cortical space that is typically reserved for {he development of the spatial faculties is not sacrificed owing to the bilateral representation of the language faculties and, 2) with the hemispheres being relatively insulated from one another, the potentially disruptive effects of interhemispheric "cross-talk'' during spatial processing can be greatly minimized. Unfortunately, the empirical findings on this issue are not consistent. For example, several studies have tested the influence of FS on language laterality via

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dichotic listening tasks and failed to find any relationship (Lake & Bryden, 1976; Piazza, 1980). In these studies actual strength of handedness was not monitored, hence the results may not be directly comparable to other research (e.g. McKeever, et al. 1983). In any event, as noted by Marino (1983), Searleman, et al. (1984) and others, it seems that of the two left-handed FS groups, it is the FS t left-handers who are generally less able with regard to spatial ability. The above results were obtained using visual and auditory tasks. Some corroborating evidence from the tactile modality comes from Tinkcom, Obrzut & Poston (1983) who assessed lateralization of language and spatial capacities by using a variant of the Whitelson (1974) dihaptic stimulation task ( i s . the palpation of three dimensional letters and nonsense forms. The effects of sex, handedness and FS were monitored. They found the right hand to be superior for all groups when performing the letter matching task, a pattern suggestive of LH lateralization for language. A different pattern was noted on the nonverbal task. Left-handed, FS t males showed superior performance with their left hand, while right-handed FS- females exhibited better performance when using their right hand. The latter finding implies that some spatial functions are localized to the LH in FS- females. Also, O’Boyle & Hoff (1987) had left- and righthanded males and females mirror trace the outline of random shapes using their dominant and nondominant hands. Females performed equally well with either hand, suggesting that the spatial capacities necessary to perform the tracing task were bilaterally represented. Males of either hand preference, however, exhibited superior performance when using their nondominant hand, as if the spatial capacities were more strictly lateralized to the contralateral hemisphere. Post-hoc analyses revealed that in left-handed males, FS+ was related to LH (rather than RH) localization of such spatial faculties. The impact of FS, hand dominance, strength of handedness, sex and brain laterality on ability level is complex and difficult to discern. Nonetheless, in a general sense there is some evidence to suggest that the presence of FS is negatively related to spatial ability, except perhaps in right-handed males. The lack of consistency among the various studies demands a cautious interpretation,however, and may reflect the fact that others assert unspecified variables are at work to complicate the relationship.

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Immune Disorders Geschwind & Galaburda (1987) have suggested that immune disorder susceptibility (ID), along with FS and left-handedness are signs of anomalous lateralization. Rich and McKeever (submitted, a) found that among left-handed females, the ID t were significantly less lateralized than ID- individuals. Yet ID had no apparent effect on language localization in left-handed males as they exhibited the prototypical pattern of LH language lateralization, irrespective of ID status. Among right-handed subjects, it was suggested that sex, FS and ID interacted to shift language laterality towards a more bilateral pattern of representation. Rich and McKeever also studied the influence of ID on spatial ability. In both left- and right-handers, those who were FS- and ID-, or those who were F S t and ID+,scored higher on tests of spatial ability than those who were positive on one dimension but not the other. Thus, they concluded that ID t may be a neurological marker for weak (or bilateral) patterns of lateralization and may affect spatial ability level by way of an interaction with FS and sex. Generally speaking, there appears to be a stronger association between lefthandedness and ID t in females than in males (cf. Rich & McKeever, submitted, b) * Several recent studies provide some support for the proposed connection between left-handedness, sex and ID susceptibility. For example, Van Strien, Bouma & Bakker (1987) found that ID+ was significantly more frequent in leftas compared to right-handed individuals. Moreover, males who are FS t and also right-handed are reported to have a lower incidence of ID than FS t , righthanded females (Searleman & Fugagli, 1987; Smith, 1987). In contrast, Bishop (1986) concluded from a large archival study that there is no apparent relationship between ID and left-handedness. Rich & McKeever (submitted, b), upon a reanalysis of the Bishop (1986) data, however, found empirical support for just such a connection. Given the conflicting nature of the findings in this area, it may be tentatively concluded that when studying ability differences between left- and right-handers, susceptibility to ID is a factor worthy of consideration.

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Age and Hand Consistency Most studies assessing FS have involved college students. Interestingly, among children less than ten years old, FS+ was found to be related to higher intelligence and spelling scores for both left- and right-handers (Wellman, 1985). No effects were reported for spatial ability. Although this finding is seemingly incongruent with the results for adults, two other studies are not. Kraft (1984) assessed the impact of age, sex and handedness on patterns of laterality and ability in a sample of students younger than six years of age. He found that sex interacts with FS to influence brain laterality and subsequent cognitive abilities and that the effect is related to age. Specifically, age four was cited as a critical point in the relationship. Similarly, Eme, Stone & Izral (1978) found that for eight year olds, FS t left-handers had poorer spatial ability that right-handers. Moreover Sherman (1979) administered the DAT Space Relations Test to high school students and discovered that for eleventh graders, left-handers performed more poorly than right-handers, irrespective of sex. for ninth and tenth graders, however, no difference was found between left and right-handed students. Kaufman, Zalma & Kaufman (1987) reported early establishment of hand dominance in pre-school and primary school children relates to precocity in cognitive skills and motor co-ordination. Interestingly, it has been suggested that early established left-handers may have somewhat lower abilities than early established right-handers (Whittington & Richards, 1987). Gottfried & Bathurst (1987) found that consistency in right hand preference during infancy and early childhood predicted advanced intellectual ability in females but not in males. Kee, Gottfried & Bathurst (1989) reaffirmed these findings when the age of the sample was increased five to nine years. Although the effects were primarily related to verbal intelligence, performance on achievement tests in reading and arithmetic computation were also positively related to hand consistency. The above findings were especially useful when juxtaposed with the results of Kee, Gottfried, Bathurst & Brown (1987). They report that hand consistency over time is related to language lateralization in females; the stronger the hand consistency, the higher the likelihood that the language facilities are localized to the LH. For males, LH language lateralization occurred irrespective of hand consistency. Intuitively, these results suggest that the degree to which language is localized to the LH may influence verbal ability level. Yet, Sheehan & Smith (1986) found that strict LH language lateralization is related to higher spatial than verbal skills.

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Reasoning Ability Level Harshman and his colleagues (Harshman, 1988; Harshman & Hampson, 1987; Harshman, Hampson & Berenbaum, 1983) have proposed another factor which may account for some of the inconsistencies in the literature concerning handedness and ability. They hypothesize that sex by handedness interactions and their subsequent relationship to ability may vary as a function of the reasoning level of the individual (Harshman, Hampton & Berenbaum, 1983). With regard to spatial ability, females who were above the median in reasoning level performed somewhat better in tasks designed to tap spatial capacities if they were left-handed rather than right-handed. For males, superior spatial ability was associated with right- rather than left-handedness. For individuals with reasoning levels which fell below the median, the sex by hand interaction was reversed, i.e in females stronger spatial ability related to right hand dominance while in males, superior spatial ability was associated with left hand dominance. This higher order interaction pattern was also evident on tests of verbal fluency and perceptual speed, although the pattern of effects differed slightly as a function of the specific ability under study and the testing instrument utilized. Harshman & Hampton (1987) report a three way interaction between sex, handedness, reasoning ability and language lateralization. Using a verbal and nonverbal dichotic listening task to index the degree and direction of hemispheric lateralization, they found that in the above average group, LH advantages were greatly correlated with superior performance on tests of verbal ability, and RH advantages were associated with superior performance on tests of spatial ability. In the moderate and low level reasoning groups, this pattern was either absent or essentially reversed. Thus, there may be a variety of brain organizations which reflect differing profiles of cognitive abilities (Harshman, 1988). In fact, Mateer, Ploen & Ojeman (1982) suggest that sex of subject and the extent to which language is lateralized to the LH has a significant positive impact on verbal IQ. In keeping with this notion, O’Boyle & Hellige (1989) have identified at least four orthogonal.dimensions upon which individuals may differ with regard to underlying patterns of lateralization: 1) degree of dominance, 2) direction of dominance, 3) characteristic arousal level and 4) complementarity of functioning. They intimate that a connection between patterns of hemispheric asymmetry and individual differences in cognitive abilities my yet be established, although in their assessment and that of others (cf. Lewis & Harris, 1988), the

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current level of empirical support precludes any strong cause and effect conclusions. In sum, differences in cognitive abilities are seemingly influenced by underlying patterns of cerebral lateralization, which may in turn be related to hand dominance, strength of handedness. FS, sex, ID, age, hand consistency over time and level of reasoning ability. At an intuitive level, the interaction of these variables (and undoubtedly other factors) would seem to contribute to a variety of talents displayed by individuals. This connection is examined in the following section.

Handedness and Talent It has been suggested that the persistence of left-handedness as a natural variant in human beings implies that some behavioural advantages are associated with sinistrality (Annett & Ockwell, 1980). If in fact some cases of lefthandedness are indeed related to enhanced spatial ability, a new insight may be reached as to why among the army of the children of Benjamin “700 choice men were left-handed; each could sling a stone at a hair and not miss.” (Judges, XX, 16; New American Standard Bible). With such insight, one might begin to account for the fact that there are numerous left-handed athletes (Annett, 1981). Left-Handedness and Athletics

Snow (1985) summarized the findings on left-handedness and its association with excellence in athletic pursuits, especially those which depend heavily on spatial abilities. In an investigation conducted by Guiard & Athenes (as cited in Snow, 1985) it was discovered that left-handers display a significant advantage in various sports, especially tennis and fencing. For example, although the percentage of left-handers involved in athletics is only about six percent of the general population of France, in tennis and fencing the frequency is about 15% and 39% respectively. In addition, when only contestants of World Championship events are considered, the proportion of left-handers increase at each successively higher level of competition (Bisiacchi, Ripoll, Stein, Simonet & Azemar, 1984 as cited in Snow, 1985). Rather than attributing these findings to the surprise effect of competing against a nonright-handed opponent (cf. Corballis, 1983), Bisiacchi at al. argue that the critical factor is enhanced spatial skill.

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Left-Handedness in Artists and Architects Higher frequencies of left-handers have been found among artists and architects (Mebert & Michel, 1980). For example, it has been noted that Leonard0 da Vinci, Michelangelo and other famous artists were left-handed (Springer & Deutch, 1989). Peterson & Lansky (1977) report that a larger percentage of left-handers enter undergraduate majors in architecture than would be predicted by their frequency in the general population and that they are more successful in completing their program of studies than are right-handers. ALSO, during their first quarter in college, left-handed males have higher scores than right-handed males on various academic predictors, like design scores, grade point averages, etc. Moreover, Peterson & Lansky (1974) found that 29% of the faculty in architecture schools were left-handed, again an unusually high frequency of occurrence. Given that art and architecture require high level spatial capacities, such findings provide backhanded support for the notion that sinistrality is positively correlated with spatial ability. Music Musical ability has also been linked to left-handedness. Byrne (1974) found a higher frequency of mixed handers in his sample of musicians than in a control group of nonmusicians. Yet, the performance of mixed-handed musicians on two subcomponents of the Seashore test of musical talents was no different than that of right-handed musicians. Deutch (1978, 1980), however, reported that weak left-handers often exhibit enhanced performance on pitch recognition tasks. In addition, Hassler & Birbaumer (unpublished manuscript) found that left and mixed-handers differed significantly from right-handers in degree of musical talent and composition ability, but only for males. They also report that musical talent was related to the strength of language lateralization with musicians exhibiting weaker patterns of laterality. These results are consistent with those of Witelson (1980), who on the basis of her dichotic listening studies hypothesized that musicians have a greater propensity towards bihemispheric representation of phonetic, sequential type information. Some studies have shown that musicians have RH specialization for the categorization of musical intervals. Specifically, musicians tend to do better in this task with their left ear/RH while nonmusicians exhibit a significant right ear/LH superiority (Kellar, 1977; Kellar & Bever, 1980). Of some interest is the fact that the effect was due primarily to the performance of one particular group:

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right-handed FS- nonmusicians who demonstrated a strong left ear/RH superiority. Right-handed, FS t musicians exhibited no ear/hemisphere advantage. Notably, each group was balanced for sex but the impact of this variable was not analyzed. In contrast, Johnson (1977) arrives at a somewhat different conclusion, suggesting that musicians rely on the LH when processing musical melodies, while nonmusicians utilize their RH. Hence, as an individual becomes more musically adept, increasing use is made of the LH for processing. Such theorizing is congruent with the results of Bever & Chiarello (1974). In their study, musicians and non-musicians were asked to recognize 12 to 18 note melodies presented dichotically. They found a right ear/LH advantage for musicians and a left ear/RH superiority for nonmusicians. Bever & Chiarello hypothesized that the former use an analytic strategy to process the melodies (i,e, a note by note analysis which is conducive to LH involvement), while the latter utilized a more holistic strategy (i.e. the analysis of the melody as a unified "Gestalt" which serves to engender RH mediation). Mathematics and Chess Xnnett & Kilshaw (1983) found a reduced incidence of right hand dominance for individuals of higher educational status. They also examined whether high level ability in mathematics is associated with hand preference and found that left-handedness was more frequent among university mathematics students and professors, especially among males. Moreover, they discovered that females who develop outstanding skills in mathematics show a reduction in right hand preference to approximately the same degree found in males. Mankin & Benbow (in preparation) studied the incidence of left-handedness among chess masters and championship "Go" players. Both games draw upon considerable spatial skill for successful play. The frequency of left-handedness in each sample was found to be higher than in the general population. Recently, Cranberg & Albert (1988) have reported a similar abundance of left-handers among chess players.

Mathematical Precocity One of the more dramatic illustrations of a connection between lefthandedness and cognitive ability comes from studies involving extremely

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intellectually precocious youths (Benbow, 1986; Benbow & Benbow, 1984). In response to the suggestion made by Geschwind & Behan (1982) that left-handers may have an unusually well developed RH, Benbow administered the Edinburgh Handedness Inventory to 340 adolescents who scored extremely high (at least the top one in 10 OOO) on the SAT-M and/or SAT-V. The same questionnaire was administered to their parents and a comparison group of modestly (top one in 20) gifted children (N = 201). Self-report data on the hand of preference of siblings was laos gathered for the extremely precocious youths. In comparison to population norms, the gifted youths were more than twice as likely to be left-handed. In addition, they were more frequently left-handed than their parents, siblings and members of the comparison group (i.e. 15% for the precocious and roughly 10% for each of the other groups). Notably, parents and members of the comparison group were twice as likely as the extremely precocious to report exclusive reliance on their right hand when performing all of the ten tasks comprising the handedness inventory. Although the incidence of left-handedness was elevated among extremely talented females, there were significantly more left-handed males than females in the precocious group. Geschwind & Behan (1982) have suggested that prenatal exposure to high levels of testosterone may enhance the development of the RH, while inhibiting the development of the LH. Given that verbal abilities are thought to be primarily mediated by the LH, a somewhat counterintuitive result reported by Benbow (1986) was that precocious individuals of high verbal ability also exhibited a tendency for increased frequency of left-handedness. In fact, the highest incidence of left-handedness was found for verbally rather than mathematically talented males (about 24% vs. 14%). Though an unexpected finding at the time, the recent work of Galaburda and his colleagues (Galaburda, Corsiglia, Rosen & Sherman, 1987) sheds some light on this result. Galaburda et al. suggest that testosterone modifies brain lateralization, not by slowing the development of one hemisphere at the expense of the other, but rather by promoting the growth of the structures on the nondominant side (e.g. the plenum temporal of the RH). Hence, in the case of the precocious youths, it may be that both the LH and RH are highly developed relative to "normal." This pattern of enhanced bihemispheric development might in part account for the fact that precocious individuals who exhibit superior math ability also tend to be superior in verbal abilities. Other perhaps less convincing explanations of this effect may be that 1) the construct of "verbal reasoning," as indexed by the SAT-V, taps several component processes that are mediated primarily by the RH (e.g. part-whole relationships,

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as opposed to the phonetic or syntactic aspects of language and/or 2) the superior verbal performance of these precocious youths is related to the "reasoning ability" variable proposed by Harshman and his colleagues (Harshman, Hampson & Berenbaum, 1983). In the Benbow (1986)) study, all of the precocious youths were, by definition, of extremely high reasoning ability. Because Geschwind and Behan (1982) found that the incidence of ID was higher for left-handers that right-handers, Benbow (1986) also examined the frequency of allergies and other immune disorders in precocious youths, their parents, siblings and a matched comparison group. Over 50% of the extremely precocious children (twice the rate of the general population) had some form of ID. The frequency of occurrence was also substantially higher that observed in their parent, siblings or the comparison group. The findings concerning left-handedness and ID both suggest that intellectual precocity may be associated with a generally weaker (perhaps bilateral) pattern of hemispheric lateralization. Some of our recent work is in keeping with this hypothesis. For example, O'Boyle & Benbow (submitted) found that gifted children failed to show the typical right ear/LH advantage for the recognition of dichotically presented syllables, a pattern due to the uncharacteristic success of their RH hemisphere in the performance of this linguistic task. In a second companion experiment, when asked to determine which of two chimeric faces is "happier" (cf. Levy, Heller, Banich & Burton, 1983), precocious youths exhibit a reliance on RH processing that is significantly greater than that of matched control children. Notably, the degree of this RH reliance correlated with their SAT performance, i.e. the greater the RH involvement, the higher the SAT score. The O'Boyle & Benbow findings taken in composite suggest that heightened RH activation during cognitive processing may contribute to extreme intellectual precocity.

A Reanalysis of Benbow (1986) In previous sections of this chapter, we identified several factors that may moderate the relationship between handedness and cognitive abilities, including strength of hand preference, FS, sex, ID and reasoning ability levels. In the Benbow (1986) study involving intellectually precocious youths, three distinct profiles of cognitive abilities characterized her sample: 1) males and females of high mathematical ability, 2) males and females of high verbal ability and finally, 3) members of either sex who were precocious on both dimensions.

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In the following section we attempt to determine if the ability patterns observed by Benbow (1986) differentially relate to one or some combination of the previously identified factors. Table 1presents some of the data reported by Benbow (1986) and additional data from our reanalysis. First, gifted males were separated into those who were math precocious (SAT-Math >700), or verbally precocious (SAT-Verbal > 630). The same classification procedure was applied to females, but because the number of mathematically talented females was quite small, they were merged with those designated as precocious in both verbal and mathematical ability. Members of each group were subsequently separated on the basis of hand preference using the Edinburgh Inventory: strongly left-handed (LQ < -a), mildly left- or mixed-handed ( -40 5 LQ 5 20), mildly right-handed (20 < LQ <70) and moderately to strongly right-handed ( L a > 70). The resultant 16 groups were then monitored for the presence or absence of FS, ID (for subjects and their parents), or some combination of these factors. This breakdown produced some cells with extremely small N’s. Thus, for each of the hypothesis tested, only trends in the appropriate or inappropriate direction can be suggested. With regard to strength of handedness, the literature reviewed in this chapter suggests that left hand dominance might relate more to enhanced verbal as compared to mathematical ability, especially in males. As can be seen in Table 1, the data are consistent with this hypothesis. In the Benbow (1986) sample, the frequency of the left- and mixed-handedness (LQ 20) is slightly higher among the verbally precocious (23,5% males, 15.8% females) as compared to the mathematically precocious ( 16.6% males, 9.5% females), particularly so for the strongly left-handed (LQ < -40) males (15.7%). As previously mentioned, two other studies have implied that left-handedness is especially detrimental to spatial ability in females (Klintenberg, Levander & Schalling 1987; Sanders, Wilson & Vandenberg, 1982). In the Benbow (1986) sample, only two mathematically talented females were left- or mixed-handed (9.5%). As a group, the mathematically gifted females were particularly right-handed (90.5%). Though difficult to draw firm conclusions from the literature on the relationship between FS and ability, indications were that FS was related to lower abilities with the possible exception that FS+ might serve to enhance spatial capacities in right-handed males (e.g. McKeever et al., 1983). As revealed in Table 1, in the Benbow (1986) sample, the incidence of FS was generally quite low among the intellectually precocious ( i s about 25% as compared to about 45% in the general population), a finding which is consistent with the idea that

362

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Table 1: Reanalysis of frequencies of physiological correlates among various talent groups reported in Benbow (1986) - Percents by talent group Math Talented Males Laterality Quotient

Strong Left

Mild Left

Mild Right

Strong Right

Hand Preference

11

6

27

57

Fami 1 ial Sinistral ity

57

29

46

29

Immune Disorders

43

43

65

51

Parent has Immune Disorders

71

71

7a

67

Anomalous Lateral izat ion*

14

a6

60

53

Verbally Talented Males Lateral ity Quotient

Strong Left

Mild Left

Mild Right

Strong Right

Hand Preference

16

a

2a

49

Fami 1 ia 1 Sin i stra 1 i ty

33

25

13

30

Imnune 0isorders

a3

50

63

45

Parent has Immune Disorders

50

50

aa

55

Anomalous Lateral ization*

17

75

50

45

FS is related to a lower (at least lower than precocious) level of intellectual ability. Moreover as can be seen in Table 1, the incidence of FS t is relatively high in right-handed, mathematically talented males (57%) as compared to the other gifted groups (33%, 33%, 31%). Although less compelling, the data are at least congruent with the McKeever, et al. (1983) notion that in right-handed males, FS t and the spatial/mathematical faculties are positively related. According to Rich & McKeever (submitted a), the presence of both FS and ID or their combined absence are thought to relate to increased spatial

Ability and Talent

363

Table 1: Continued

Verbally Talented Females Laterality Quotient

Strong Left

Mild Left

Mild Right

Strong Right

9

7

28

56

33

33

17

31

Imnune Disorders

0

67

33

50

Parent has Imnune Disorders

0

100

50

65

33

33

67

65

Hand Preference Familial Sinistrality

Anomalous Latera 1 izat ion*

Both Mathematically & Verbally Talented Females Laterality Quotient

Strong Left

Mild Left

Mild Right

Strong Right

Hand Preference

9

0

5

86

Familial Sinistrality

0

0

0

24

Imnune Disorders

0

0

100

48

Parent has Imnune Disorders

0

0

100

73

100

0

0

53

Anomalous Latera 1 izat ion*

* Familial Sinistrality and has Imnune Disorders, or Familial Sinistrality and Does not have Imnune Disorders.

ability. The trends in our data are not entirely consistent with this view. Lefthanded females who were both verbally and mathematically precocious do show a tendency toward this pattern (100%). However, the actual number of subjects comprising this cell (N = 2) is too low to be seriously considered. Although ID of subject did not systematically relate to enhanced spatial ability, ID+ among their parents and siblings did. As can be seen in Table 1, 71% of strongly lefthanded, mathematically precocious males had parents or siblings who exhibited

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some form of ID. With further investigation, parental ID t may yet prove to be related to mathematical precocity. Finally, Annett and colleagues, in light of her right-shift theory (Annett, 1085; Annett & Kilshaw, 1982) have postulated that a reduced preference for the right hand may be indicative of enhanced RH cognitive development. Such selective enhancement might then serve to promote the spatial/mathematical faculties. In the Benbow (1986) sample, some support for this hypothesis is found as three of the four groups (i.e. all but the mathematically talented females) exhibit evidence of reduced right hand preference.

Concluding Comment In this review we have attempted to pull together a selection of studies relating handedness to cognitive ability and talent. Unfortunately, the findings concerning this proposed connection have a distinctly piecemeal flavour to them and are often complicated and confusing. To us, it seemed that for each piece of data confirming a relationship between hand preference and a corresponding ability or talent, there appeared to be at least one other result that either a) failed to replicate the original finding, b) postulated some new second-order variable (e.g. sex, FS, ID, etc.) that served to moderate the connection or at least, confuse the issue, or c) flatly contradicted the notion that any such relationship existed. Needless to say, the variability of these findings speaks to the likelihood of a Type 1 error. We make no bones about the fact that our review does not paint a very unified explanatory picture of the proposed connection between handedness, ability and talent. We believe, however, that it accurately reflects the fragmented nature of our knowledge at this time. Presently, there are bits and pieces of evidence in the literature, with some pointing towards a connection, for example, the fairly consistent association between left-handedness and superior talents (e.g. athletics, architecture, mathematics and chess). Only after additional research, using a variety of experimental techniques to systematically converge on this issue, will we come to better understand the complex relationship between handedness, cognitive ability and talent.

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Whittin ton, J.E., & Richards, P.N.(1987). The stability of children’s laterality re erences and their relationship to measures of performance. British ! o i o l of Educational Psychology, 57, 45-55. Witelson, S.F. (1974). Hemispheric specialization for linguistic and nonlinguistic tactual perception using a dichotomous stimulation technique. Cortex, 10, 317. Witelson, S.F. (1980). Neuroanatomical asymmetry in left-handers: A review and implications for functional asymmetry. In J. Herron (Ed.), Neuropsychology of left-handedness. New York: Academic Press. Yen, W.M. 1975 . Inde endence of hand preference and sex-linked genetic effects o spatial per ormance. Perceptual Motor Skillsl 41, 311-318. Yeo, E.B., & Cohen, D.B. (1983). Familial sinistrality and sex differences in cognitive abilities. Cortex, 19, 125-130. Zurif, E.B., & Bryden, M.P. (1969). Familial handedness and left-right differences in auditory and visual perception. Neuropsychologia, 7, 179-187.

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LER-HANDEDNESS Behavioral Implications and Anomalies, S. Coren (Editor) 0 Elsevier Science Publishers B.V. (North-Holland), 1990

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Chapter 13

Familial Sinistrality and Cerebral Organization Walter F. McKeever Northern Arizona University

It is well-established that left-handers have a different cortical organization of some higher functions than do right-handers. Diverse data indicate, for example, that left-handers are less exclusively dependent on the left hemisphere for language functions. The incidence of persistent aphasia following brain lesions is lower in left-handers (Goodglass and Quadfasel, 1954; Segalowitz and Bryden, 1983), and they appear to recover speech functions more quickly than do right-handers following cerebral insult (Subirana, 1958; Luria, 1970). Wada Test data suggest that as many as %% of right-handers, but only 70% of lefthanders, are clearly left hemisphere dominant for speech (Rasmussen and Milner, 1977). Similarly, Borod, Carper, Naeser, and Goodglass (1985) found that only one percent of right-handers they studied were aphasic in response to right hemisphere lesions, but 24% of right-lesioned left-handers were aphasic. The possibility that language functions may also be less strictly left hemisphere-dependent in those with a history of left-handedness in their families is much less well-established. Various investigators have suggested, on the basis of limited case studies of aphasics, that positive familial sinistrality (FS + ) portends a favourable prognosis for more rapid and complete language recovery following aphasia onset (Subirana, 1958; Luria, 1970), and that right hemisphere language in dextrals is more likely to occur if the individual is FS t (Zangwill, 1960). Hardyck (1977) has proposed that lateralizations of language and visuospatial processing vary along a continuum as a function of handedness and FS

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characteristics. According to this model, the right-hander with no familial sinistrality (RHFS-) typifies the markedly left hemisphere language/right hemisphere spatial lateralization type, while the left-hander with familial sinistrality (LHFS t ) possesses a bilateralized type of language and spatial function organization. The right-handed person with familial sinistrality (RHFS t) is hypothesized to be less lateralized for both functions than are the RHFS-, but more lateralized than the LHFS t . According to Hardyck (1977), the left-hander without familial sinistrality (LHFS-) does not fit neatly into this handedness-FS model. Hardyck, citing the work of Hecaen and Sauguet (1971), suggests that the LHFS- person has a cerebral organization resembling that of the RHFS- person. Orsini, Satz, Soper, and Light (1985), on the other hand, have questioned the putative role of FS in cerebral organization. These investigators failed to find any relationship of the FS variable to language laterality as inferred from a dichotic verbal task and two concurrent activities (verbal output/manual tapping) tasks. The fact that the sample size was quite large and that significant effects of handedness (lesser language lateralization in left-handers) were obtained on all three tasks supplied credibility to the investigators’ negative conclusion regarding the importance of FS. Orsini et al. (1985) went on to suggest that FS may have been erroneously implicated in cerebral organization through two types of rystematic errors. According to the authors, the erroneous conclusion could reflect the fact that left-handedness is positively correlated with FS, so that effects due to left-handedness may have been wrongly attributed to FS. The second error could be publication bias in favour of positive effects of FS. It would seem that the first suggested source of error cannot account for positive FS effects where both handedness and FS have been carefully assessed. Indeed, assessment of FS without the accompanying assessment of handedness has seldom been conducted. The second suggested source of error is, of course, possible. Anyone familiar with the literature regarding the FS variable and cerebral organization can readily understand the frustration attending efforts to characterize the role of FS. The literature on the subject is diverse and resistant to nice generalization. In this paper I shall review data regarding the relationship of FS to a number of other variables. I shall not attempt to review every study which might be relevant, but shall review studies which seem to me to be the best in relation to a particular question, or which illustrate points or approaches which might ultimately prove fruitful. I can say at the outset that the final conclusion will riot be that FS influences are non-existent. I believe that FS

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is probably an important variable. The problems associated with isolating and documenting any effects of FS, however, are many, and I shall discuss some of these near the end of the paper, and suggest some strategies that might prove useful for research on the relation of FS to cerebral organization.

Clinical Studies I have already mentioned the case studies of Subirana (1958) and Luria (1970) which suggest a better prognosis for speech recovery in FS t individuals. These indicate that FS, regardless of the handedness of the patient, is associated with a more bilateral organization of language functions, the greater bilaterality resulting in less chronic disruption of language mechanisms by unilateral lesions. More formal and ambitious clinical studies have been conducted by Hecaen and Sauguet (1971), Newcombe and Ratcliff (1973), Warrington and Pratt (1973), and Hecaen, DeAgostini, and Monzon-Montes (1981). Because the Hecaen and Sauguet results were generally consistent with those of the more comprehensive Hecaen et al. study, I will review only the later study. Newcombe and Ratcliff (1973) reported a limited but interesting study of the effects of left and right lesions in 28 left-handed men who had suffered unilateral missile injuries more than 20 years earlier. A total of nine LHFS- men, three with left lesions and six with right lesions, and 19 LHFS t men, seven with left and 12 with right lesions, were examined. Assessments included performances on vocabulary, spelling, object naming, fluency, block designs, visual closure, and mazes. The data showed the left-lesioned LHFS t veterans to be more impaired on verbal tasks than were the left-lesioned LHFS- veterans, yet the LHFS t had higher scores on the spatial tasks, particularly mazes, showing that they were not more impaired generally. This suggests that the LHFS t were the more left hemisphere dominant for language, and this conclusion was reinforced by the observation that five of the 12 LHFS- men with right lesions had been dysphasic in the acute recovery period according to the medical records, while none of the six LHFS t men with right lesions had ever been dysphasic. These findings are not consistent with the Hardyck (1977) model. Results showing n o difference in inferred language laterality between LHFSand LHFSt individuals were presented by Warrington and Pratt (1973). Patients were 24 left-handers who received unilateral left and right hemisphere electro-convulsive therapy for depression. The authors noted that all patients were considered free of neurological disorder. Language laterality was inferred

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from dysphasic responses to four questions put to the patient immediately after the patient was able to respond to the question "What is your name?" Left dominance was inferred from greater dysphasia following left than right treatment, and right dominance was inferred from greater dysphasia following right treatment. This method, applied earlier to 55 right-handed patients (Pratt and Warrington, 1971), had indicated left dominance in 98% of the righthanders. Results showed that seven of the eight LHFSt patients were left hemisphere dominant; of the 16 LHFS- patients, 11 were judged to be left dominant, three right dominant, and two were indeterminant for language, is., their scores for dysphasia were equal after left and right treatments at several different doses of current. It is reasonable to regard these two patients as bilateral for language, and if one does, 87.5% of the LHFS t and 68.8% of the LHFS- were left hemisphere dominant. While this difference is not statistically significant, the direction of the difference suggests greater left dominance in the LHFS t . Again, this result is contrary to that hypothesized by Hardyck (1977). Finally, the most ambitious clinical study was conducted by Hecaen, DeAgostini, and Monzon-Montes (1981). Data were presented on 141 left and 130 right-handers with unilateral lesions. Six verbal functions were assessed: judged speech non-fluency; judged articulatory defect; confrontation naming; auditory comprehension shown in response to oral commands; visual comprehension shown in response to written commands; and writing, both to dictation and spontaneously. Four spatial functions were tested: spatial dysgraphia (repetitions in writing and margin size increase); unilateral spatial agnosia (neglect); constructional apraxia; and spatial agnosia (loss of topographic concepts and topographic memory). Results for the LHFS t showed only two verbal functions to be more strongly impaired by left than by right lesions. These were articulatory and naming performances. The LHFS-, however, were more impaired by left than right lesions on all six verbal functions. These results are consistent with the Hardyck model. On the spatial function measures, the LHFS t showed significantly greafer impairments on all four measures following right than left hemisphere lesions; the LHFS- showed no significant differences in response to left or right lesions. Indeed, LHFS- patients showed non-significantly higher incidences of impaired performance after left than right lesions, with the difference on constructional apraxia approaching significance (p< .07, computed by the writer). These spatial function results are not compatible with the Hardyck (1977) model, and suggest greater right hemisphere visuo-spatial function lateralization in the LHFS t than in the LHFS-.

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The RHFS- showed significantly greater impairment of all verbal functions, except articulatory disorder, following left than right lesions. The direction of the non-significant difference for articulatory disorder was consistent, however, i.e, a higher incidence of disorder following left than right lesions. The RHFS t , however, showed significantly greater impairments in response to left lesions on only two functions -- naming and auditory comprehension. The RHFS + showed greater frequencies of impairment of all four spatial functions in response to right as opposed to left lesions, but in no case was the difference significant. The RHFS-, however, showed significantly greater impairment following right lesions on three of the four spatial tasks, the only exception being constructional apraxia, where the difference was just short of significance (Fisher Exact Probability =.059, computed by the writer). The data for right-handers therefore were highly congruent with the Hardyck model (1977) expectations, showing greater bilaterality of both verbal and visuo-spatial functions in RHFS t as opposed to RHFS- persons. A final important aspect of the study concerned differences in response to anterior versus posterior lesions. Posterior lesions were defined as postRolandic, including the temporal lobes; anterior lesions were frontal and Rolandic. Only in RHFS-patients did the frequency and severity of deficits differ consistently between anterior and posterior lesion groups. Posterior lesions were significantly more disruptive than anterior lesions for three verbal functions and three visuo-spatial functions in the RHFS-. The RHFS t showed no significant differential effects of anterior versus posterior lesions on verbal functions, and only two significant differences on visuo-spatial functions. Again, posterior lesions were more disruptive when differences were found. Left-handers, regardless of FS status, showed little evidence of anterior-posterior differences, giving rise to the assertion of the authors that left-handers have a more diffuse intrahemispheric organization of functions. The same assertion would be appropriate for the RHFS t . In addition to analyses of specific language and spatial impairments, Hecaen et al. provided data on the incidence of aphasia and spatial disorders (any manifestation, severity ignored) following left and right hemisphere lesions. I have arranged these figures as shown in Table 1. The only statistical analysis conducted by Hecaen et al. on these frequencies tested for a sex difference within FS-handedness groups. The more interesting question is whether the various patient groups showed differential impairments in response to left versus right lesions. Fisher Exact Probability tests show that four of the eight handedness-FS groups did not show a significantly greater incidence of aphasia

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Table 1: Incidence of left and right brain-lesioned patients in each handednessFS-sex group who suffered ("YES")or did not suffer ("NO")aphasia or spatial disorder and significance of differences of incidences (Adapted from Hecaen et al., 1981)

Aphasia Data Left Les ioned Left -Handed FS- Male FS- Female FS+ Male

FS+ Female

All Patients

Significance Level

Right Les ioned

YES

NO

%YES

YES

NO

%YES

18 14 23 11 66

9 0 9 3 21

67 100 72 79 74

5 2 9 2 18

21 4 7 4 36

19 33 56 33 32

,006 .003 .919 .078 .001

18 6 5

11 11 9 3 34

62 35 36 70 51

0 3 1

18 13 11 13 55

0 19

.ooo

Right-Handed Male Female Male Female All Patients FSFSFS+ FS+

7 36

1 5

a

7 8

.251 .ii7 ,002 .001

Spatial Disorder Data Left Les ioned

Significance Level

Right Les ioned

Left-Handed

YES

NO

%YES

YES

NO

%YES

FS- Male FS- Female

10 7 12 5 34

17 7 20 9 53

37 50 38 36 39

11 2 11 4 28

15 4 4 2 25

42 33 73 67 53

,456 ,426 ,023 ,217

5 3 2 1 11

24 14 12 9 59

17 18 14 10 16

11 4 5 4 24

7 12 7 10 36

61 25 42 29 40

,003 .463 .130 .283 .002

FS+ Male FS+ Female All Patients

.115

R i ght -Handed FS- Male FS- Female FS+ Male

FS+ Female

All Patients

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379

following left than following right lesions. These were the LHFS+ males and females, and the RHFS- females and RHFS t males. For spatial disorders, only two of the eight groups showed a significantly higher incidence of disorders following right than left lesions. These were the LHFS t and RHFS- males. Ignoring the FS and sex variables, the data ("all patients" rows) show that both left and right-handers suffered higher frequencies of aphasia following left than right lesions. The right-handers suffered a significantly higher incidence of spatial disorders following right lesions, but left-handers suffered spatial disorders with similar frequencies after left or right lesions. Finally, one can contrast the incidence of disorders in left and right-handers following left and right lesions. These contrasts reveal that aphasia was more common in left-handers than righthanders following left hemisphere lesions (X' = 10.2, df 1,p < .002) and following right lesions (X' = 10.1, df 1, p < .002). Spatial disorders were more common in left-handers than right-handers following left lesions (X' = 10.4, df 1, p < .002), but the frequency of spatial disorders following right hemisphere lesions did not differ between handedness groups (X' = 1.86, df 1, p c.175). Critique of the Clinical Studies

All of the studies have flaws. The small sample sizes of Newcombe and Ratcliff (1973) and Pratt and Warrington (1973) are obvious weaknesses. Another flaw, common to all, concerns the definition of handedness. In a footnote, Hecaen et al. (1981) mention that the hand used for writing was not considered in determining strength of hand preference since the investigators had never encountered a case of a left-handed writer in France who was over the age of 35. Most of the patients studied were over 35. Only 12 of the 24 patients of Warrington and Pratt (1973) wrote with the left hand. Social pressures to write with the right hand were probably less severe in the British samples, but again, most of the patients would have attended elementary school during the 1930's, when pressures presumably existed for right hand writing. It is certainly possible that the neurological organization of language functions could be influenced by the early adoption of right hand writing. The problem of defining handedness is magnified if one is asked to identify the handedness of relatives when hand used for writing cannot be employed as a criterion. Thus, especially in the Hecaen group studies, the validity of the FS- and FS+ designations is problematic. A related problem also concerns the definition of FS status. While FSt is typically based only on first degree relatives, because of the greater difficulty of securing information regarding the handedness of more distant

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relatives, the Hecaen group counted a diverse set of relatives in arriving at FS classifications. On the one hand, this approach can have its strengths, especially where the number of first degree relatives is very small. On the other hand, the Hecaen group included aunts, uncles, and cousins. Left-handedness in cousins has a 50% chance or arising from a family having no biological relationship to the patient, and the same is true of aunts and uncles unless the requirement that they be siblings of the patients’ parents is stipulated, and the Hecaen group did not indicate that this stipulation was made. Additional serious flaws of the Hecaen et al. (1981) study are (1) the abse,nce of data regarding the time of assessments in relation to the time post-onset of lesions; and (2) the absence of evidence that lesion sites were comparable in the various groups compared. It is well-known that left-handers show more transient disruptions of neuropsychological functions following brain lesions. The very high rate of symptoms, particularly verbal ones, in the left-handed as opposed to the right-handed patients suggests that the assessments characterize relatively early symptoms. Secondly, there is no assurance that lesion sites were comparable. Data on the anterior versus posterior lesion sites of part of the sample (surgically verified cases, Table 2 of the article) are reported and show that, at least for those patients, both left and right lesions were more frequently posterior lesions in the LHFS t than in the LHFS-. Among the right-handers, left lesions were more often posterior in the RHFS t than in the RHFS-, while right lesions were more often anterior in the R H F S t than in the RHFS-. Examination of the data shows that posterior lesions were generally more disruptive of both verbal and spatial functions. If these distributions characterized the whole sample, one might argue that left language function is overestimated in LHFS t and RHFS t patients in relation to LHFS- and RHFSpatients, while right hemisphere spatial function is overestimated in LHFS + relative to LHFS- patients and underestimated in RHFS t relative to RHFSpatients, Given the serious barriers to unequivocal interpretation of the data, the clinical studies can be regarded only as illustrating a (contradictory) range of possible influences of FS on cerebral organization for language and visuospatial functions. Because of the greater problems in defining handedness of relatives than of patients, the data would seem to provide better information regarding differences between handedness groups than between FS groups.

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381

Experimental Studies of Normal Subjects Studies of possible differences in language laterality as a function of handedness and FS characteristics have often employed dichotic listening and lateralized tachistoscopic presentation methodologies. The rationales for these methods are detailed in Springer (1986) and McKeever (1986a). The studies to be reviewed all employed normal subjects (Ss), most of whom were college undergraduates. Other behavioural techniques, such as the concurrent activities paradigm (Kinsbourne and Hiscock, 1977) and tactile paradigm (e.g. Witelson, 1974), have been applied to the study of language laterality. It is my view that to date these techniques have less claim to credibility as measures of language laterality, and given page limitations, and the fact that they provide no substantial reinforcing or countervailing evidence to that supplied by dichotic and tachistoscopic studies, I shall forego a consideration of them. I have also omitted studies in which FS was thoroughly confounded with handedness, handwriting posture, or strength of hand preference, or which employed very small samples, or reported data on only a single subset of FS Ss, such as RHFS t males. Dichotic Listening Studies of Language Lateralization

The results of a number of studies are indicated in Table 2. The table indicates which handedness-FS or handedness-FS-sex group(s) failed to show the right ear advantages (REA) indicative of left hemispheric specialization or showed a reduced REA relative to the most strongly REA group (usually righthanded Ss). Zurif and Bryden (1969) found significant REAs in their right-handed Ss (all were FS-) and in their 10 LHFS- Ss, but the LHFS t tended to recall more digits from the left ear. Higenbottom (1973), employed a dichotic words task and found a significant REA for right-handers, but non-significant ear differences for left-handers, regardless of FS status. Hines and Satz (1974) found a significant REA across groups, with no differential asymmetry as a function of FS in righthanded Ss (FS was not assessed in left-handers). Briggs and Nebes (1976) classified Ss according to degree of hand preference (right, left, and mixed) as well as to FS. No differential aspmatries were found between any groups, including the handedness groups. Lake and Bryden (1976), found evidence of reduced REAs in FS t females, regardless of handedness, and in LHFS- males. It should be noted, however, that their dichotic task produced a low level of

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McKeever

Table 2: Summary of dichotic listening studies involving assessment of FS effects, indicating groups showing weak or absent Right Ear Advantages (WAREA) ss

FS

Task

Z u r i f & Bryden (1969)

20 RH 20 LH

20 FS-/ 0 FS+ 10 FS-/lO FS+

Dichot i c Digits

LHFSt

Higenbottom (1973)

33 RH 56 LH

NA 27 FS-/29 FS+

D i c h o t ic Words

LH

Hines & Satz (1974)

60 RH 30 LH

30 FS-/30 FS+ NA

0 ichot ic

None

Briggs & Nebes (1976)

40 RH 40 MH 40 LH

20 FS-/20 FS+ 20 FS-/20 FS+ 20 FS-/20 FS+

D ichot i c Digits

None

Lake & Bryden (1976)

72 RH 72 LH

36 FS-/36 FS+ 36 FS-/36 FS+

D i c h o t ic Syll.

FS+ Fe LHFS- Ma

Lishman & McMeekan (1977)

22 RH 20 LH

26 FS-/16 FS+

D ichot ic Digits

FS+ SLH

McKeever & VanDeventer (1977a)

80 RH 71 LH

44 FS-/36 FS+ 34 FS-137 FS+

Dichot i c Digits

LHFS+

Geffen & Traub (1970)

39 LH

14 FS- males 25 FS+ males

Dichot i c Monitor.

LHFS- Ma

Piazza (1980)

32 LH 32 RH

16 FS-/16 FS+ 16 FS-/16 FS+

cv

Dichot i c Syll.

None

Sear leman (1980)

117 LH 256 RH

? FS-/ ? FS+ ? FS-/ ? FS+

cv

D i c hot ic Syll.

None

O r s i n i e t a1 (1985)

257 LH 215 RH

141 FS-/116 FS+ 145 FS-/ 70 FS+

Dichot i c Words

LH

60 LH 60 RH

30 FS-/ 30 FS+ 30 FS-/ 30 FS+

Dichot i c Syll.

LH

McKeever (1986b)

134 LH 225 RH

67 FS-/ 67 FS+ 127 FS-/ 98 FS+

D ichot i c

LH

Rich & McKeever (1989)

64 LH 64 RH

32 FS-/ 32 FS+ 32 FS-/ 32 FS+

Study

Bryden (1986)

WAREA

Digits

cv

cv

cv

Syll.

Oichot i c Syll.

cv

LH

Abbreviations: NA = not assessed; LH = left-handed; RH = right-handed; MH = mixed-handed; SLH = s t r o n g l y left-handed; Ma = males; Fe = females.

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383

REA overall. Even among RHFS- females, only 67% of the Ss had REAS. Lishman and McMeekan (1977) found somewhat lower REAs in FS t than in FS- left-handed writers (not significant, however), but found significantly lower REAs in strongly left-handed F S t as opposed to strongly left-handed FShanders. Geffen and Traub (1980) employed the dichotic monitoring task in a study which found that 76% of LHFS t males, and only 35% of LHFS- males had REAs (Fisher Exact Probability = .0013). The number of Ss in other sexhandedness-FS groups were inadequate to permit meaningful analysis with respect to FS. McKeever and VanDeventer (1977a) found a non-significant REA in LHFS t females, but significant REAs in all other handedness-sex-FS groups. Piazza (1980) analyzed the data for male and female Ss separately. There was a significant REA across handedness-FS groups of males and no FS influences. Among females, there was no significant REA and no significant effects involving FS. Despite this, it is possible that an FS by ears interaction may have been found had male and female data been included in the same analysis. The FS t Ss had lower REAs than the FS- Ss in every handedness-sex group, although the REA of RHFS t males was nearly as large as that of RHFS- males. Rather than speculate about such possibilities, however, the best approach would be to accept the conclusions as presented. Searleman (1980), despite the fact that he employed very large samples of left and right-handers, failed to find effects of FS, or even of handedness. Degree of right hand preference, however, was found to relate positively to REAs. He concluded that REAs are pervasive. Four more recent studies, Orsini et al. (1985), Bryden (1986), McKeever (1986b), and Rich and McKeever (1989) all studied rather large samples and found highly significant REAs for right-handers and significantly smaller REAs for lefthanders. Despite these clear differentiations of right from left-handers, no effects of FS were seen. Conclusions from the Dichotic Verbal Studies. The best conclusion from these studies would seem to be that FS has shown no dependable influence on the language laterality processes involved in the mediation of dichotic listening tasks. The most recent, and in terms of methodology, the most sound studies, provide consistent negative data regarding an influence of FS. This conclusion is very strong with respect to right-handed persons. Two studies, those of Zurif and Bryden (1969) and McKeever and VanDeventer (1977a), employed dichotic digits and did find lesser left lateralization in LHFSt Ss. Piazza (1980) found no FS effects. The Lishman and McMeeken (1977) data showed LHFS t Ss to have significantly smaller asymmetry, but only if they were strongly left-handed

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McKeever

according to a handedness inventory. Contrary to the outcomes reported by Zurif and Bryden (1%9) and McKeever and VanDeventer (1977a), Lake and Bryden (1976) and Geffen and Traub (1980) found lesser REAs in LHFS- males, consistent with the Newcombe and Ratcliff (1973) clinical data. Given that the two studies showing straightforward differences between LHFS- and LHFS + Ss employed dichotic digits tasks, one might speculate that a memory load is necessary to show the difference. Other dichotic digit studies, however (Hines and Satz, 1974; Brigs and Nebes, 1976), found no FS effects. Additionally, if one were to suggest that the digits tasks are more likely to find an FS effect because they involve a memory load, it must be noted that the Orsini et al. task also involves memory for words, the only difference being that the words are not digit names. In conclusion, the data show no hint of FS influences in righthanders, and only occasional findings of FS influence have obtained in lefthanded samples. It is possible that subtle differences in tasks exist, and that lesser lateralization for some aspect of language processing exists in the LHFS + , but the data merely suggest this possibility. Tachistoscopic Studies of Language Lateralization Table 3 presents the main features and findings of a number of tachistoscopic verbal studies in which possible FS influences were assessed. Three of the studies found no FS effects at all (Hines and Satz, 1974; Piazza, 1980; Seitz, 1986). The table also shows the word "none" after the Seitz and McKeever (1984) study, i.e., none of the groups had weak or absent right visual field advantages (RVFA). This experiment found a peculiar result, in which all handedness-FS groups had large and significant RVFAs, but FS + males had by far the largest, with the result that a significant handedness-sex-FS interaction was obtained. The RHFS + males, however, were the smallest group in the study (9 Ss) and an attempted replication (Seitz, 1986) with balanced and larger samples showed all groups having large RVFA of the same magnitude. Six studies found weak or absent RVFA in RHFS+ Ss generally. These included the Hines and Satz (1974) digit recognition accuracy study, the McKeever, VanDeventer, and Suberi (1973) and the McKeever and VanDeventer (1977a) studies of recognition of masked letters, the Hannay and Malone (1976) study of the recognition and recall of nonsense words, the McKeever and Jackson (1979) study of naming latencies for pure color patch stimuli, and the Sullivan and McKeever (1985) study of word naming latencies. Two other studies (McKeever and Hoff, 1982; and McKeever, Seitz, Hoff, Marino, and Died (1983)

Cerebral Organization

385

found interactions of sex and FS status in right-handed Ss. Both studies found lesser RVFA in RHFS- females and RHFS t males in latencies for naming five recurring drawings of objects (the Object Naming Latency Task). While this interaction was initially unexpected, it is interesting that it parallels the sex-FS interaction in aphasia frequencies following left lesions in right-handers as seen in the Hecaen et al. (1981) data (see Table 1). In summary of the twelve tabled studies regarding FS and language laterality in right-handed Ss, then, four found no FS effects, six found that FS reduced left lateralization generally, and two others found that FS reduced left lateralization in RHFS t males but not females. Turning to the studies of left-handed Ss, two found lesser left language dominance in LHFSt Ss. One of these was the study of Zurif and Bryden (1969), which included only ten FS- and ten F S t males, and no female Ss; the other was that of Schmuller and Goodman (1979) which included both sexes, but only eight Ss of each FS designation. The latter study, using simple word recognition accuracy, found LHFS+ Ss to show a significant LVFA; LHFS- Ss showed no significant asymmetry. The outcome for LHFS t Ss is surprising given the very small sample sizes, since significant LVFAs have never been found in much larger samples. The procedure was somewhat unusual, in that Ss reported first the word that a furation point directional symbol indicated, and then reported the other word if they could. The LHFS- resembled, generally, the LHFS + Ss on first reports, but resembled the right-handers on second reports, is., right-handers and LHFS- Ss had more second report errors on LVF than RVF words. The basis for the significant LVFA across report order for LHFS t Ss was due to their having higher error rates on both first and second reports of RVF words. Two studies found no FS effects. One of these (McKeever et al., 1973) found a significantly smaller RVFA in left-handers, but no differential effect of FS within the left-handed group. It might be noted, however, that the sample was small. Seitz (1986), despite an adequate sample size, found no significant effects due to either handedness or FS. Six studies found lesser RVFAs in LHFS- Ss, and five of these (McKeever et al., 1973; Higenbottom, 1973; McKeever and VanDeventer, 1977a; Bradshaw and Taylor, 1979; McKeever, 1979) used letter or word stimuli. The sixth study (Experiment 1, of McKeever, 1979) employed the Color Naming Latency Task. It is perhaps worth noting that three of the studies finding lesser lateralization in LHFS- Ss also found extremely low levels of word recognition for this group, and that Bradshaw and Taylor (1979) explicitly raised the question as to a possible word naming deficit in LHFS- Ss. Piazza’s (1980) data, however, showed no such trend.

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McKeever

Table 3: Summary of tachistoscopic language task studies involving assessment of FS effects, indicating groups showing weak or absent right visual field advantages (WARVFA)

Study

Subjects

FS

Task

WARVFS

Z u r i f & Bryden (1969)

20 RH 20 FS-/ 0 FS+ Ma. 20 LH 10 FS-/lO FS+ Ma.

L e t t e r Recognition Accuracy

LHFS+

Hines & Satz (1971)

84 RH 66 FS-/18 FS+

D i g i t Recognition Accuracy

RHFS+

McKeever e t a l . (1973)

48 RH 24 FS-/24 FS+ 23 LH 14 FS-/ 9 FS+

Masked L e t t e r Recognition

RHFS+ LH

Uni- & B i l a t e r a Word Recognition

LHFS-

Higenbottom (1973)

33 RH NA 56 LH 29 FS-/27 FS+

L e t t e r Recognition Accuracy

LHFS-

Hines & Satz (1974)

60 RH 30 FS-/30 FS+ NA 30 LH

D i g i t Recognition Accuracy

None

Andrews (1977)

48 ?

Trigram Recognition

FSt

McKecver & VanDeventer (1977a)

80 RH 44 FS-/36 FS+ 71 LH 34 FS-/37 FS+

Masked L e t t e r Recognition

RHFS+ LHFS-

15 FS-/33 FS+

Hannay & Malone 30 RH (1979)

15 FS-/15 FS+

Nonsense Word Recognition

RHFS+

McKeever & Jackson (1979)

24 RH

12 FS-/12 FS+

Colour Naming Latency Task

RHFS+

Piazza (1979)

32 LH 32 RH

16 FS-/16 FS+ 16 FS-/16 FS+

B i l a t e r a l Word Recognition Accuracy

None

Bradshaw & Taylor (1979)

36 LH 36 RH

18 FS-/18 FS+ 36 FS-

U n i l a t e r a l Word Naming Speed

LHFS-

McKeever & Hoff (1979)

64 RH 32 FS-/32 FS+

Object Naming Latency Task

McKeever (1979)

51 LH

Colour Naming Latency Task

15 FS-/36 FS+

RHFS- Fe RHFS+ Ma LHFS-

387

Cerebral Organization Table 3: Continued

Study

Subjects

FS

Schmuller & Goodman

16 LH 8 RH

8 FS-/ 8 FS+ 8 FS-/ 0 FS+

McKeever (1979)

25 LH

1C FS-/15 FS+

McKeever e t a1

50 RH 27 FS-/23 FS+

(1983)

Task

WARVFS

B i l a t e r a l Word Recognition

LHFS+

Uni & B i l a t e r a l Word Recognition

LHFS-

Object Naming

RHFS- F .

Latency Task

RHFS+ M .

Sullivan & McKeever (1985)

40 RH 21 FS-/19 FS+

Word Naming Latency Task

RHFS+

Seitz & McKeever (1984)

50 RH

29 FS-/21 FS+

B i l a t e r a l Object Naming Latency

None

Seitz (1986)

48 RH 48 L H

24 FS-/24 FS+ 24 FS-/24 FS+

B i l a t e r a l Object Naming Latency

None

A b b r e v i a t i o n s : NA = n o t assessed; LH = Left-handed; males; Fe = females

RH = right-handed;

Ma =

Conclusions from the Tachistoscopic Language Laterality Studies. The majority of the evidence suggests that FS acts to reduce left hemisphere language lateralization in right-handers, particularly males. In left-handers, the balance of the evidence suggests that LHFS- Ss are the group who are most apt to be less left hemisphere dominant for language. Exceptions to this finding come from the very small sample studies of Zurif and Bryden (1969) and Schmuller and Goodman (1979). The Schmuller and Goodman study was the only one assessing FS in left-handed Ss and requiring the memory storage of the second to-be-reported word, and it indicated the LHFS t Ss had a Bradshaw significant LVFA. A replication of their finding would be important. Again, however, the sample was small and significant LVFAs for bilateral word stimuli is an unusual finding. The lessened left hemisphere specialization in LHFS- suggested by the majority of the studies may be particularly for letter stimuli and brief presentations, although naming latencies for colour stimuli, a non-demanding task in perceptual terms, have also shown lesser left language specialization.

388

McKeever

Thus, unlike the situation in regard to dichotic listening studies, FS effects for the processing and reporting of visual language stimuli have been obtained with some regularity in the tachistoscopic studies. The direction of the effect most often obtained in right-handers is consistent with the view that FS acts in the direction of bilateralization in right-handers (Hardyck, 1977). The results for left-handers, however, are not consistent with the Hardyck model. The tachistoscopic studies agree more with the Newcombe and Ratcliff (1973) conclusion that LHFS- Ss are less left dominant for language function than are the LHFS t . Studies of FS and Lateral Dissociations of Language Processing. The Hecaen group studies suggest that one effect of FS on language might be to laterally "dissociate" different language processes. Whereas LHFS- and RHFSpatients were found to have virtually all tested language functions more impaired by left than by right lesions, the LHFS t and RHFS t had only two of the six functions more impaired by left than by right lesions. This suggests that the FS- persons are left hemisphere dominant for the various verbal functions, while the F S t are left hemisphere dominant for some but not other verbal functions. Information regarding the lateral coherence of language functions can be gained from the degree of correlation between multiple language laterality tasks administered to the same Ss. A few such studies have been reported, though the motivation has not been to study the lateral coherence of language organization, but rather to address the "problem" of apparent low correlation between dichotic and tachistoscopic verbal task results. The problem has been seen to exist on the logic that if a person is "diagnosed" as left hemisphere language dominant on a dichotic task, a tachistoscopic task purporting to measure "language lateralization" should provide the same diagnosis. Bryden (1965) failed to find any correlation of asymmetries from a dichotic digits task and a task involving unilateral tachistoscopically presented letters. He speculated that the result could be due to differences in the two tasks or to a dissociation of laterality effects for the two modalities. In an effort to make the tasks more similar, Zurif and Bryden (1969) devised a letter recognition task which presented four letters bilaterally on each trial. The dichotic digits task presented four digits per stimulus item to each ear. Ss were 20 RHFS-, 10 LHFS-, and 10 LHFS t male undergraduates. No significant cross-modal correlations were found. Indeed, the largest cross-modal correlation (between the bilateral ordered administrations) was only t .18, well short of significance.

Cerebral Organization

389

Zurif and Bryden (1969) did not examine the cross-modal correlation in the different handedness or FS groups. Hines and Satz (1974) addressed the question of cross-modal correlation by testing a larger sample than tested by Zurif and Bryden (1969), and assessing the reliability of the two tasks. Both tasks presented series of digits on each trial. The cross modal correlation was significant for RHFS- Ss (r = +.39). The authors stated that the correlation for RHFS + Ss (r = + .34) was also significant at the .05 level, but actually a correlation of .36 would be required for significance with the 28 df available. The correlation for left-handers was clearly non-significant (r = t .02). They concluded that auditory and visual language processing were "dissociated" in left-handers. Since the correlation for RHFS + Ss was actually non-significant, the same conclusion would be justified in relation to RHFS- SS. Fennell, Bowers, and Satz (1977a; 1977b) did not directly correlate ear difference scores with visual field difference scores, but correlated left ear scores with left field scores, and right ear scores with right field scores. Such correlations would not necessarily be informative regarding the communality of language laterality in the two modalities, but, in any event, the correlations were non- significant. Smith and Moscovitch (1979) correlated half field recognition accuracy score differences with ear difference scores of 15 non-inverted handwriting posture (NHP) left-handers, 15 inverted handwriting posture (IHP) left-handers, and 15 right-handers. The visual task presented vertically-spelled CVC trigrams, unilaterally, and the auditory task was a standard dichotic consonant vowel discrimination task (DCVT). Correlations were negative and non-significant for all three groups. Dagenbach (1986) administered a tachistoscopic task similar to that employed by Smith and Moscovitch (1979), and a dichotic "fused word" task to 230 Ss. He computed cross-modal correlations for right-handed, ambidextrous, and lefthanded groups. Contrary to the thrust of the findings of Hines and Satz (1974), Dagenbach found no correlation for right-handers (r = -.08), but low positive correlations for ambidextrous and left-handed groups (r = + .20 and r = + .26, respectively). An important requirement for assessing the degree of lateral coherence from correlations between tasks is that the measurements be reliable. The studies reviewed above provided no assurance that this requirement was met. Indeed, those investigators who did assess the reliability of their measurements of asymmetry found them to be rather poor. Hines and Satz (1974) obtained split-

390

McKeever

half reliability data for their dichotic and tachistoscopic tasks. They found the split-half reliability of the dichotic task to be good (rtt = t .%), but the reliability of the tachistoscopic task was poor (r,, = t.46). Dagenbach (1986) also calculated split-half reliabilities for his tasks, but did not report them. Since he presented both raw correlations and correlations corrected for attenuation for his major groups, however, one can calculate that the mean reliability of his tasks was approximately .65. If tasks possess adequate reliability, lack of correlation between dichotic and tachistoscopic task asymmetries can be interpreted as implying that different neural networks are involved in the processing of the two tasks. We have been concerned with the need to develop reliable measures of functional asymmetries for dichotic and tachistoscopic tasks. We found that requiring Ss to identify only the syllable they were "most certain of hearing" on the DCVT yielded much higher reliability than had been reported for the task when Ss had been encouraged to report both syllables presented on a trial (e.g., Teng, 1981), a procedure that results in a great deal of guessing. Further, we have the S indicate her/his response by bracketing, with both forefingers, the printed syllable from the list of syllables displayed on the table at which he/she is seated. The split-half reliability for the DCVT, thus administered, was found to be .88 (McKeever, Nolan, Diehl, and Seitz, 1984). Two additional tasks with excellent reliability, developed in our laboratory, are the Bilateral Object Naming Latency Task (BONLT) and the Dichotic Object Naming Latency Task (DONLT). The BONLT (Seitz and McKeever, 1984) is a bilateral version of the Object Naming Latency Task (ONLT) originated by McKeever and Jackson (1979). The BONLT was developed in hopes of providing a visual task which might show a larger asymmetry than the ONLT. Seitz and McKeever (1984) found that the BONLT yielded much larger asymmetries than did the ONLT, and in addition, unpublished data show the split-half reliability of the BONLT, calculated from the performances of 146 Ss, to be .87, much better than that of the ONLT (.42 to .55 in various studies). Finally, Krutsch and McKeever (1987) developed the DONLT in order to have an auditory task highly similar to the BONLT which could be used along with the BONLT to assess the question of cross-modal correlation of language laterality. The DONLT delivers the names of two of the five objects from the BONLT (apple, clock, lamp, moose, shoe) on each of 180 trials, with precisely the same stimulus orders as on the BONLT. Krutsch and McKeever (1987) found that the DONLT yielded a split-half reliability of .90 in a sample of 40 Ss.

Cerebral Organization

391

In addition to reliability, verbal laterality tasks should provide robust rightside superiorities in right-handers. These tasks are rather exemplary in this regard. All three tasks show highly significant right-side advantages in righthanded samples, and the percentages of right-handers showing these advantages for the DCVT, BONLT, and DONLT are 83.1% (McKeever, 1986b), 95.9% (composite data, 98 Ss), and 92.5% (Krutsch and McKeever, 1987), respectively. The percentages of right-sided advantages in left-handers for the three tasks have been found to be 73.9% (McKeever, 1986b), 83.0% (Seitz, 1986), and 54.2% (Krutsch, 1989), respectively. Two studies in our laboratory have administered the BONLT and one of the auditory tasks just described to the same Ss. Krutsch (1989) administered the BONLT and the DONLT to 27 right-handers and 24 left-handers. The correlation between the asymmetries across all Ss revealed a small but significant correlation (r = t.28, 49 df, p < .025). The correlations for right and lefthanded Ss were highly similar ( t .27 and t .%, respectively). Thus, the data suggest that language processings of these tasks are equally "dissociated in right and left-handers. Because of the data of the Hecaen group indicating that FS t persons may have less focal, or more laterally dissociated, language functions, correlations between asymmetry measures were computed for FS t and FS- Ss across and within handedness groups. The correlation for FS t Ss was -.02, nonsignificant; that for FS- Ss was t.54, df 25, p < .004). For LHFS- Ss the correlation was t .52 (11 df, p < .07), and for RHFS- the correlation was t .66 (12 df, p < .Ol). For the LHFS t and RHFS+ Ss, the comparable correlations were -.08 and -.07, respectively. These data suggest that FS t Ss, regardless of handedness, had quite different asymmetries for the processing of the DONLT and BONLT. The FS-, however, regardless of handedness, have substantially similar degrees of asymmetry for processing these tasks. The strongest correlation was obtained for FS- males ( t .82, df 11, p < .OOl). A second study (VanEys and McKeever, 1988) had administered the DCVT and the BONLT to 64 right-handed Ss, all of whom had been classified for FS. Although the study had manipulated several variables, essentially no effects of these manipulations on asymmetries had been observed. Having found the crossmodal correlation results just cited, we were interested in whether cross-modal correlations might be found in the VanEys and McKeever data. The results showed no relationship across the 64 right-handed Ss (r = t.13, p < .307). Thus, the data suggest that the BONLT and DCVT are processed less similarly than are the BONLT and DONLT. When the cross-modal correlations were computed for F S t and FS- Ss separately, the correlation was again non-

392

McKeever

significant for the RHFSt (r = t.01, df= 30,p <.95), but the correlation for the RHFS- was significant (r = t.36, df = 30,p c .045). The data from these two studies would seem to illustrate that cross-modal correlations can be shown for well-constructed tests, that the degree of correlation can be expected to differ as a function of the nature of the processings required, and that a greater lateral coherence of language processes in FS- than FS t persons may be characteristic. Further study of this question, with reliable tests designed to tap different aspects of language function may contribute importantly to our understanding of possible FS influences on cerebral organization. Tachistoscopic Studies of FS and Spatial Processing Laterality Although there is a substantial literature on FS and visuo-spatial ability in neurologically intact Ss, relatively little work has been done with regard to possible differential influences of FS on the lateralization of visuo-spatial processing. This is partially due, no doubt, to the fact that few tachistoscopic spatial tasks yield clear asymmetries, and it is generally believed that spatial processing ability is much less lateralized than is language (Bryden, Hecaen, and DeAgostini, 1983). Five lateralized tachistoscopic studies which employed adequate sample sizes and were designed to assess possible FS effects can be described. Gilbert (1977) administered a lateralized face recognition task to 37 FS t and 27 FS- Ss, the FS classifications ignoring the handedness of the Ss themselves. The task produced a significant LVFA, but no differential asymmetry between FS groups. The FSt were, however, significantly inferior to the FS- in a separate, non-lateralized test of face recognition ability. Piazza (1980) also administered a lateralized face recognition task to Ss of varying sex-handedness-FS categories. The task showed a significant LVFA across groups. Left-handednessand FS t status were associated with very small half-field differences, the FS effect being marked among right-handers and quite small among left-handers. Schmuller and Goodman (1980) employed bilateral presentation of drawings of familiar objects, with a directional symbol at futation to indicate which object to report first. Eight Ss of each handedness-FS status were included. Results showed a significant LVFA for RHFS- Ss and a significant RVFA for LHFS t Ss, with non-significant asymmetries for the other two groups. These findings are the only ones showing a significant RVFA for a left-handed FS-classified

Cerebral Organization

393

group on a task regarded by the investigators as a visuo-spatial laterality task. The small sample size, plus the fact that the recognition of bilaterally presented familiar object drawings yields RVFAs in right-handers (Seitz and McKeever, 1984) must suggest caution in accepting these findings without replication. The data do suggest, nonetheless, that RHFS- Ss showed the “expected LVFA, while LHFS + Ss showed a “reversed asymmetry.” Marino (1983) studied the performances of 75 left-handed Ss on a lateralized spatial task originally employed by Berlucchi et al. (1979). The task, the Clockface Reading Latency Task (CRLT), requires that Ss report, as quickly as possible, the time shown on a clockface having no numerals on it. Berlucchi et al. had shown that, despite the vocal nature of the final response, the spatial processing demands of the task are sufficient to elicit a left visual field advantage (LVFA). When FS status was defined according to left-handedness in both first and second degree relatives, Marino (1983) found a significant LVFA in both latency and error data, and a trend toward a significant FS by visual field interaction, in which LHFS + Ss showed little asymmetry while LHFS- Ss showed a LVFA. When the FS+ definition was based on left-handedness in first degree relatives only, a significant visual field by FS interaction obtained in error data, but the interaction was just short of significance in the latency data (p < .07). Marino concluded that the CRLT provided some evidence for a greater right hemispheric specialization of spatial function in LHFS- than in LHFS + Ss. Marino and McKeever (1988) also studied the performances of 74 FSclassified right-handed Ss on the CRLT. Data were analyzed using two different definitions of FS status. The first definition classified Ss as FS+ if they had a first-degree relative, or two or more second degree relatives, who was (were) left-handed for writing. This analysis showed a significant LVFA. No other effects were significant, although there was a tendency for FS- Ss to show a greater LVFA than did FS+ Ss. In the second analysis, Ss were classified as FS+ only if they had a first degree left-handed relative, and as FS- only if they had no left-handed second degree relatives at all. This eliminated eight Ss whose previous FS+ status was based on second degree relatives only and it also excluded seven Ss whose previous FS- status had ignored the presence of a single left-handed second degree relative. This “strict FS” definition revealed a main effect of field, as in the previous analysis, but also a highly significant FS by field interaction. The FS+ Ss showed minimal LVFS while the FS- showed a substantial LVFA. The authors concluded that FS+ is associated with reduced right hemispheric specialization for visuo-spatial processing in right-handers.

394

McKeever

These few studies suggest that FS t status is associated with reduced right hemisphere specialization for spatial processing in right-handers. Of the four experiments assessing possible FS effects in right-handers, only that of Gilbert failed to find reduced asymmetry in RHFS t as opposed to RHFS- Ss. The two studies of left-handers obtained somewhat disparate findings, although both found LHFSt Ss to be less right hemisphere dominant for their tasks. Schmuller and Goodman (1980) found a significant RVFA for LHFS t Ss; the data of Marino (1983) suggested right hemispheric specialization in the LHFSSs and no asymmetry for LHFS t Ss. FS and Handwriting Posture Searleman, Tweedy, and Springer (1979) assessed handwriting posture in 60 LHFS t and 53 LHFS- Ss and found no relationship between these variables. Parlow and Kinsbourne (1981) also reported no relationship between FS and handwriting posture in a sample of 73 left-handers. Both studies based FS status designations on left-handedness in first degree relatives only. McKeever (1979), however, considered both first and second degree lefthanded relatives in defining FS and reported 3 positive relationship between the inverted handwriting posture and FS t status in 143 left-handers. Most recently, we have reported the results of an analysis of the incidence of FS- and FS t status, and the number of left-handed (writing hand) first and second degree relatives of 471 left-handed Ss (McKeever and VanEys, 1989). Our findings showed that the incidence of FS t status was not significantly different between non-inverted hand posture (NHP) and inverted hand posture (IHP) Ss when FS status was defined only in terms of first degree relatives. When FS status was defined in terms of left-handedness in second degree as well as first degree relatives, IHP Ss had a significantly higher incidence of FS t designations than did NHP Ss. In addition, we also employed Bishop’s (1980) suggested definition of FS status, which considers only parents and grandparents as a means of avoiding possible confounding of FS status with number of relatives. This definition yielded a still greater incidence of FS t designations in IHP than in NHP left-handers. In addition, we found that left-handedness is significantly more common in the relatives of IHP than NHP left-handed Ss, and that this relationship is principally a function of a much higher rate of left-handedness in the maternal family line of the Ss. Specifically, only 6.2% of the maternal relatives (mother, her siblings, and her parents) of NHP Ss were left-handed, while 13.7% of the maternal relatives of IHP Ss were left-handed. The incidence

Cerebral Organization

395

of left-handed relatives was also significantly higher in the paternal families of IHP than NHP Ss, but the difference was significantly less dramatic (10.6% to 7.1%, respectively).

FS and Hand Skill, Eye Dominance, and Degree of Hand Preference The degree of difference between the hands, in terms of skill, and of eye and hand preferences have received some attention as possible markers for cerebral language lateralization. The question of the possible relationship of these variables to FS has been addressed to some extent. McKeever and VanDeventer (1977a; 1977b) compared handedness-FS groups on finger tapping, grip strength, and pegboard tasks, as well as on the Edinburgh Handedness Inventory. They also tested Ss for monocular and binocular sighting dominances. Data were presented in terms of the percentages of Ss who did not show superiority or preference for the same side as their writing hand. The only significant difference was for the tapping test, where LHFS+ Ss were more frequently superior on the side of their preferred writing hand. It might be noted also, that except for the tapping test, the LHFS- group had slightly higher percentages of Ss whose superior or preferred side was congruent with their writing hand side on all other measures (pegboard, grip strength, inventory, monocular and binocular sighting dominances). Thus, the LHFS t were significantly more left hand dominant on tapping, but were not generally more left-sided on the other measures. Porac and Coren (1979) showed that right hand and right ear preferences of offspring were significantly lower in families where one or both parents had left hand and ear preferences, than where both parents had right sided preferences. Eye and foot preferences were not differentially affected by differences in parental preferences for eye and foot. Birkett (1979) presented data showing left-handers generally preferring the right eye for sighting, while right-handers had no preference for monocular sighting dominance. The data also showed that among right-handers, but not left-handers, there was an FS effect wherein RHFS- males were more often right eyed while RHFS t males were more often left eyed; and RHFS- females were more often left eyed and RHFS t females were more often right eyed. These results are highly inconsistent with those of McKeever and VanDeventer (1977a), especially with respect to the total percentage of left eyed preference. The latter found only 18.2% of RHFS- Ss and 25.0% of the RHFS t used the left eye for monocular sighting. The comparable figures for Birkett’s Ss were 56.3% and 52.6%. The fact that these figures are so high, and that they are higher than in

396

McKeever

the left-handers, raises questions as to the representativeness of the Birkett sample. In addition to the McKeever and VanDeventer data, Annett (1985) has cited several studies showing that the majority of right-handers are right eye dominant for sighting. Finally, we have gathered a substantial amount of data on the relationship of FS to laterality quotients on the Edinburgh Handedness Inventory. Separate two-way ANOVAs (sex, FS) were computed on the Edinburgh Handedness Inventory laterality quotients (LQs) of 1157 right-handed and 133 left-handed writers. LQs can vary from -100 (complete left hand preference) to t100 (complete right hand preference). The mean LQs of the right-handed FS groups were as follows: RHFS- females (N = 469) = t 79.5; RHFS t females (N = 215) = t 77.5; RHFS- males (N = 324) t 74.4; and RHFS t males (N= 149) = + 71.4. Among right-handers, the females were significantly more right hand preferent, but these differences are extremely small and their statistical significance reflects the power of the large sample size more than the size of the difference in mean LQs. The FS t had slightly lower LQs than the FS-, but this difference was not significant (p’ .lo). Among left-handers, the mean LQs of the female LHFS(N=44) and LHFS t (N=40) were - 47.5 and -36.5; the mean LQs of the male LHFS- (N=31) and LHFS t (N= 18) were -49.55 and -52.78. There were no significant sex or FS effects among left-handers. We are currently increasing the sample sizes substantially, particularly of left-handers, in order to be able to draw a firmer conclusion regarding possible LQ differences between LHFS- and LHFSt Ss. At present, the conclusion must be that FS is unrelated to degree of hand preference. Conclusions regarding FS influences on manual skill differences are less certain, but available data suggest a weak relationship, if any. These findings argue that FS and direction/degree of hand preference/skill are largely independent factors which could exercise independent influences on cerebral organization. The Incidence of FS in Left-and Right-Handers

We have accumulated data regarding the frequency of FS in 4,031 college students, not selected for FS status. The FS designations were based on firstdegree relative handedness for writing. Handedness of the college students themselves was also based on hand used for writing. Table 4 shows the incidence of FS- and FS t status for the sex-handedness groups. The incidence of F S t designations was very slightly higher,in RH

Cerebral Organization

397

Table 4: Percentages of female and male right-handers and left-handers having at least one first degree (FS+) or having no first degree (FS-) left-handed relatives

Group

FS+

FS-

Total N

Right-Handed Females Males

36.8 33.2

153.2 66.8

2315 1381

Left-Handed Females Males

52.0 47.4

48.0 52.6

202 133

females than RH males, but because of the large sample size, the difference is significant (X’= 4.81, df 1, p < .03). Among left-handers, FS t designations were, again, significantly, though slightly, more common in females than males (X’ = 6.73, df 1, p < .01). These data suggest that studies not assessing FS and contrasting the performances of left-handed males and females, may also be contrasting possible FS influences without realizing it. In addition, we have looked at the frequency of FS t in left-handed college students for whom we have data regarding the handedness of second degree relatives as well as first. Somewhat surprisingly, fully 30.0% of left-handers have absolutely no left-handed parents, siblings, biologically related aunts or uncles, or grandparents.

FS and Abilities Although Hardyck and Petrinovich (1977) published a highly influential review which concluded that no differences in abilities between left and right-handers could be found, there are now a number of studies which show relatively slight but significant superiorities of right-handers over left-handers on visuo-spatial tasks (see McKeever, 1989). A number of studies have attempted to identify a subgroup of left-handers who might be responsible for the lower scores of heterogeneous samples of left-handers. Familial handedness, sometimes in combination with strength of left hand preference, has received the most at tent ion.

398

McKeever

Bradshaw, Nettleton, and Taylor (1981) administered the Australian version of the Wechsler Adult Intelligence Scale (WAIS) to 48 left- and 48 right-handed Ss. All Ss had strong preferences for the preferred hand. They found that LHFS t Ss scored significantly lower than LHFS- Ss or the right-handed Ss on the Performance Scale, but not on the Verbal Scale of the WAIS. Since the Performance Scale contains a number of "spatial" subtests, they suggested that positive FS was unfavourable for spatial ability in left-handers. They saw no influence of FS in right-handers. Burnett, Dratt, and Lane (1982) administered a spatial test (mental rotation) to 353 undergraduates at Rice University. They gathered information on FS and also established five levels of hand preference of Ss from the Edinburgh Handedness Inventory. Results showed that males outperformed females at all levels and that the highest scores were obtained by the moderately, as opposed to the strongly, right-handed Ss. Familial sinistrality was unfavourable for spatial ability among the nonright-handers, and clearly favourable among right-handed males. Yeo and Cohen (1983) studied the question of possible differences between sex-FS groups in spatial ability, perceptual speed, and verbal fluency. Ss were 124 right-handed undergraduates at the University of Texas (31 Ss in each of the four sex-FS groups). Results indicated that the FS t showed relative decrements on 50th spatial tasks employed (mental rotation and disembedding), and on perceptual speed, but not on verbal fluency. The authors concluded that FS t Ss, especially females, show decrements in spatial ability, and they suggested that the poorer performance of the FS t could be due to a relative bilateralization of language processing in this group. Searleman, Hermann, and Coventry (1984) studied only left-handed undergraduates whose Scholastic Aptitude Test (SAT) scores were available. The Ss were given a hand preference questionnaire. Searleman et al. found that strongly (exclusively left preferent) left-handed FS t students had scored markedly lower on the combined verbal and mathematics sections of the SAT than did strongly left-handed FS- or weakly left-handed Ss. The strongly lefthanded FS- Ss showed a particularly marked advantage over the other lefthanders on the verbal section of the SAT. The groups involved in this study were rather small (only 13 strongly left-handed FS t and 11 strongly left-handed FS-, for example), but the effects were highly significant. The authors noted that the LHFSt Ss of Bradshaw et al. (1981) who performed poorly on the Performance Scale of the WAIS were also strongly left-handed, and they

Cerebral Organization

399

suggested that FS+ status in combination with strong left hand preference may be a marker for lower abilities in left-handers. A total of 343 right-handers, classified for FS, were individually administered The Stafford Identical Blocks Test (Stafford, 1961) in our Bowling Green State University laboratories over a period of about ten years. The Stafford Identical Blocks Test (SIBT) is a test of visuo-spatial ability involving mental rotation. This data has been reported by McKeever, Seitz,, Hoff, Marino, and Diehl(l983). The principal findings were that males scored significantly higher than females and that FS had a substantial effect among females. Specifically, RHFS- females scored significantly higher than RHFS + females. Indeed, the superiority of males on the SIBT was a function of the poor performances of FS+ females, with the FS- females scores being comparable to those of males generally. Additionally, the performances of FS-classified left- and right-handers on the SIBT have been compared (McKeever, 1986b). A total of 134 left-handers and 225 right-handers were studied. This investigation found that males and righthanders scored significantly higher than females and left-handers on the SIBT. Again, a substantial superiority of RHFS- over RHFS+ females was seen, and the main effect of FS (FS + unfavourable) approached significance across sex and handedness groups (p = .06). The FS+ scored lower than the FS- in all sexhandedness groups except among left-handed females. Recently, Rich and McKeever (1989) looked at SIBT and Minnesota Paper Form Board (MPFB) performances of 64 left and 64 right-handers who had been classified for FS status. In addition to FS, the factors of sex, handedness, and immune disorder were included as counter-balanced independent variables. Again, males scored significantly higher than females on the SIBT (but not on the MPFB) and right-handers scored significantly higher than left-handers on both spatial tests. N o direct effects of FS were found in this somewhat unusually constituted sample, but FS was found to interact with history of immune disorder. Those Ss who were negative or positive for both FS and immune disorder status (FS- without a history of immune disorder and FS+ with a history of immune disorder) scored significantly higher on both spatial tests than those Ss who were negative for one of the factors (FS or immune disorders) and positive for the other factor. This interaction held across both sex and handedness factors. The explanation for this interesting and unexpected interaction is uncertain. We have suggested that FS and immune disorder status may be equally and oppositely correlated with endogenous steroid levels, which in turn, may be related to spatial ability (Klaiber, Broverman, and Kobayashi, 1967; Petersen, 1976; Hampson and Kimura, 1988). The hypothesis requires that

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the superior spatial ability groups would have intermediate levels of endogenous steroids, and that intermediate levels would be optimal for spatial ability. While this is highly speculative, the interaction of FS with immune disorder history is an interesting finding which should be pursued in future work. Finally, Murray (1988), in his Masters Thesis research at Northern Arizona University, compared the performances of 24 left-handed and 24 right-handed college students on the Halstead Reitan Neuropsychological Test Battery (HRNB). Somewhat surprisingly, Murray found left-handers to score significantly lower than right-handers on both the Memory and the Time (for both hands together) measures of the Tactile Performance Test (TPT). The TPT is a ten hole form board test which Ss perform with each hand separately and then with both hands together, while blindfolded. Most interestingly, this effect was due to FS t Ss. When the performances of RH and LHFS t Ss were compared on all tests, it was found that the F S t had lower scores (not significantly) on all 14 HRNB measures, and significantly lower scores on TPT time scores for the non-dominant hand and both hands together, TPT total time, TPT memory score, finger tapping dominant hand score, tapping difference score between hands, and the Impairment Index. These findings suggest mild general deficits in LHFS t as compared to both the RH and LHFS- Ss, with the deficits most apparent on tests involving manual performances (TPT and Finger Oscillation). It should be borne in mind, however, that these findings are from small samples (12 LHFS- and 11 LHFS t ) ,and they clearly require replication. Furthermore, the poor performance of LHFS t Ss on the Finger Oscillation Test stands in contrast to the data of McKeever and VanDeventer (1977a), although the finger tapping test administration in the latter study was not the same as that employed in the HRNB. Murray did administer a screening instrument for history of possible neurological insult to his Ss and found no differences between handedness or between LHFS- and LHFS t groups. In sum, data from this area of investigation do indicate that left-handers perform somewhat less well than right-handers, particularly on visuo-spatial tasks. There is some evidence to suggest that this relative decrement may be largely a function of the LHFS t group or some subset of the LHFS t group. Reduced spatial ability in RHFS t females has also been suggested by a number of the studies. Studies differ with respect to FS effects in right-handed males, Burnett et al. (1982) finding FS t associated with high spatial ability in right-handed males, and Yeo and Cohen (1983) finding FS unfavourable to spatial ability in RHFS t males. Our own investigations initially suggested positive influences of FS (McKeever et al., 1983)

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but later found no influences of FS on spatial ability in right-handed males (McKeever, 1986b; Rich and McKeever, 1989).

Summary of Demonstrated and Suggested Correlates of FS

A brief overall summary can be offered. Findings suggest that FS is probably an important variable for understanding cerebral organization, but the data are rather messy. One cannot escape the conclusion that there is considerable noise in the data and that the factor of FS is probably confounded, in some critical but unknown ways, with other effective variables. The dichotic language task studies fail to suggest clear differences between FS- and FS t persons, regardless of their handedness'. The tachistoscopic studies, though far from perfectly congruent, suggest lesser left hemisphere "dominance" for language function in both RHFS t and LHFS- persons. Recent studies in our laboratory suggest that different aspects of language function may be more laterally "dissociated in the FS t than in the FS-, but further study of this is needed. Tachistoscopic studies also suggest that FS t acts to reduce right hemisphere specialization for visuo-spatial function in right-handed persons, and may do so in left-handers, as well. Little relationship of FS to hand preference, hand skill, or eyedness has been found, suggesting that motor laterality characteristics are largely independent of FS. We have found that FS is related to handwriting posture in left-handed persons, with the inverted writers having significantly more left-handers among their first plus second degree relatives. This greater incidence of left-handedness occurs mainly within the maternal family line. The incidence of FS, when FS status (FS t or FS-) is defined by the presence of at least one first degree relative is significantly, but very slightly, greater in right-handed females than males (about 37% versus 33%) and in lefthanded females than males (about 52% to 47%). Clearly, FS t status is more frequent in left than in right-handers (50% versus 35.5%, respectively). Fully 30% of left-handed college student Ss report no left-handed parents, siblings, biologically related aunts or uncles, or grandparents. Finally, there is evidence

'

Although it is outside the scope of this paper, it should be noted that there are a number of studies which show relationships of dichotic language task asymmetries and degree of hand preference, with left ear advantages more common in strongly left handed persons.

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that FS t status may be negatively related (as is left-handedness) to viduo-spatial ability in both left and right-handers. A tentative general inference, that FS t may influence visually mediated processes more than auditory and motor processes, may be advanced in view of this set of conclusions.

The Difficulty of Assessing FS Influences: Suggested Strategies

If the reader is struck, as she must be, by the lack of consensus regarding the influence of FS on cerebral organization, even after twenty years of active investigation with rigorous techniques and careful observations, it must be reemphasized that the problem is exquisitely difficult. First, the effects of handedness itself on cerebral organization are not entirely clear, although the generalization that left-handers are less lateralized for more functions than are right-handers is supportable. To expect that FS effects within handedness groups should be readily demonstrable is unrealistic. I believe there are two major problems which we must solve if we are to further our understanding of the influence of FS on cerebral organization. These are definitional problems, including most crucially some means of identifying "genetic left-handers," and problems regarding the comprehensiveness of measured attributes. The prejudice against left hand writing has abated in Western countries, but in defining FS one has to assess handedness mainly from information about the writing hand of parents of Ss, and the pressures for right hand writing were greater for those individuals. Efforts to assess FS in grandparents are frustrated by the still greater pressure for right hand writing which was prevalent in their school days. Indeed, I believe that efforts to understand FS influences in cultures such as France are, with all due respect to the Hecaen group, who have made noble efforts with respect to the question, largely useless at present. Securing valid measures of both overall hand preference and hand preference for writing are mandatory for research in the area. While problems of defining subject and familial handedness are not inconsequential, the most serious problem centers about a different aspect of the problem of defining FS. This concerns the issue of pathological left-handedness

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(PLH), which is discussed at length in Coren's and Searleman's chapter of this volume. The absolutely basic questions in research on FS, in my opinion, are whether a non-pathological and non-accidental genetically determined form of left-handedness exists, and, if it does, what are its effects on cerebral organization? If pathological/accidental determinants of left-handedness are universal, or common, or even less than major but still not trivial, say 10% of the cases, the problem of assessing the cerebral organizational consequences of "genetic left-handedness" is fraught with difficulty. Bakan (1971) has suggested that all left-handedness is pathological. McManus (1983) denies the existence of PLH in left-handers who have no obvious neurological deficits, but Bishop (1983) has suggested that PLH may account for five percent of ostensibly normal left-handers. Annett (1985), though generally of the opinion that PLH is not a significant phenomenon in seemingly normal left-handers, nonetheless isolated the data of the children of families in which both parents were left-handed when she suspected that one or both parents could be PLH. A total of five of 29 parent pairs were judged as possible cases of PLH, though the number of instances in which both parents were so judged was not reported. Based on this incidence, one might estimate that PLH could exist in somewhere between 8.5% and 17% of left-handers. That the 15 children of the parent pairs Annett thought to be possible cases of PLH had right hand superiorities on her peg moving task, while the other children showed no asymmetry suggests some validity to Annett's "diagnoses" of PLH. Corballis (1983, p. 145) has estimated, within the framework of Annett's (1985) "right shift theory," that the combined incidence of PLH and unusual fortuitous causes could be responsible for 11% of left-handers, i.e., that 11% of left-handers possess the right shift gene. It is conceivable that an even higher incidence could obtain. As noted earlier, fully 30% of college student left-handers report no left-handed persons among first degree relatives nor among grandparents and biologically related aunts and uncles. If, furthermore, subtle pathological effects are often due to heritable anatomic/physiological characteristics which increase reproductive casualty probabilities, as suggested by Bakan (1971), the problem of defining FS+, as though it meant "genetic left-handedness," is greatly increased. Not only does one have to deal with PLH Ss, but with FS t designations based on the PLH of relatives. In other words, the view of Satz (1972) that the likely PLH case is the FS- left-hander may be quite wrong. Some evidence for an association of FS + , PLH, and mental retardation has been reported (Bradshaw-McAnulty, Hicks, and Kinsbourne, 1984; Pipe, 1987;Searleman, Cunningham, and Goodwin, 1988).

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If we assume that both genetic left-handedness and familial pathological lefthandedness exist, then the question "how can we distinguish between them?" becomes the critical question. The basic assumption which underlays much of our interest in the question of possible FS influences on cerebral organization is that genetic left-handednessexists and that it probably has specific effects on the organization of the brain. It seems a good assumption. When we ask what are the influences of FS on the brain, we think in terms of effects of genetic left-handedness, since pathological effects will simply be diverse consequences of the sites and extents of brain damage. I suggest that the major goal of research in the area is to establish the effects of this variety of FS, which, for convenience I shall label FS + B, or positive genetic familial sinistrality. There may well be another type of positive familial sinistrality, FS + @, or positive genetic-pathological familial sinistrality. This variety of FS would be heritable, the inherited traits being those conducive to pregnancy risk and birth stress events. In addition to these two varieties of possible FS + persons, there are the FS-, who have no left-handers in either first or second degree relatives and who as mentioned earlier, represent about 30% of left-handers. These individuals have traditionally been regarded as the prime candidates for PLH. I would suggest, however, that it is possible that while some of the FS- may be cases of PLH, the group would be diverse. First, some FS- cases may have the "right shift gene," but manifest left-handedness in response to fortuitous pressures. In addition, I suggest that more than one genetic blueprint can eventuate in nonpathological left-handedness. One can imagine, for example, a form of lefthandedness, with its own cerebral organization and aptitude correlates (or lack of them), which is of low frequency and is due to some comparably infrequent assortment of polygenes in individual cases. Such cases would be apparent FScases, because of the low incidence of left-handedness in the families, but would actually be genetic left-handers. If the reader will grant these possibilities, no less than five distinct FS groups can be specified: FS+,, FStP, F S t , (persons who suffer identifiable trauma which likely caused their left-handednessbut who also have left-handedness in their families), FS-,, and FS-,. Future research should concentrate on trying to specify these varieties of FS. The groups which are easiest to identify are probably the FS + and FS-, groups. These individuals could be largely identified through their histories of neurological insult/and or particularly poor performance of non-dominant hand on tests of manual speed and dexterity. Their laterality and ability patterns would depend largely on the sites and extents of injury. The FS-, could be largely identified by default, i.e., they would be the FS- for whom on? could find

,

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no history or performance indication of pathology. There could be two forms of FS-,, as suggested above, but for the present, we can ignore that difference. The remaining two groups may not be distinguished by procedures designed to identify PLH (medical histories, unusually poor performance of the nondominant hand, lateralized trophic “shortfall” of the hands or feet, or other potential markers for genetic or non-genetic left-handedness). Both have lefthanded relatives and pathology can be presumed to be so subtle in the FS t gp as to escape detection. I would suggest that the best, though only partially effective, approach to isolating the group of most interest, i.e, the FS t *, would be to constitute groups differing only according to whether the maternal or paternal family line contained left-handers. Such a strategy is powerful in the case of left-handed Ss, but less powerful in the case of right-handed Ss. If heritable pathological traits, manifesting in increased subtle pregnancy risk or birth stress proclivities, are the major cause of FSt,, then it is possible to achieve some control over the likelihood of FS t gp in Ss. If the left-handedness in the father’s family is FS t gp he would not be able to pass that form of left-handedness to his children. Thus, left-handed children would be FSt,, since there would be no FS+ in the maternal line and the father could not pass heritable pathological left-handedness to his children. Right-handed children, however, would be of uncertain type. They would be FS t but could be either RHFS t or RHFS t gp, depending on the origin of the left-handedness in their fathers’ families. However, a group of RHFS t could be secured by selecting those right-handed Ss who have lefthandedness only in the father’s family and who have a left-handed sibling with no history or assessment evidence which would indicate PLH. A second group of left-handed Ss, for research purposes, would consist solely of those whose FS t status was based on FS t in the mothers’ families and whose fathers’ families contained no left-handers. Children, right or left-handed, would be of uncertain type where left-handedness exists in the mothers’ families and not in the fathers’. The segregation of true FS t , Ss from FS t would not be perfect, but the strategy has the potential for greatly clarifying the basic question regarding the influence of genetic left-handedness on cerebral organization. Thus, through a combination of stringent assessment, to rule out PLH, and selection according to side of the family having left-handedness, a much better isolation of possible “types”of left-handers can be achieved. This could allow us to establish, definitively, the influence of both genetic left-handedness and FS + on cerebral organization.

,

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It might be noted that in an earlier study (McKeever and Hoff, 1982) a posthoc separation of RHFS t Ss according to paternal versus maternal FS status showed that the FS t paternal Ss had a substantially lower RVFA than did FS t maternal Ss on the ONLT. Even though right-handers could be either FS + or FS t gp, as just noted, the probability of FS + g would be greater for paternals than for the maternals. Looking back at this finding, I feel we did not adequately appreciate its implication with respect to PLH. Securing groups of pure "maternals" and "paternals" is feasible. For example, in looking over family data from 453 left-handed Ss on whom we have handedness information for all first and second degree relatives, I found that 22.9% of them would fall in the "maternal" group and 20.6% in the "paternal" group. Thus, although many lefthanded Ss have left-handed on both sides of the family, or only in siblings, ample numbers who meet the criteria for maternal or paternal FS t designations exist. The final inadequacy of research in this area, as in much of psychological research, is the lack of breadth of independent and dependent measures and consequent lack of knowledge regarding the interrelationships of attributes. As just suggested, a finer isolation of "varieties" of left-handers and FS groups is needed, and thorough assessments of hand preferences and skill, and handwriting posture in left-handers, should always be incorporated. At the dependent variable level, the typical laterality study administers one dichotic, tachistoscopic, concurrent activities, blood flow, or other task to each S. It is undeniably true for most of us, that we require one, or at most, two hours of our Ss because they are unpaid, except for course credit in introductory psychology, and it is therefore not feasible to administer a wide array of counterbalanced or randomized tasks to them. As suggested in the section on cross modal correlations of asymmetries, however, it may be that it is more in the coherence of lateralities or neural networks than in the lateralities of specific functions that different FS groups differ most importantly. Some strategies, or resources, most obviously subject pay, which will allow the experimenter to secure and maintain valuable subject groups over long enough periods to allow extensive assessments are needed. The problem of securing an ideal set of FS/PLH groups as suggested above is sufficient that, once secured, appropriately broad assessment is mandatory not only in terms of desirability, but in terms of practical efficiency as well. I hope these prescriptions will prove beneficial in pursuing the elusive answer to the deceptively simple question "What is the influence of FS on cerebral organization?"

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SECTION k PSYCHOLOGICAL AND SPATIAL IMPLICATIONS

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LEFT-HANDEDNESS Behavioral Implications and Anomalies, S. Coren (Editor) 0 Elsevier Science Publishers B.V. (North-Holland), 1990

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Chapter 14

Sinistrality and Psychopathology Pierre Flor-Henry Alberta Hospital and University of Alberta

The patterns of cerebral organization which determine sinistrality, although partially understood in certain instances, remain obscure and enigmatic in others. A variety of psychopathological disorders are associated with an excess of lefthanders in the afflicted individuals: autism, certain forms of schizophrenia, bipolar (but not unipolar) affective illnesses, susceptibility to dysphoric mood states, certain types of criminal psychopathy (particularly if recidivistic) and epilepsy with psychosis. Intellectual retardation is also associated with increased sinistrality: Bradshaw-McAnulty et al. (1984) for example found that right hand preference varied inversely with the severity of the mental retardation. Pipe (1987) reported that the incidence of nonright-handedness in developmentally retarded individuals and in patients with Down's syndrome is approximately twice that of a normal comparison sample. Subsequently Pipe (1988) reviewed the evidence which shows that the excess sinistrality in Down's syndrome cannot be the result of "pathological left-handedness:" left hemisphere damage evoking compensatory sinistrality, since in Down's syndrome, as in infantile autism, the subjects with increased right hemisphere language functions indicated by left ear superiority on dichotic listening are dextral. The same unusual association has been found in autistic children (Prior and Bradshaw, 1979). That the pathological left-handedness hypothesis has to be applied with circumspection in special pathological populations is further shown by the fact that increased sinistrality occurring with mild mental retardation is linked to increased familial sinistrality (Searleman et al. 1988). Taylor (1975) in his study of temporal lobe epileptics who underwent temporal lobectomy notes a significant excess of

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sinistrality in the group as a whole (20%), an effect for which patients with “alien tissue” lesions arising during embryogenesis were responsible (28% sinistral), rather than subjects with post-natally acquired mesial temporal sclerosis (12% sinistral). Moreover as he remarks “It is hard to relate this to ‘pathological sinistrality’ (Satz, 1972), since it was equally likely whether the lesion was in the left or right temporal lobe.” At the same time there are undoubtedly some populations where sinistrality, compensatory to left hemisphere pathology, arises. Vargha-Khadem et al. (1985) investigated the consequences of unilateral brain disease, acquired pre- and/or post-natally in 28 children with left hemisphere pathology, 25 with right-sided lesions, comparing these two groups with each other and with 15 normal children. Whereas 100% of the children with right hemisphere lesions were dextral 87% of the patients with left hemisphere lesions were strongly left-handed and all patients with prenatal or early post-natal left hemisphere lesions (two months to five years) were strongly left-handed. The familial incidence of sinistrality was similar in the left and right brain-damaged groups (29% vs 38% respectively). The theme of this paper is psychopathology. However it is important, in order not to lose sight of the complexity of the issues raised by the sinistral brain, that its correlates can be with intellectual-cognitive or manipulo-spatial superiority. Hicks and Dusek (1980) observed that gifted childrm (I.Q. 132 or higher) were significantly less dextral than non-gifted children (I.Q. 132 or less). The fact that Leonard0 da Vinci was left-handed is well known. Perhaps even more remarkably the greatest violinist of all times, Paganini, even although he played the instrument as if he was right-handed, was also left-handed. The number of geniuses who were left-handed is quite extensive, for example, the painters Michelangelo, Rapheal, Holbien, Picasso; in music C.P.E. Bach and Raveli; in the performing arts Charlie Chaplin, Greta Garbo, Marcel Marceau, and Harpo Marx; and political leaders include Alexander the Great, Charlemaine, and Napolean Boneparte. Hkcaen (1984) cites astonishing figures on how, in unimanual sports such as fencing or tennis, the top positions at the level of world or Olympic competition are dominated by sinistrals. In 1981 the 20 tennis players classified as best in the world included 25% of sinistrals. The top 8 places in the Mexico games of 1979 for fencing were all won by sinistrals; the same was true in the Moscow Olympics of 1980. Further in both these sports the number of sinistrals increases proportionally with ranking: for tennis in 1980 there were 17% sinistrals in the first 200, 24% in the first 25, 40% in the first 10 and 75% in the first 4. For fencing in the same year 48% in the first 25, 80% in the first 10 and 100% in the first 4! Similar trends were observed in the world championships for fencing, 1981.

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Turning to neurological diseases there are curious observations where in some cases sinistrals are at an advantage, others at a disadvantage: early onset primary degenerative dementia of the Alzheimer type (onset before 65) affects selectively the left hemisphere, compared to the right and the prevalence of left-handedness in this group is 22%. This contrasts with the late onset senile dementias in whom there is an excess of dextrality (Seltzer et al. 1984). On the other hand (no pun intended!) in schizophrenic patients sinistrality protects against the emergence of persistent tardive dyskinesia (Barr et al. 1989).

Sinistrality and Psychosis Taylor (19 5) extracted from a series of 255 patients H.-O ..ad undergone temporal lobectomy for the relief of intractable psychomotor epilepsy all the patients with ‘alien tissue’ (small tumours, hamartomas, focal dysplasias) and compared them with all the cases of mesial temporal sclerosis. There were 47 of the former and 41 of the latter. Complex interactions between type of neuropathological lesion, sex, handedness and laterality of the epilepsy significantly determined the probability of psychosis which, characteristically, was high in the total sample since 13 of the 88 subjects exhibited a schizophrenic disorder (15%). The probability of psychosis was highest in females, with left sided epilepsy, who were sinistral and who had alien tissue neuropathology. Dextral males with right hemisphere epilepsy and mesial sclerotic lesions were least susceptible to a psychotic evolution. 7 of the 13 psychotics, or almost 54% were left-handed. The five published series where handedness frequencies are considered all show a remarkably high frequency of sinistrality in the special group of schizophrenias associated with temporal lobe epilepsy. The range of sinistrality in the schizophrenic psychoses of TLE lies between 17% and 71.4%, as opposed to 5.3% and 14.6% in control groups: temporal lobe epilepsy without psychosis (Taylor, 1975; Kristensen and Sindrup, 1978; Toone and Driver, 1980; Sherwin et al. 1982; Trimble and Perez, 1982). These several studies cumulate 179 epileptic schizophrenics, with an overall sinistrality of 22.3% whereas for the 238 controls the overall sinistrality was 9% (see Table 1). There are important conclusions which emerge from a consideration of those variables which determine a schizophrenic evolution in temporal lobe epilepsy. As table 1 illustrates sinistrality plays a part, but cannot be necessarily attributed to “simplistic theories of the genesis of left-handedness,” as Taylor repeatedly

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Table 1: Sinistrality in temporal lobe epilepsy with psychosis Author

Controls

Psychotic %

n

total n

%

n

total n

Taylor, 1975

53.8

7

13

14.6

11

75

Kristensen & Sindrup, 1978

17.6

16

91

5.3

5

95

Toone & Driver,l980

17.0

10

57

-

Sherwin et al.1982

71.4

5

7

7.3

5

68

Trimble & Perez,1982

18.0

2

11

-

40

179

21

238

Total n Overall sinistra 1

22.3%

8.8x

emphasises (Taylor, 1975, 1977) in spite of the fact that left hemisphere epilepsy is here another crucial determinant of psychosis. The observation that lesions of the dominant hemisphere arising during embryogenesis, rather than thoseoccurring post-natally are responsible for a later psychotic evolution emphasises the importance of the developmental epoch at which pathological events disrupt the organization of the central nervous system. It is not only what happens where in the CNS which influences subsequent (dis)organization sometimes after a prolonged latency - but when it happens which may be the factor of overriding importance. The onset of seizures with mesial sclerosis is earlier - mostly before the age of two - in contrast to the alien tissue lesions which lead to seizures manifested for the first time much later - after the age of 10. Furthermore there is evidence that those temporal lobe epileptics who subsequently become psychotic have a tendency to first manifest their seizures during puberty: another critical developmental period which, of course, is also shared by the endogenous schizophrenias. It has been shown, formally, that the mental symptomatologyis identical in "epileptic"and "endogenous"schizophrenia (Perez and Trimble, 1980). A substantial body of evidence confirms that schizophrenia is associated with increased incidence of sinistrality. Indeed, although much less studied, this also appears to be true for bipolar affective syndromes. Let us exami9e the evidence for schizophrenia first. One of the first studies, with an impressive

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sample size, is that of Dvirsky, (1976) in the U.S.S.R. He compared 660 male and 610 female schizophrenics against 2,150 healthy male and 2,190 healthy female controls. The continuous forms of schizophrenia were associated with a significant increase in sinistrality. The effect was more pronounced in males than in females, and left-handedness was associated with the more malignant forms of the illness. Dvirsky concluded that sinistrality in schizophrenia was a modifying factor that carried the probability of increased severity of illness. In a later investigation (Dvirsky, 1983) 1,177 right-handed and 93 left-handed schizophrenics were compared in order to establish the possible relationships existing among handedness, age of onset and form of illness in these patients. In dextrals the age of onset was later, around the ages of 35-44 with a high incidence of psychoses with cyclothymic and/or affective delusional features, or catatonic episodes - schizophrenic psychoses with intermittent or only moderately progressive course which the Soviet classification labels "shift-like'' forms. In sinistrals the onset was earlier, around 20-24, with a high prevalence of progressive paranoid forms of chronic schizophrenia. Related findings are reported by Katsanis and Iacono (1989) in the study of 63 schizophrenic patients, of whom 56 were male, where 12 (or 19%) were left-handed. A systematic comparison of ventricular size and neuropsychological performance of the sinistral and dextral schizophrenics showed that the left-handed patients had significantly larger lateral ventricles and were significantly more impaired on the Wisconsin Card Sorting test as well as scoring significantly lower on the WAISR IQ than the right-handed schizophrenics. In the United States Gur, (1977) compared 200 schizophrenics with 200 controls and found an excess of lefthanded responses in the schizophrenics. Eye acuity, eye dominance and handedness-footedness were independent measures which showed no significant associations in either the schizophrenics or the normals. In a series from the United Kingdom, (Fleminger et al. 1977) 800 psychiatric patients and 800 controls were studied. The psychiatric group included 102 schizophrenics and 120 affective psychotics. There were significantly more dextrals (consistent dextrality according to Annett, 1970) among female psychotics than controls, with a similar tendency towards increased dextrality in male psychotics. There were significantly more left-handed writers in male than in female schizophrenics and more mixed-handedness (right hand preferred for writing with left hand responses in one or more of the 12 items of hand preference) in women with personality disorders. Taylor et al. (1980) replicated these results in a sample of 272 schizophrenics who exhibited a significant excess of dextrality (consistent) when compared to the 800 controls of Fleminger et al. (1977). In a subsequent

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study Taylor et al. (1982) reanalysed the Fleminger et al. (1977) investigation by applying the actual Annett handedness classification instead of the Fleminger modification which had been utilized in the earlier reports of this group. Then, the excess dextrality in the functional psychoses was no longer significant. With respect to schizophrenia, the males now showed a trend towards excess sinistrality and the females towards excess full dextrality. Nasrallah et al. (1981) examined 79 consecutive male patients between the ages of 18 and 50, admitted to an in-patient unit over a period of one year and who satisfied DSM-I11 criteria for schizophrenia. They were compared to a control group consisting of 75 hospital staff members. There was a significant excess of sinistrality in the schizophrenic patients (19%) compared to the controls (5%). Paranoid schizophrenia had significantly more left-handed subjects (33%) than nonparanoid forms of schizophrenia (11.5%). Nasrallah and McCalley-Whitters (1982) found no difference in handedness in 88 manic patients (all males) compared to 86 age and sex-matched normal controls (8 and 7% sinistrality respectively). Piran et al. (1982) reviewed the handedness and eye preference in 25 early onset schizophrenics (average age 18 years), 24 brain-damaged subjects, 16 non-psychotic psychiatrically disturbed subjects and 16 healthy controls. All groups were comparable in age and 77% of the schizophrenics were males, the psychiatric controls having an excess of females and the other two groups a sex ratio around unity. With 23% sinistrality for writing and 73% left eye dominance the schizophrenics were significantly more sinistral than all other comparison groups (i.e. 11% left-handed for writing and 27% left eye dominance in the healthy controls, with the other two groups showing lower values). Luchins et al. (1979) found 17% non-dextrals in 66 schizophrenic patients. The full dextral patients were significantly more chronic than the sinistrals. Luchins also administered the Torque test to a subgroup of 55 of these patients. In this test three circles are drawn with each hand. Torque is present when the circles are drawn clockwise with either hand. Torque is associated with sinistrality and, in disturbed children, with a later predisposition to schizophrenia (Blau, 1977). All 34 schizophrenics without torque were chronic whereas the six acute patients all showed Torque. Kameyama et al. (1983) reported on almost 600 Japanese schizophrenics who were diagnosed according to Research Diagnostic criteria. No difference from normals in terms of hand preference was found although the younger patients had a significantly higher frequency of right eye dominance than age-matched controls. Taylor et al. (1983) had also observed a significant excess of consistent dextrality in a combined sample of 179 male prisoners (personality disorders, neurotic and

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violent) as well as in 34 male schizophrenics. In an investigation carried out in the People’s Republic of China (Yan et al. 1985) we found a significant excess of non-dextrality in 200 schizophrenics (20%). This was true for both male and female patients when compared against 432 healthy controls in whom the incidence of non-dextrality was identical to that of Western populations (7%) when cultural variables were taken into account (very strong pressure in China to impose the right hand for writing and chopstick use irrespective of innate preference). The pattern of hand preference in 56 manic-depressive patients did not differ from the controls. The manic-depressives and the schizophrenics had a significant excess of left eye dominance and an increasing divergence between eye and hand dominance when compared to the controls. Andreasen et al. (1982) had found 15% sinistrals in a cohort of 51 schizophrenics. However positive symptomatology schizophrenia was 100% dextral, whereas negative symptomatology schizophrenia was significantly non-dextral, with only 67% dextrality. Furthermore the sinistrals in this series had a significantly greater degree of ventricular dilatation than the dextral schizophrenics (Andreasen and Olsen, 1983). Table 2, which summarizes the evidence reviewed shows clearly that, not withstanding four studies finding excess dextrality in schizophrenia the implications of which will be discussed later, a significant association with sinistrality emerges overall. If the schizophrenic syndrome or bipolar affective disorders are correlated with the sinistral pattern of brain organization it might be expected that mental symptoms might themselves show significant associations with sinistrality or dextrality. There is evidence that this is, indeed, the case. In 70 psychotics, irrespective of diagnosis, Lishman and McMeekan (1976) found a strong association between left-handedness and delusions, a negative association with hallucinations and an over-representation of young males. Pogady and Friedrich (1975) reported an excess of sinistrality in a representative sample of 650 psychiatric patients in a psychiatric hospital: 20% were sinistral or ambidextrous. Paranoid delusions and incoherent thinking were more frequent in sinistrals than in dextrals or ambidextrous patients, whereas emotional lability, depression, visual-auditory hallucinations and obsessions were more frequent in ambidextrous subjects. Manschreck and Ames (1984) found that anomalous motor laterality was significantly higher in schizophrenics than in affectives or normals and very strongly correlated with thought disorder and neurological deficits. These consisted principally of sensory errors on the right side of the body. Further thought disorder and right sided graphesthesia were strongly associated. Manoach et al. (1988) examined the relation between language dysfunction

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Flor-Henry

Table 2: Sinistrality in schizophrenia: Representative research Schizophrenia Sin istra 1 total n % n

Study Yan et al. 1985

Norma 1 s Sinistral total n

%

n

225

20

45

432

7

30

1,270

7

93

4,340

4

175

102

13

12

800

15.4 107

66

27

11

**(Chaugule and Master, 1981) 93

68

63

150

50

76

Nasrallah et al. 1981

79

19

15

75

5

4

Andreasen et al. 1982

51

15

8

Piran et al. 1982 (early onset)

26

23

6

16

11

2

Taylor et al. 1982 (Maudsley sample)

26

15

4

Kameyama et a1 . 1983

584

15

86

686

18

121

**Gur, 1977

200

70

139

200

56

112

Manoach e t al. 1988

58

Dvirsky, 1976 Fleminger et al. 1977* Luchins et al. 1979

Totals

2,487

X2 = 60.03

*

**

31 12%

18 298

6,349

7%

439

( p <0.001)

Handedness as reclassified according to Annett (Taylor et al. 1982) Excluded from overall analysis since classifications that find 50% or more sinistrals i n normals are not satisfactory

(verbal incoherence, derailment, illogical thinking, poverty of information in speech and neologisms) in 58 male schizophrenics. 31% of the sample was lefthanded for writing. All the left-handers were thought disordered, but only 70% of the dextrals, a significant difference at the p ~ 0 . 0 2level of probability. Although the various measures of thought disorder were highly intercorrelated, dextrals and sinistrals did not differ for the items of illogical thinking and neologisms. Some studies, which will be discussed later, also indicate that sinistrals have increased mood instability and are more prone to anxiety than dextrals.

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Sinistrality in Autism and Childhood Schizophrenia Profound affinities link the autistic and schizophrenic syndromes: in both there is a striking excess of males, in both there is an over-representation of first and last-born in the sibship, in both vestibular abnormalities are found, in both there is an excess of sinistrality and in both there are genetic-constitutional (‘idiopathic’) and acquired forms. Satz et al. (1985), who discuss handedness subtypes in autism, review twelve studies published after 1975 which indicated that the prevalence of manifest sinistrality across the twelve studies was 34%. In their own investigations they observe 20% sinistrality and 40% non-dextrality (mixedhandedness). Walker and Birch (1970) studied lateral preference and right/left awareness in 80 male schizophrenic children between the ages of eight and eleven and found an enormous increase in sinistrality. Eighty percent of normal children of comparable age and IQ were right-handed, compared with only 32% of the schizophrenic children, who, in addition, had impaired right/left orientation. Many investigations have noted a striking excess of sinistrality in autistic children; for example, Colby and Parkinson (1977) showed a frequency of 65% non-dextrality in 20 autistic children compared to 12% in normal children. High functioning autistic children have been found to have a specific decrement in verbal, as opposed to performance, subtests of the Wechsler Scales and to exhibit a significant impairment in the left hemisphere in neuropsychological testing, scoring in the normal chronological age level for right hemisphere indicators (Hoffman and Prior, 1982).

Sinistrality in Monozygotic and Dizygotic with Schizophrenia Boklage (1977) analyzed concordance for schizophrenia and handedness in those published twin series that gave the information. In dextrals, if both twins were dextral and one schizophrenic, the probability of concordance for schizophrenia was of the order of 90%. If the twins were discordant for handedness however, the concordance for schizophrenia fell to about 25%, and then the sinistral twin was likely to be schizophrenic, with a milder illness than that occurring in dextral monozygotic twins. Boklage notes that monozygotic twinning is itself an anomaly of embryonic symmetry formation, which takes

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Table 3: Autism and childhood schizophrenia Normals S inistra 1

Schizophrenia S in ist ra 1 Study

total n

Walker and Birch, 1970 80 20 Colby and Parkinson, 1977 Satz et al. 1985 387 (Average across 12 studies)

%

n

68x 54 65% 13 33.6x 130

total n 69 25

%

n

20%

14 3

12%

place extremely early in ontogenesis, before amniogenesis, some 8 or 9 days after conception, with the appearance of the prochordal plate and primitive streak, which define a dorso-ventral axis, thus an antero-posterior gradient, and therefore, right and left. Boklage concludes that abnormalities of embryonic symmetry development are reflected simultaneously in the twinning process itself, in the abnormal motor laterality found in all monozygotic twins (schizophrenic or normal), and in the etiology of schizophrenia in this population. Incidence of sinistrality was 12% (8/66) in dizygotic and 34% (19/56) in monozygotic schizo+hrenic twin pairs. Luchins et al. (1980) are generally in agreement with Boklage since they report in the study of M Z twins with schizophrenia, that in twinships with at least one left-handed twin the sinistrals tend to suffer from a mild schizophrenia and the dextrals are not schizophrenic. These authors, pooling their own sample with that of Gottesman and Shields (1972) and of Pollin and Stabenau (1968) cumulate 20 1-2 L.H. twinships. 17 of the 23 (or 74%) of the sinistrals are schizophrenic as opposed to only 8 of the 17 (or 47%) dextrals. Thus concordance for schizophrenia is much higher in twin pairs where both are righthanded, but the probability of a schizophrenia type psychosis is greater in sinistrals. Contrary to Boklage when these authors examined the Gottesman and Shields sample for dizygotic twins a higher concordance for schizophrenia emerged in the 1-2 L.H.twinships than in the 2 R.H. pairs with 2 of 7 (or 28.6%) concordant for schizophrenia as contrasted to only 1 or 26 (or 3.8%) in the 2 R.H.pairs. This is the exact opposite of the situation found in monozygotic twins. The fact that in monozygotic twins discordant for schizophrenia and discordant for handedness, it is the sinistral member of the pair that is at risk for psychosis, at first glance appears to fit well with the with the CT scan investigations of twins discordant for schizophrenia undertaken by

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Reveley et al. (1987) who found significantly lower left relative to right hemisphere density in the schizophrenic twin than in their healthy co-twin or in normal control twins. But, not in keeping with the pathological left-handedness hypothesis, in these 11 pairs of identical twins discordant for schizophrenia and 18 pairs of control twins, only one of the schizophrenics, none of their co-twins, and three of the control twins were left-handed. It should be noted that the frequently quoted notion that monozygotic twins are more likely to be lefthanded than the general population is not supported by a critical review of the evidence (McManus, 1980) which indicates that the incidence of sinistrality is the same in monozygotic, dizygotic or singletons. However, if a family history of sinistrality is present then the twins are discordant for handedness in 41% of instances, this falling to 16% in the absence of familial sinistrality. The figures cited are for monozygotic twins but the picture is similar for dizygotic pairs. Further Lewis et al. (1989) could not confirm in a recent English series the interactions described by Boklage. In this series of 44 psychotic twins 7 or 16% of the schizophrenics (ICD-9 criteria) were left-handed as opposed to 1 of 10 or 10% with other psychiatric diagnoses and 3 or 27 or 11% in normal twins. The criterion used was the hand preferred for writing. In their total sample of twins analyzed data was available for 125 subjects: 60 complete pairs, half monozygotic and half dizygotic. 14 of the 125 were left-handed. There was a trend towards higher rates in the schizophrenic subjects - however in the monozygotic pairs, discordant for psychosis, the proportion of left-handedness was similar in the discordant pairs (1 of 17 discordant) as opposed to 4 of 14 discordant in the twins concordant for schizophrenia. The authors point out that their failure to replicate Boklage, 1977 may be in part because of the criteria for hand preference. Their choice for definition of handedness is particularly unfortunate since with such a scheme the frequency of ambiguous handedness cannot be determined and it is largely in this area that there is an over representation of atypical handedness in schizophrenia. For example Satz et al. (1989) in the examination of 93 carefully diagnosed schizophrenics compared to 105 normal controls find 60% right-handed, 39% mixed and 2% left-handed in the patients as opposed to 82% right-handed, 14% mixed and 4% left-handed in the normal subjects. Satz et al. conclude that the excess of mixed-handedness in schizophrenia is robust (p<0.005). Lewis et al. (1989) in an important footnote report that in re-analysis of the Gottesman and Shields, (1972) series, at a 20year follow-up, less than half of the original probands fulfill RDC or DSM-I11 for schizophrenia. Retrospectively thus it would appear that the preponderance of

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sinistrals in the earlier series is brought about by schizo-affective or manicdepressive variants.

Sinistrality and Mood Moscovitch et al. (1981) documented excess dextrality (100%) in 52 patients with severe unipolar depressions requiring electroconvulsive therapy (ECT). Moreover, the incidence of familial sinistrality in first-degree relatives was only 5.7%, as opposed to 28.5% in the general population reviewed by Annett (1970) (p <0.01). In the study of bipolar psychoses, however, Sackeim and Decina (1983) encountered 29% sinistrality in the children of bipolar affective parents, as opposed to a 5% incidence in control children. The 113 parents had an incidence of sinistrality of 24.7%; this was even higher in bipolar I subjects (32.5% sinistral); bipolar I1 subjects showed 15.7% sinistrality. Similarly, Green et al. (1983), in the analysis of children born to schizophrenic parents, found 38% sinistrality,which contrasts with a 7.5% incidence in control children matched for age, sex and verbal IQ. Davidson and Schaffer (1983) confirmed earlier American studies that indicated that in the general population, independent of familial sinistrality, high anxiety subjects are significantly more sinistral than low anxiety groups. These authors measured anxiety in 538 college students, as a function of sex and handedness. Dextral females were significantly more anxious than dextral males and sinistrals were very significantly more anxious than dextrals. 4/7 (or 57%) of the subjects who were rated as most anxious were left-handed as opposed to 2/33 (or 6%) as least anxious (p cO.OoO6). Earlier Hicks and Pellegrini (1978) had obtained very similar results in 266 college students in whom handedness characteristics were correlated with scores on the Taylor Manifest Anxiety Scale. In the comparison of the 23 lefthanded and 12 mixed-handed subjects with the 35 students who were totally dextral the sinistral group scored significantly higher for anxiety than the purely dextrals. Orme (1970) measured emotional instability in 300 school girls from an approved school and contrasted the 23 (7.6%) who were left-handed with the 277 who were right-handed, both groups being of comparable and normal intelligence. Although the dextrals, here, were significantly more unstable than 143 control girls, the sinistrals were emotionally more unstable than the dextrals. Several authors in various countries have documented a curious interaction in which males with low monoamine oxidase activity were characterized by extraversion, impulsivity, ‘sensation-seeking’behaviours, suicidal attempts and

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dysphoric mood, i s . Von Knorring et al. (1984) in Sweden and Demisch et al. (1982) in Germany, (see Von Knorring et al. 1984 reference five other replications). Further Buchsbaum, (1977) showed that in the normal population sinistral males have significantly lower platelet MA0 activity whereas no male with high platelet MA0 activity was left-handed. In addition Von Knorring, (1984) found that in a large series of 1,129, 18 year old boys selected from the general population, low M A 0 activity subjects were very clearly left-handed. The table below shows how pronounced is the effect: Dextral

Low MA0 (30%) Normal or High MA0

x2

= 5.3,

S i n is t r a 1

290 (29.2%)

30 ( 4 2 . 2 % )

702 (94.5%)

4 1 (5.5%)

p < 0.02

von K n o r r i n g (personal comnunication, 1989)

Porac and Coren (1981) further discuss a number of studies undertaken in the 1920’s and 1930’s, all of which find evidence of increased sinistrality in emotionally disturbed children. It was noted above that Sackeim observed the unusually high incidence of 25% sinistrals in bipolar affective psychoses but that Moscovitch found 100% dextrality in severe unipolar depressions. Lishman and McMeekan (1976) found a slight but significant excess of sinistrality in 70 psychotics brought about principally by the manic-depressive and schizo-affective patients, i.e. bipolar states. Young psychotic males, irrespective of diagnosis were also more sinistral than expected by chance. Taylor et al. (1982) similarly reports that in both the Fleminger series and the Maudsley sample there is a trend whereby male schizophrenics are more sinistral and female schizophrenics more fully dextral. In a replication involving 114 consecutive schizophrenics and manic-depressives Flor-Henry and Yeudall (1979) found exactly the same increase in sinistrality in manic-depressive and schizo-affective psychoses as that reported by Lishman and McMeekan: 12% showed strong sinistrality largely because of an increase in inconsistent sinistrality (Annett, 1970) - i.e. twice the frequency found in the general population. That the excess in the bipolar psychoses should be of inconsistent, rather than consistent, sinistrality is of particular interest in the light of the report of H k a e n and Sauguet (1971) according to which the non-familial types of consistent sinistrality are similar to

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consistent dextrals in exhibiting an absence of language deficits after right brain lesions, while inconsistent sinistrals have bilateral language representation. In a series of papers I have argued the evidence that psychosis, the manifestation of altered bilaterally asymmetrical hemispheric organization, will necessarily be accompanied by alterations of lateral motor preference (FlorHenry, 1979a; 1983; Flor-Henry and Koles, 1980; Yan et al. 1985). The dextralitysinistrality dimension interacts in a complex manner with psychopathological expression and motor laterality and is a much more subtle indicator of unusual or altered hemispheric processes than is generally supposed. The description of a few single cases, admittedly exceptional, is illuminating. Lewis C. Bruce published in Brain in 1895 "notes of a case of dual brain action." This described a 47 year old Welsh sailor who suffered from a manic-depressive illness. When in the excited phase "talkative and mischievous" he was dextral and understood both Welsh and English. In the phase of melancholia he no longer understood English and became exclusively sinistral. During the transitions between depression and mania he was ambidextrous. When asked to write with his left hand during the manic phase (when he was dextral) he produced mirror writing from right to left. We have seen two related personal cases. The first was of a young man with unipolar depressive psychosis. When well he was ambidextrous; when depressed he lost the manual skill of his left hand, becoming completely dextrd, except for writing. Our second case was of a woman in her early fifties who proved to be 100% sinistral during a manic episode, becoming 100% dextral when asymptomatic. These last two examples suggest that depression, altering the organization of the right hemisphere, interferes with left hand dexterity and that mania, altering the organization of the left hemisphere, interferes with dextrality. The patient of Bruce shows the opposite: dextral when manic and sinistral when depressed, ambidextrous in the intervals. The picture here is further complicated by the fact that he was bilingual and it is now well established that the patterns of cerebral organization of bilinguals or polyglots is different to that of monolinguals. Notwithstanding, Corballis and Beale (1976) perhaps provide the clue: sinistrals or ambidextrous subjects capable of automatic mirror writing with the left hand are often right brain dominant, or have left hemisphere damage. In this context, it is also of relevance that 47% of patients with Multiple Personality shift handedness in the course of their transformations (Putnam et al., 1983). Since Lombroso (1903) associated criminality with left-handedness this issue has remained controversial. Hare and Forth (1985), commenting that research with adult criminals has produced inconsistent results, some showing an

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association of left-handedness with criminality, others failing to do so, find in a sample of 258 prison inmates no difference in hand preference distribution against a large normative sample of 1,211 control males. This was true for psychopathic and non-psychopathic criminals. If anything criminals were more right sided dominant than non-criminals. Porac and Coren (1981) however point out that in an earlier investigation (Hare, 1979) where the base rate of sinistrality in two criminal groups was similar to that seen in age-matched male controls (13.3% versus 11.4%) the non-psychopathic criminals have an incidence of lefthandedness of 8.3% whereas the psychopathic criminals with a frequency of 17.8% are significantly more left-handed. Wardell and Yeudall (1980) found that it was a sub-group of criminal psychopaths who were prone to sinistrality: those with a large verbal/performance IQ discrepancy (reduced verbal relative to performance abilities) and with psychopathy and schizophrenia elevations on the MMPI. Nachshon and Denno (1987) investigated the correlates of lateral preference and criminality, starting with a cohort of 2958 black children studied in Philadelphia in a perinatal project between 1959-1962. Selecting only males, with complete laterality data taken at age 7 and who were resident in Philadelphia up to the age of 18, a final sample of 1066 black males was obtained. Police records showed that 313 (or 29%) had been criminally charged and 10% violent. Unexpectedly, left-handedness was significantly more frequent in non-offenders than in offenders, there were no differences in foot or eye preference. Violent and non-violent offenders were similar for hand and foot preference but there was a significant difference for eye preference: 60% of the non-offenders and 64% of the non-violent offenders showed right eye preference whereas only 40% of the very violent offenders (n = 57, murder, rape, aggravated assault) were right eye preferent (p <0.008). 63% of the very violent and 60% of the violent offenders had cross preferences as opposed to 47% in all other groups. Citing evidence indicating that in males, birth stress is associated with left eye (but not left hand) preference and that visual evoked potentials are of higher amplitude from the dominant than the non-dominant eye and thus that there may be a hemisphere-eye association Nachshon and Denno conclude that their results suggest the presence of left hemisphere dysfunction in violent criminals, possibly the resuit of birth trauma. An important, and methodologically rigorous investigation on the association between sinistrality (left-handedness) and delinquency was published in 1980 by Gabrielli and Mednick. In this prospective study 265 children were intensively examined in 1972. They were extracted from the Danish peri-natal cohort of

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9,125 children consisting of all the children born between 1959 and 1961 at the Rigshospitalet in Copenhagen. All children with schizophrenic mothers or fathers (n = 72) were included and were matched against a group of children with psychopathic fathers, or character disordered mothers (n = 72). The remainder, 121 matched controls had parents free from psychiatric disorder. The delinquent individuals were identified from the Danish police register in 1978. Because, characteristically, only 7% of the girls were registered as delinquent they were discarded from the analysis. The final group, thus, consisted 36 boys with a schizophrenic parent, 36 with psychopathic father or character disordered mother and 57 with parents free from psychiatric disturbance. The offender group was considerably more sinistral, and this was independent of the presence or absence of mental illness in the parent. For instance 65% of the definitely left-handed boys were later arrested at least once but only 30% of the righthanded group. Low verbal IQ was correlated with criminality, but not with sinistrality. Neither neurological impairment nor social interaction factors (11 measures) correlated either with criminality or sinistrality. 33% of criminals with multiple arrests were sinistral; of those with a single crime 11% and in nonoffenders 7%. The evidence reviewed indicates that sinistrality is a modifying variable in a number of psychopathological syndromes. Given the fact that psychopathological syndromes are themselves heterogeneous, are not always similarly defined and that the methods of evaluating dextrality or sinistrality are often astonishingly dissimilar, there is no surprise that conflicting findings are frequent in this area of research. The majority of studies report an increased incidence of sinistrality in schizophrenia. Averaging the studies reviewed in this paper (Table 2) the incidence of sinistrality for the schizophrenics is 12%, i s . almost twice that of the combined controls (7%). There is a tendency for a male preponderance, early onset and progressive paranoid forms of psychosis with structural cerebral changes in this group. Some sub-populations of schizophrenics appear to exhibit excess dextrality. The available evidence suggests that these are the more acute schizophrenic syndromes, with florid, positive symptomatology, intermittent course and favourable outcome - as opposed to the chronic, deficit, negative symptomatologyschizophreniaswhere sinistralityis over-represented. Depending on the pathogenesis and its timing in development, intrauterine or post natal, the implications of a dextral of sinistral brain organization for schizophrenia will be different. Under certain circumstances subtle dysfunction in the left hemisphere will determine both a progressive schizophrenia and sinistrality. In &her

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circumstances, the more lateralized dextral brain will have less compensatory neural plasticity than the sinistral brain to a primary, or induced left hemisphere dysregulation. I suggested a few years ago (Flor-Henry, 1983) that the acute schizophrenias, with positive symptoms and frequent affective features are largely a variant of the affective psychoses but, in contrast to the purer manic-depressive bipolar psychoses that are associated with sinistrality, are more likely to exhibit first rank symptomatology through induced left hemisphere overactivation than the less lateralized classical bipolar syndrome. Even although not necessarily expressed through changed patterns of motor laterality there is evidence, both neurophysiological and neuropsychological, of altered left hemispheric functions during acute psychosis. Wexler and Heninger (1979) found in acute schizophrenia, acute mania and depression a temporary loss of right ear advantage to verbal dichotic stimuli during the psychotic episode. Hommes and Panhuyssen (1970) demonstrated by carotid barbiturization that dextral depressed patients no longer manifested aphasic responses after dominant hemisphere injections, this effect being significantly correlated with the intensity of the depression. Moreover the emotional reaction was euphoric, normally a feature of non-dominant hemisphere barbiturization. Mood regulation, recent evidence suggests, hinges on a complex reciprocity between the left and right frontal limbic zones, stability maintained by mutually interacting contralateral inhibition, with different emotions having different lateralization. The overall regulation of mood, however, appears to be the result of left frontal inhibitory regulation of right hemispheric systems. (Flor-Henry, 1979b, 1986). If this representation is, in its essential aspects, accurate then the interaction between mood instability and sinistrality immediately follows at the theoretical level. As we have seen the empirical evidence supports such an association, both in the general population, in a particular personality type and in the bipolar psychoses. A sub-population of criminal psychopaths, on the evidence to date, are clearly more sinistral than the general population. Only some express, through altered motor laterality, the subtle dysfunction of dominant hemispheric functions that is a fundamental aspect of the cerebral disorganization of the male psychopath (See Flor-Henry, in Press for review).

Conclusion The evidence reviewed shows that the incidence of certain psychiatric disorders is increased in sinistral populations. In a few others there appears to

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be an increase in full dextrality. These observations are theoretically important. The origins and consequences of sinistrality in schizophrenia are different in monozygotic twins, dizygotic twins and in singletons. The concordance for schizophrenia is much higher in R.H. - R.H. pairs than in 1-2 L.H. pairs monozygotic twinships but the reverse is true for dizygotic twins with a higher probability of illness in both twins when discordant for handedness. The severity of illness is most severe in RH-RH monozygotic when both are likely to be affected, but the frequency of schizophrenia is much higher in 1-2 LH pairs who exhibit a milder form of illness. Given the fact that the original diagnosis of schizophrenia in the original series could only be maintained in less than 50% of cases, it seems probable that these latter are largely manic-depressive or schizo-affectivevariant of the syndrome; which, in singletons are also associated with excess sinistrality. It cannot be overemphasized that being a twin is biologically extremely hazardous, both during intrauterine and post-natal life. Not only are general pregnancy complications such as toxaemia greater, but problems specific to twin pregnancy arise: foetal crowding, unequal distribution of blood supply, the so called ‘placental transfusion syndrome’ and shorter gestation, with lower birth weight. 55% of twin births are premature and a third of the deliveries are breech - as opposed to 3% in the general population. Low birth weight is a risk factor for schizophrenia in singletons and breech for autism. One in six of all multiple pregnancies in England terminate with the death of one or both twins (Dunn, 1965). Numerous studies over the last 50 years have shown that twins have a specific deficit in language abilities, corresponding to about 6 months developmental delay when compared to controls (reviewed by Mittler, 1971). Supporting Boklage, this can be viewed as the expression of disturbed symmetry fundamental to the twinning process: one twin is derived from a partially differentiated left and the other from a partially right differentiated half of a single embryo (Newman, Freeman and Holzinger, 1937). Why should sinistrality be a modifying variable associated with a mild illness in monozygotic twins, but with a severe illness in singletons? The origins of schizophrenia are multiple as are the determinants of sinistrality. Both schizophreniaand sinistrality are fundamentallyrelated to a functional alteration of dominant hemispheric systems. A sinistral pattern of brain organization may be the result of 1- genetic influences; 2- compensatory to left hemisphere damage, especially if sustained before the age of five years. In an important communication O’Callaghan et al. (1987) have drawn attention to a third mechanism: a failure in the development of normal right/left temporal neuroanatomical asymmetries which occurs around the 31 week of pregnancy.

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Studying at the age of four the handedness of extremely low birth weight infants, born at 26 and 29 weeks gestation and all weighing less than 1,OOO g, it was found that the handedness preference was random, with 54% sinistral. In children over 1,OOO g handedness was similar to control children (8% and 15% respectively). None of the very low birth weight children had cerebral palsy and they were of average intelligence. That their sinistrality was not the result of ‘pathological left-handedness’ was indicated by the absence of a gradation in frequency of left-handedness from the very low birth weights to the heavier infants. Pollin and Stabenau (1968) found, in 100 monozygotic twins discordant for schizophrenia that birth complications were four times more frequent in the schizophrenic than in the healthy co-twin. Chitkara et a]. (1988) compared the diagnostic distribution in 20,895 patients at the Maudsley Hospital with that of 504 patients born twins, including 117 twins (or 23%) where the co-twin had died before the age of 15. There was a significant excess of schizophrenia, personality disorders and substance abuse in the sub-group where the co-twin had died at birth or in early childhood: a subgroup which also had significantly more birth and perinatal complications and more males. When both twins survive - and one remains healthy, and only one monozygotic twin becomes schizophrenic, the hypodensity of the left hemisphere in the affected is not associated the sinistrality. The inference is that the pathogenic factors, if developmental, operated after 31 weeks of gestation, or if, as is more probable, of brain damage type, were sufficient to induce pathological changes in the left hemisphere, but not severe enough to induce pathological sinistrality. Hence, perhaps the reason for the more benign illness, in the left-handed subject in monozygotic pairs discordant for schizophrenia and discordant for handedness. In the right-handed monozygotic twins concordant for schizophrenia there is no birth trauma effect in evidence (Boklage, 1977). The pathogenic influences must therefore lie essentially in the consequences of the twinning process itself, leading to a more severe illness because of their greater lateralization. In this situation the sinistral twin would be protected from schizophrenia by the more bilateral pattern of brain organization - hence the high concordance for schizophrenia in dextral pairs with the malignant syndrome. Thus, the paradox that in monozygotic twins sinistrality protects against schizophrenia in the absence of external cerebral insult but if the latter is present increases the probability of schizophrenia. An accentuation of this process would produce increased sinistrality. The absence of sinistrality in this group is striking, if we recall that the incidence of sinistrality in monozygotic twins with a history of familial sinistrality is very high: 41% and that in the general population 30% - 35% have first degree relatives who are

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left-handed. Hence the very high concordance for schizophrenia since these subjects are identical, with the same ‘flaw’ in lateral symmetry organization; unmodified by external intrauterine or perinatal events. Dizygotic twins are essentially singletons sharing the same uterus, and therefore are especially at risk for acquired brain-damage during the latter part of gestation and early postnatal life. Should these adverse events occur, both twins would be exposed to the increased risk factors: hence the greater concordance for schizophrenia in dizygotic pairs with at least one left-handed member, and the greater risk for the sinistral: the pathological left-handedness effect. The brain-damage model also fits the general population of schizophrenics with severe manifestation of the illness: early onset males, with absence of family history for psychosis, and structural pathology specifically involving the left hemisphere (see for review Flor-Henry, 1989). In the absence of brain-damage the more lateralized dextral brain is more susceptible to induced dysfunction leading to acute schizophrenia, positive symptomatology with first rank symptoms, which have been correlated with left-lateralized changes in regional cerebral circulation (Uchino et al. 1987). Hence the excess dextrality in this group, and in the subgroup of chronic severe depressions. This sub-group, described by Moscovitch et al. (1981) consisted of unipolar depression, is 80% female and exhibits a striking absence of sinistrality, both in the patients and their first degree relatives. Modern evidence suggests that the left hemisphere, through contralateral inhibitory regulation, modulates emotional and aggression related neural subsystems in the right hemisphere. Given the relative vulnerability of the left hemisphere in males, the excess sinistrality seen in male psychopathy and in males with low MA0 activity and ‘sensation-seeking’behaviours is theoretically to be expected. Similarly the association of left-handedness with emotional instability, in both sexes, and with the bipolar psychoses immediately follows. Finally, it is perhaps worth observing that, in the sinistrality-psychopathology interactions discussed, the modifying influence of gender is clearly evident in the psychiatric categories discussed - but, except for the special case where the cotwin does not survive, it is absent in twins. This is another illustration of the exceptional and neurobiologically hazardous situation intrinsic to twinship and which overrides the more subtle, gender related, cerebral effects which can be manifested in singletons.

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Dvirsky, A.E. (1983). Clinical manifestations of schizophrenia in right-handed and left-handed patients. Journal of Neuropathology, Psychiatry (Korsakov), 83, 724-727 (in Russian). Fleminger, J.J., Dalton, R., & Standage, K.F. (1977). Handedness in psychiatric patients. British Journal of Psychiatry, 131, 448-452. Flor-Henry, P. (1986). Observations, reflections and speculationson the cerebral determinants of mood and on the bilaterallv asmmetrical distributions of the major neurotransmitter systems. Acta Neboiogical Scandinavica, 74(Suppl 109), 75-89. Flor-Henry, P. (1983). Commentary and synthesis. In P. Flor-Henry & J. Gruzelier (Eds), Laterality and Psychopathology pp. 1-18, Amsterdam: Elsevier Science Publishers B.V. Flor-Henry, P. (1979a). Laterality, shifts of cerebral dominance, sinistrality and psychosis. In J. Gruzelier, & P. Flor-Henry, (Eds), Hemisphere Asymmetries of Function in Psychopathology pp. 3-19,Amsterdam: Elsevier/North Holland Biomedical Press. Flor-Henry, P. (1979b). On certain aspects of the localization of the cerebral systems regulating and determining emotion. Biological Psychiatry, 14, 677698. Flor-Henry, P. (In press). Observations on the influence of gender in schizophrenia and some other psychopathological syndromes. Schizophrenia Bulletin, special edition edited by Dr. J.M. Goldstein. Flor-Henry, P. (In press). Psychopathology and hemispheric specialization. In F. Boller, J. Grafman, & G. Gainotti (Eds) Handbook of Neuropsychology, Section M, Emotional Behaviour and its Disorders, Amsterdam: Elsevier Science Publishers Biomedical Division. Flor-Henry, P., & Koles, ZJ. (1980). EEG studies in depression, mania and normals: evidence for partial shifts of laterality in the affective psychoses. Advances in Biological Psychiatry, 4, 21-43. Flor-Henry, P. & Yeudall, L.T. (1979). Neuropsychological investigation of schizophrenia and manic-depressive psychoses. In J. Gruzelier & P. FlorHenry (Eds), Hemisphere Asymmetries of Function in Psychopathology, pp. 341-362, Amsterdam: Elsevier/North Holland Biomedical Press. Gabrielli, W.F., & Mednick, SA. (1980). Sinistrality and delinquency. Journal of Abnormal Psychology, 89, 654-661. Green, P., Hallett, S., & Hunter, M. (1983). Abnormal interhemispheric integration and hemispheric specialization in schizophrenics and high-risk children. In. P. Flor-Henry & J. Gruzelier (Eds), Laterality and Psychopathology, pp. 443-469, Amsterdam: Elsevier Science Publishers BV. Gottesman, I.I., & Shields, J. (1972). Schizophrenia and Genetics: A Twin Study Vantage Point. Orlando, Florida: Academic Press. Gur, R.E. (1977). Motoric laterality imbalance in schizophrenia. Archives of General Psychiatry, 34, 33-37. Hare, R.D. (1979). Psychopathy and laterality of cerebral function. Journal of Abnorntal Psychology, 88, 605-610.

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Hare, R.D., & Forth, A.E. (1985). Psychopathy and lateral preference. Journal of Abnormal Psychology, 94, 541-546. HBcaen, H. (1984). Les gauchers. Etude neuropsychologique, pp. 43-53, Paris: Presses Universitaires de France. HBcaen, H., & Sauguet, J. (1971). Cerebral dominance in left-handed subjects. Cortex, 7, 19-48. Hicks, RA., & Dusek, C.M. (1980). The handedness distributions of gifted and non-gifted children. Cortex, 16, 479-481. Hicks, RA. & Pellegrini, R.J. (1978). Handedness and anxiety. Cortex, 14, 119121. Hoffman, W.L., Prior, M.R. (1982). Neuropsychological dimensions of autism in children: a test of the hemispheric dysfunction hypothesis. Journal of Clinical Neuropsychology, 4(1), 27-41. Hommes, O.R., & Panhuyssen, L.H.H.M. (1970). Bilateral intracarotid amytal injection. Psychiatry, Neurology and Neurochirurgia, 73: 447-459. Kameyama, T., Niwa, S.-I., Ken-ichi, H., & Saitoh, 0. (1983). Hand preference and eye dominance patterns in Japanese schizophrenics. In P. Flor-Henry & J. Gruzelier (Eds), Laterality and Psychopathology pp. 163-180, Amsterdam: Elsevier Science Publishers BV. Katsanis, J. & Iacono, W.G. (1989). Association of left-handednesswith ventricle size and neuropsychological performance in schizophrenia. American Journal of Psychiatry, 146, 1056-1058. Kristensen, O., & Sindrup, E.H. (1978). Psychomotor epilepsy and psychosis. 11. Electroencephalographicfindings (Sphenoidal electrode recordings).Acta Neurologica Scandinavica, 57, 370-377. Lewis, S.W., Chitkara, B., & Reveley, A.M. (1989). Hand preference in psychotic twins. Biological Psychiatry, 25, 215-221. Lishman, WA., & McMeekan, E.R.L. (1976). Hand preference patterns in psychiatric patients. British Journal of Psychiatry, 129, 158-166. Lombroso, C. (1903). Left-sidedness. North American Review, 170, 440-444. Luchins, D., Pollin, W. & Wyatt, R.J. (1980). Laterality in Monozygotic schizophrenic twins: an alternative hypothesis. Biological Psychiatry, 15, 87-93. Luchins, D.J., Weinberger, D.R., & Wyatt, R.J. (1979). Anomalous lateralization associated with a milder form of schizophrenia. American Journal of Psychiatry, 136, 1598-1599. Manoach, D.S., Maher, BA., & Manschreck, T.C. (1988). Left-handedness and thought disorder in. the schizophrenias. Journal’of Abnormal Psychology, 97, 97-99. Manschreck, T.C., & Ames, D. (1984). Neurologic features and psychopathology in schizophrenic disorders. Biological Psychiatry, 19, 703-719. McManus, I.C. (1980). Handedness in twins: a critical review. Neuropsychologia, 18, 347-355. Mittler, P. (1971). The Study of Twins. Harmondsworth,England: Penguin Books Ltd.

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Moscovitch, M., Strauss, E., & Olds, J. (1981). Handedness and dichotic listening performance in patients with unipolar endogenous depression who received electroconvulsive therapy. American Journal of Psychiatry, 138, 988-990. Nachshon, I., Denno, D. (1987). Violent behaviour and hemisphere function. In S. A. Mednick, & T. E. Moffitt (Eds), Biology and Antisocial Behaviour, Cambridge: Cambridge University Press. (In Press). Nasrallah, HA., & McCalley-Whitters,M. (1982). Motor lateralization in manic males. British Journal of Psychiatry, 140, 521-522. Nasrallah, HA., Kuperman, S., McCalley-Whitters, M., Kellor, K. (1981). Laterality in paranoid and non-paranoid schizophrenia. Presented at American Psychiatric Association annual meeting. New Orleans, May, New Research Abstracts, p.32. Newman, H.H., Freeman, F.N., & Holzinger, K.J. (1937). Twins: A Study of Heredity and Environment, University of Chicago Press. O’Callaghan, M.J., Tudehope, D.I., Dugdale, A.E., Mohay, H., Burns, Y, & Cook, F. (1987). Handedness in children with birthweights below lo00 g. The Lancet, May 16, 1155. Orme, J.E. (1970). Left-handedness, ability and emotional disability. British Journal of Social & Clinical Psychology, 9, 87-88. Perez, M.M. & Trimble, M.R. (1980). Epileptic psychosis-diagnostic comparison with process schizophrenia. British Journal of Psychiatry, 137, 245-249. Pipe, M.-E. (1988). Atypical laterality and retardation. Psychological Bulletin, 104, 343-347. Pipe, M.-E. (1987). Pathologicalleft-handedness: is it familial?Neuropsychologia, 25, 571-577. Piran, N., Bigler, E.D., & Cohen, D. (1982). Motoric laterality and eye dominance suggest unique pattern of cerebral organization in schizophrenia. Archives of General Psychiatry, 39, 1006-1010. Pogady, J. & Friedrich, J. (1975). Left- and right-handedness as a manifestation of functional asymmetry and brain hemisphere specialization and psychopathology. Czechoslovakian psychiatry, 71, 273-278 (in Czech.). Pollin, W. & Stabenau, J.R. (1968). Biological, psychological and historical differences in a series of monozygotic twins discordant for schizophrenia. Journal of Psychiatric Research, 6, 317-332. Porac, C., & Coren, S. (1981) Special populations, chapter 8, Lateral Preferences and Human Behavior, pp 128-146, New York: Springer-VerlagNew York Inc. Prior, M.R. & Bradshaw, J.L. (1979). Hemisphere functioning in autistic children. Cortex, 15, 73-81. Putnam, F.W., Post, R., Guroff, J., Silberman, E., & Barban, I. (1983). 100 cases of multiple personality disorder. Presented at the American Psychiatry Association New York City Annual Meeting. (new research). Reveley, MA., Reveley, A.M., & Baldy, R. (1987). Left cerebral hemisphere hypodensity in discordant schizophrenictwins.Archives of General Psychiatry, 44, 625-632. Sackeim, HA., & Decina, P. (1983). Lateralized neuropsychological abnormalities in bipolar adults and in children of bipolar probands. In P.

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Flor-Henry & J. Gruzelier (Eds), Laterality and Psychopathology, pp. 103128, North Holland: Elsevier Science Publishers BV. Satz, P. (1972). Pathological left-handedness: an explanatory model. Cortex, 8, 121-135. Satz, P., Soper, H.V., Orsini, D.L., Henry, R.R., Zvi, J.C. (1985) Handedness subtypes in autism. Psychiatric Annals, 15, 447-450. Satz, P., Green, M.F., Lyon, N. (1989) Some pathological substrates in manual laterality. Brain Dysfunction, 2, (In Press) Searleman, A., Cunningham, T.F., & Goodwin, W. (1988). Association between familial sinistrality and pathological left-handedness: a comparison of mentally retarded and nonretarded subjects. Journal of Clinical and Experinrental Neuropsychology, 10, 132-138. Seltzer, B., Burres, M.J.K., & Sherwin, I. (1984). Left-handedness in early and late onset dementia. Neurology, 34, 367-369. Sherwin, I., Peron-Magnan, P., Bancard, J., Bonis, A. & Talairach, J. (1982). Prevalence of psychosis in epilepsy as a function of the laterality of the epileptogenic lesion. Archives of Neurology, 39, 621-625. Taylor, D.C. (1977). Epilepsy and the sinister side of schizophrenia. Developrnental Medicine & Child Neurology, 19, 403-407. Taylor, D.C. (1975). Factors influencing the occurrence of schizophrenia-like psychosis in patients with temporal lobe epilepsy. Psychological Medicine, 5, 249-254. Taylor, P.J., Dalton, R., Fleminger, J.J., & Lishman, W.A. (1982). Differences between two studies of hand preference in psychiatric patients. British Journal of Psychiatry, 140, 166-173. Taylor, P.J., Brown, R., Gunn, J. (1983). Violence psychosis and handedness. In P. Flor-Henry, & J. Gruzelier (Eds). Laterality and Psychopathology. pp. 181194, Amsterdam: Elsevier Science Publishers BV. Taylor, P.J., Dalton, R., & Fleminger, J.J. (1980). Handedness in schizophrenia. British Journal of Psychiatry, 136, 375-383. Toone, B.D. & Driver, M.V. (1980). Psychosis and epilepsy. Research Clinical Forums, 2, 121-127. Trimble, M.R. & Perez, M.M. (1982). The phenomenology of the chronic psychoses of epilepsy, In W. Koella, M.R. Trimble (Eds), Seizures, Psychosis, Mania and the Hippocampal Link, Basel: Kager. Uchino, J., Nakane, Y., Ohta, Y., Hirota, N., Yonekura, M., & Mori, H. (1987). Correlations between regional cerebral blood flow and clinical symptoms in schizophrenics. In R. Takahashi, P. Flor-Henry, J. Gruzelier & S. Niwa (Eds.) Cerebral Dynamics, Laterality and Psychopathology, pp. 513-518, Amsterdam: Elsevier Science Publishers B.V.(Biomedical Division). Vargha-Khadem, F., O’Gorman, A.M., & Watters, G.V. (1985). Aphasia and handedness in relation to hemispheric side, age at injury and severity of cerebral lesion during childhood. Brain, 108, 677-696. Von Knorring, L., Oreland, L. & Winblad, B. (1984). Personality traits related to monoamine oxidase activity in platelets. Psychiaty Research, 12, 1116.

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Walker, HA., & Birch, H.G. (1970). Lateral preference and right-left awareness in schizophrenic children. The Journal of Nervous and Mental Disease, 151, 341-351. Wardell, D., & Yeudall, L.T. (1980). A multidimensional approach to criminal disorders. The assessment of impulsivity and its relation to crime. Advances in Behavioral Research and Therapy, 2, 159-177. Wexler, B.E., Heninger, G.R. (1979). Alterations in cerebral laterality during acute psychotic illness, Archives of General Psychiatry, 36: 278-284. Yan, S.-M.. Flor-Henry, P.,Chen, D., Li, T., Qi, S., & Ma, Z. (1985). Imbalance of hemispheric functions in the major psychoses: a study of handedness in the People’s Republic of China. Biological Psychiatry, 20, 906-917.

LEFT-HANDEDNESS Behavioral Implications and Anomalies, S. Coren (Editor) 0 Elsevier Science Publishers B.V. (North-Holland), 1990

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Chapter 15

Autism and Anomalous Handedness Susan E. Bryson University of Guelph

Autism is a severe developmental disorder beginning early in life and marked by life-long disability. The syndrome is defined by a triad of impairments: deviant social development, impaired communication, and various repetitive, ritualistic behaviours (Denckla, 1986; Wing, 198lb). Although autism occurs in children of normal or even above normal intelligence, the vast majority of cases (approximately 75%) score within the retarded ranges (Bryson, Clark, & Smith, 1988; Lotter, 1966; Wing & Could, 1979). Prevalence has recently been estimated to be as high as 16 per lO,OOO, with boys being affected four times more often than girls (Bryson, et al., 1988; Sugiyama & Abe, 1989). Prognosis is generally poor. Most autistic individuals require long-term care and supervision (Lotter, 1974; Wolf & Goldberg, 1986). Currently, no treatment or prevention is possible, as any aetiology and neuropathology specific to autism remains to be identified.

Chapter Overview This chapter begins with a brief description of current thought on autism. We then turn to the research on handedness, which has only begun to address some of the outstanding questions. The main finding is that autistic children have an increased incidence of nonright-handedness. A shift toward the left in this and other developmentally disabled populations has traditionally been attributed to left cerebral pathology, or to incomplete or atypical cerebral

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lateralization. The critical question is whether handedness might mark something important about autism. Existing data do not allow any firm conclusions. However, it will be argued that evidence implicating genetic factors favours theories which invoke the mechanism of cerebral lateralization. I will end by suggesting how future research might elucidate the biological significance of anomalous handedness in children with autism and other developmental learning disorders.

Current Thought on Autism Accumulating evidence suggests that the social impairments distinguish autism most from all other developmental disorders (see, e.g., Bartak, Rutter, & Cox, 1975; Fein, Pennington, Markowitz, Braverman, & Waterhouse, 1986). In the early stages of development, autistic children are inattentive to other people, and perhaps even more importantly, are essentially nonresponsive to social overtures. The children express little emotion, and appear insensitive to the emotional expressions of others (Hobson, 1986). Extreme passivity and/or frank resistance to any social contact is common. Later in development, many autistic children become more socially aware, but their social relations remain extremely impaired throughout adulthood. Deviance in the language of autistic children appears specific to pragmatics, that is, the social or communicative aspect of language (Baltaxe, 1977; TagerFlusberg, 1986). Approximately one third of autistic children do not speak (Bryson, et al., 1988; Lotter, 1966). Language, when present, is characterized by marked echolalia and by limited comprehension. However, empirical findings indicate that the structure of autistic children’s language is consistent with their developmental levels, nor does their understanding of words and their underlying concepts (although semantics has been the subject of few systematic investigations; Eskes, Bryson, & McCormick, in press; Menyuk & Quill, 1985; Tager-Flusberg, 1986). Indeed, in some high-functioning autistic children, the more formal aspects of language (is., structure and semantics) are essentially normal. Even in these cases, however, language is dysprosodic and non-emotive, and is not used for social purposes, or as a vehicle for acquiring knowledge about the world. The final defining feature of autism is a tendency to engage in repetitive behaviours. These may include motor stereotypes (e.g., rocking, flapping the arms or spinning), and obsessional and/or compulsive behaviours such as

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ordering objects or events in an inflexible manner. Exceptional ("savant") skills typically involving reading, mathematics or music are common (Rimland, 1978). They are frequently associated with, but not necessarily explained by, an unusually good memory (Hermelin & O'Connor, 1986; Frith & Snowling, 1983). Such activities predominate in the absence of more socially productive behaviour. Autism is now viewed by many as a spectrum of disorders, the final manifestation of a number of different aetiologies. A negligible minority of cases have documented histories of medical conditions such as rubella, phenylketonuria or tuberous sclerosis (see Coleman, 1979, for a review), although the aetiological significance of such co-occurrences remains controversial (Chess, 1979). Otherwise, the aetiologies are generally assumed to be genetic. As many as 15 percent of autistic or autistic-like children are positive for Fragile-X, a sex-linked form of mental retardation, which predominates in males (see Folstein & Rutter, 1987, for a review). Several lines of evidence suggest that other, perhaps related, but as yet unidentified genetic disorders may be present in other cases. Folstein and Rutter conclude from available data that it is not autism per se that is inherited, but rather a predisposition toward a cognitive or language disorder. They hypothesize that autism results only when some additional factor such as birth complications is overlaid on this genetic predisposition (but see Bryson, Smith, & Eastwood, 1988, for data suggesting that obstetrical complications may be an effect rather than a cause of abnormal development). In Folstein and Rutter's (1977) research on twins, 36 percent of M Z and no DZ pairs were concordant for autism, virtually all discordant MZ (82% versus 10% DZ) twins were mentally retarded or language impaired, and the presence of birth complications distinguished the autistic from the nonautistic in the discordant pairs. The idea of multiple aetiologies does not, of course, preclude the possibility that a common neuropathology underlies the core symptoms of autism. Nonetheless, the marked heterogeneity within autistic children has led several authors to suggest that there may be identifiable subgroups, each possibly with a different aetiology. Studies focusing on differences in clinical symptomatology and/or cognitive abilities (Fein, Waterhouse, Lucci, & Snyder, 1985; Siegel, Anders, Ciaranello, Bienenstock & Kraemer, 1986) may meet with limited success, in part because each varies, not only across children, but also within the lifespan of a given child. One possibility is that handedness might prove to be a useful biological marker. Handedness is of particular interest because of its relationship to neuropathology and cerebral organization.

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Handedness and Autism Both McCann (1981) and Fein, Humes, Kaplan, Lucci, and Waterhouse (1984) have summarized the data on handedness in autistic children (excluding the recent work of Satz, Soper, Orsini, Henry, & Zvi, 1985, and Soper, Satz, Orsini, Henry, Zvi, & Schulman, 1986). As they indicate, the studies vary in several respects, most notably, the developmental levels of the individuals in question. The studies are also open to various criticisms, including the methods used to ascertain handedness, small sample sizes and inadequate control groups. Nonetheless, a consistent pattern emerges: autistic children have an increased incidence of both left-handedness and a failure to show a preference (not established). Excluding those whose handedness is not established, Fein, et al. (1984) have calculated that on average (across twelve studies) 18 percent of autistic children are left-handed. They also note that this estimate is about twice that observed in the normal population (Hardyck & Petrinovich, 1977), but similar to that reported in studies of mentally retarded and epileptic groups (Pipe, 1988). The other main finding is that a substantial proportion of autistic children (X = 36%; Fein, et al., 1984) fail to show a preference for one hand. The more recent findings of Satz and colleagues (Satz, et al, 1985) are instructive in this regard. Satz, et al's data derive from eight handedness items (90% criterion for establishing right or left-handedness), administered three times on two separate occasions. Two groups of autistic children were tested: young, high functioners (X = 11 yrs; most were of dull normal/borderline intelligence; none had histories of seizures) and older, lower functioners (X = 21.5 yrs; most were severely mentally retarded, with associated CNS disorders). Test-retest reliability was high (.87),and the distributions of handedness did not differ between the two autistic groups. Overall, 44 percent were right-handed, 22 percent were lefthanded, and 36 percent showed no preference, which essentially replicates the combined results of previous studies (Fein, et al., 1984). The procedure of repeating measures (both within and across sessions) provided valuable information about this latter group. Autistic children who showed no preference were inconsistent within tasks ( e.g., cutting) and not merely across tasks (e.g., writing versus throwing). Thus, Satz, et al. conclude that handedness in this subgroup of autistic children is truly not established, rather than being of the "mixed" variety seen in normal, mentally retarded or epileptic populations. Autistic children with no hand preference score more poorly than autistic right- or left-handers on measures of language and general intelligence (Fein,

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Waterhouse, Lucci, Pennington, & Humes, 1985;Soper, Satz, Orsini, Henry, ZVi, & Schulman, 1986; Tsai, 1983). No psychometric differences have been found between autistic right and left-handers. This finding stands in contrast to data recently reported for the mentally retarded. Lucas, Rosenstein and Bigler (1989) found that boti2 left-handedness and inconsistencies in handedness are associated with poor language in mentally retarded individuals. They also note that evidence of brain damage is more common in the severely than in the mildly mentally retarded (see McLaren & Bryson, 1987, for a review), suggesting that nonright-handedness in that population may be related to cerebral pathology (but see Bradshaw-McAnulty, Hicks, & Kinsbourne, 1984, and Pipe, 1987, for evidence of familial sinistrality in the severely retarded). Research has focused on the question of whether the parents of autistic children have an increased incidence of left-handedness. Several studies have failed to find any deviations from the right (Boucher, 1977; Fein, et al., 1985; Tsai, 1982). Boucher reports that the parents may be even more predisposed to right-handedness, although data derived from questionnaires may reflect a rightsided reporting bias (cf., McCann, 1981). In any event, most authors have concluded that anomalous handedness in autistic children can not be explained by familial sinistrality. However, evidence from other lines of research suggest that this conclusion may be premature. First, several authors have reported that the siblings of autistic children have an increased incidence not only of autism, but also of mental retardation and language disorders (see Folstein & Rutter, 1987, for a review), each of which is associated with nonright-handedness. Healy, Aram, Horwitz, and Kessler's (1982) work on hyperlexia (unusually good decoding skills in the presence of poor comprehension) provides even stronger evidence for familial sinistrality in at least some autistic children. Their comprehensive analysis of twelve hyperlexics, all of whom showed autistic symptomatology, reveals a shift toward the left hand in both the cases (at least 33%) and their fathers (50%), but not the mothers. In addition, virtually all of the fathers had a history of reading difficulties, with most being delayed in acquiring reading skills. The siblings were also prone to various learning disorders, including both hyperlexia and delayed reading. Autistic children who are hyperlexic tend to be more intelligent and to have a better prognosis (Burd, Fisher, Knowlton, & Kerbeshian, 1987). Highfunctioning autistic children are also less likely to have frank evidence of brain damage (Lotter, 1966; Rutter, Greenfeld, & Lockyer, 1967), but are not necessarily less "autistic" (see Smith & Bryson, 1988). Taken together, these findings suggest that there may be a subgroup of autistic children, a large

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proportion of whom may be higher functioning, whose handedness reflects both familial sinistrality and a predisposition toward various learning disorders. Findings such as those reported by Healy, et al. (1982) also raise interesting questions about families who appear at risk for various developmental learning disorders. Hyperlexia is essentially the converse of reading failure; decoding skills exceed other language (and even some non-verbal) abilities. One important question is why some family members have difficulty learning to read (dyslexia), while others in the same family show exceptional reading skills (hyperlexia)? Accounts of the significance of nonright-handedness need to consider the apparent heterogeneity in cognitive disabilities manifested within the families of autistic children. In summary, autism is associated with an increased incidence of both lefthandedness, and a failure to show a hand preference. Cases in whom handedness is not established score more poorly on measures of general intelligence and language, but no such differences have been found between autistic right and left-handers. Evidence of family sinistrality is not provided in unselected samples of autistic children. However, existing data do suggest that in mine autistic individuals, both their handedness and associated cognitive/language disabilities are caused by genetic factors. Families of autistic children appear at risk for virtually all developmental learning disorders found disproportionately in left-handers.

Theoretical Accounts The shift toward the left hand in autistic children, like other developmentally disabled populations, has been attributed to left cerebral pathology, or to incomplete or atypical cerebral lateralization. Fein, et al. (1984) have critically reviewed the evidence for left cerebral dysfunction, and conclude that at best the data are only suggestive. One of their main points is that functions normally mediated by the right hemisphere, specifically, speech prosody and emotional responsivity, are extremely impaired in autistic children. They provide several grounds for assuming either no damage, or bilateral impairment, which may be asymmetrical (is., predominantly left or right sided), and may involve the left brain more often than the right. The same conclusion has been reached in other recent reviews of the neuropsychological and neurophysiological evidence, which also implicate subcortical as well as cortical systems in autism (see, e.g., DeLong & Bauman, 1987; Hetzler & Griffin, 1981).

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Satz’s most recent model (Soper & Satz, 1984), revised specificallyto account for his autism data, assumes that deviations in handedness are pathological (versus natural, or genetically determined), and reflect the site of the underlying lesion. Following more traditional thought, an increased incidence of lefthandedness, (as seen in autistic populations) is attributed to pathological lefthandedness resulting from left cerebral pathology. Their model assumes further that damage to the right hemisphere will, conversely, produce pathological righthandedness in some natural left-handers. Such cases would presumably be rare, given the low frequency of naturally occurring left-handedness (see Fein, et al., 1984, for an estimated frequency in autistic children). Finally, the failure to establish a hand preference, which appears common in autistic children (30 40% of cases), is attributed to bilateral cerebral dysfunction. While the Soper-Satz model may account for deviations from the right hand in some developmentally disabled children (e.g., Lucas, et al., 1989), the claim that handedness marks subgroups of autistic children with different lesion sites appears problematic in several respects. First, left hemisphere dysfunction in left-handed autistic children does not explain the abnormal rhythm of their speech, or their apparent insensitivity to emotion (Fein, et al., 1984). Nor does it account for the differences in symptomatology between left-handed autistic children and left-handers with other developmental disorders. Moreover, evidence to date does not support Satz, et al’s prediction that autistic left-handers should perform worse than their right-handed counterparts on at least some psychometric measures. Indeed, Fein, et al. (1985) report a nonsignificant trend toward better language in autistic left-handers. In addition, Dawson, Warrenburg, and Fuller (1982) report that handedness in autistic children does not predict cerebral dominance, as indicated by EEG measures of hemispheric activation, although language competence does. Approximately 70 percent of autistic individuals showed a right hemisphere advantage for the processing of verbal material; those remaining showed the more typical left hemisphere advantage, which was associated with better language skills (also see Dawson, Finley, Phillips, & Galpert, 1986, for corroborative evidence using average evoked cortical responses). These findings converge to suggest that left-handedness in autistic children does not simply reduce to poor language, and by implication, to left cerebral dysfunction. Healy, et al’s (1982) findings also raise questions about the assumption that handedness distinguishes the site of the underlying lesion. Assuming that reading is a highly lateralized skill, good decoders (hyperlexics) would be expected to present with a different pattern of cerebral organization than poor readers

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(dyslexics), irrespective of hand preference. The co-occurrence of different learning disorders within the same families, each associated with a shift toward the left hand, suggests an alternate explanation of the handedness data. Nonright- handedness in at least some autistic children appears to be genetically determined, reflecting both familial sinistrality and a predisposition toward various developmental disorders. It may also be significant that in the families examined by Healy, et al., paternal (and not maternal) sinistrality predicted nonright-handedness in the offspring. Estimates from the general population indicate that left-handedness in the offspring is about twice as likely when the rnofher rather than the father is left-handed (McGee & Cozad, 1980). Gender effects have also been reported in the laterality literature (McGlone, 1980), and virtually all developmental learning disorders are more common in males than females (see, e.g., Silva, 1980; Werry, 1968). Such findings would appear consistent with theories which invoke the mechanism of cerebral lateralization. The idea that incomplete or atypical lateralization underlies the shift toward the left hand in learning disabled populations has been developed most extensively by Geschwind and Galaburda (1987).

The Geschwind-Galaburda Hypothesis Geschwind and Galaburda (1987) propose a relationship between lefthandedness, male gender, immune disorder and developmental learning disabilities, including autism (see Geschwind & Behan, 1982,1984, for supporting evidence in dyslexics). This four-way relationship is assumed to result from the parallel effects of anomalous dominance and immune system dysfunction, each of which are thought to arise from genetically determined abnormal levels of, or sensitivity to, foetal sex hormones. The hypothesized role played by the male hormone testosterone accounts for the preponderance of male learning disordered children. The variability in both the severity and kind of cognitive disability associated with left-handedness is explained not only by the extent and site of the pathology, but also by its time of onset. This latter assumption allows for the range of developmental learning disorders manifcsted, without positing different aetiological mechanisms. A single aetiology could, depending on the point in development that it took hold, explain both the variability and commonalities in cognitive disabilities across different populations. Thus, one distinct advantage of the Geschwind-Galaburda hypothesis is that it provides a parsimonious account of phenomena that do not seem strictly categorical.

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Leboyer, Osherson, Nosten and Roubertoux (1988) have recently reviewed the evidence for this hypothesis from studies of autistic children. They conclude that there are grounds for optimism, but that existing data do not allow firm conclusions. In particular, the evidence for anomalous dominance is equivocal. Following Fein, et al, (1984), Leboyer, et al. focus on the methodological problems inherent in laterality studies of autistic children. They also emphasize that the findings on language (left hemisphere) suggest a delayed development, whereas some functions normally mediated by the right hemisphere appear frankly deviant. Their other major point is that studies to date have focused on separate aspects of the predicted cluster of symptoms ( k . , either anomalous dominance or familial learning disabilities or susceptibility to immune disorders). While substantive questions remain, existing data do not appear inconsistent with the assumptions of Geschwind and Galaburda. For example, their claim that left-handedness indexes atypical language lateralization does not preclude the possibility that language is only delayed, or that functions normally mediated by the right hemisphere are also, or even more, atypically represented (cf., Marx, 1982). The most problematic cases for virtually all theories are those of normal or above normal intelligence, whose language is also age appropriate, but who nonetheless are autistic. One possibility is that this particular subgroup has an increased incidence of right- rather than left-handedness, in which case evidence of anomalous lateralization may be restricted to right hemisphere functions. Alternatively, handedness in autistic children may be related to subcortical rather than cortical dysfunction (Fein, et al., 1984). While the evidence is largely circumstantial, Dawson and Lewy (1989) provide a strong case for the idea that the basic problem in autism is one of right hemisphere hyperarousal and inattention, particularly to novel, complex and unpredictable (i.e., social) stimuli. They suggest that this subcortically-mediated pathology results in functionally unstable, but not necessarily structurally anomalous, patterns of lateralization. Hypotheses implicating subcortical dysfunction (also see Kinsbourne, 1987) are conceivably compatible with the claims of Geschwind and Galaburda. Such hypotheses provide an alternate interpretation of deviations in handedness, including left-handedness, in intelligent, linguistically-competent autistic individuals. It is possible, for example, that hyperarousal of the right hemisphere results in shifts toward the left hand in both high- and low-functioning autistic persons.

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Summary and Conclusions Current findings raise a number of interesting questions about the mechanisms underlying anomalous handedness in autistic children. While the disorder is undoubtedly biologically determined, Satz, et al’s (1985; also see Soper, et al; 1986) claim that handedness distinguishes subgroups of autistic children with different lesion sites was questioned on several grounds. It would appear more likely that a common neuropathology underlies the core symptoms of autism, and in particular, the social impairments. Moreover, existing evidence suggests that handedness in at least some autistic children is related to both familial sinistrality and a predisposition toward various learning disorders. Such findings seem more consistent with the claims of Geschwind and Galaburda, although the data are by no means conclusive. The Geschwind-Galaburda hypothesis also suggests a somewhat different interpretation of the increased incidence of autism and other learning disorders in the families of autistic children. As noted previously, Folstein and Rutter (1987) have argued that a cognitive disability is inherited, and that autism results only when some other aetiological factor (e.g., birth complications) is overlaid on the cognitive vulnerability, In contrast, the Geschwind-Galaburda hypothesis assumes that autistic and related developmental disorders result from anomalous patterns of cerebral organization, and does not preclude the possibility that a single aetiology might give rise to different, although overlapping phenomena. The phenotypic manifestation is thought to depend on both the severity and site of the pathology, and on its time of onset during foetal development. This hypothesis, unlike that of Folstein and Rutter, appears capable of accounting for cases in which autism exists in the apparent absence of cognitive disability (is., mental retardation and/or language disorder). Anomalous cerebral dominance also provides a viable explanation for the cognitive deficits as well as the exceptional abilities found within the same autistic child.

Directions for Future ‘Research Leboyer, et al. (1988) suggest that future research determine whether the four-way relationship predicted by Geschwind and Galaburda exists within the same autistic individual. Three related points may warrant consideration. The first is that Leboyer, et al’s criteria for verifying the Geschwind-Galaburda hypothesis may be overly stringent. Assuming that the onset, site and severity

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of the pathology vary across individuals, including those in the same family, it may be sufficient to demonstrate that the cluster of symptoms merely congregate in the same genetic pool (i.e., the family unit). The second point is that research efforts might profitably restrict the study population to relatively high-functioning autistic children (i.e., those of near normal to normal intelligence). This would reduce the possible confounding effects of associated disorders such as epilepsy, which are common in severely retarded autistic individuals, and, in the case of epilepsy, appear to be associated with a pattern of lateral preferences specific to epilepsy rather than autism (Bryson, Porac, & Smith, in preparation). Note further that associated neurological disorders may in part explain the more equable male-female ratio reported for low- than high-functioning autistic children (2-3:l versus 9:1, respectively; Wing, 1981~).Conversely, the sex ratio for high-functioning autistic individuals suggests that genetic factors loom particularly large in that subgroup. Finally, future studies might determine whether the families of autistic children with essentially normal language also show evidence of Geschwind and Galaburda’s predicted symptom cluster. In them, however, anomalous dominance may be restricted largely to right hemisphere functions, and thus may be associated with an increase in righthandedness. Leboyer, et al. also argue for additional studies designed to examine the hypothesized relationship between left-handedness and anomalous dominance in autistic children. To this I would add that such research should include a careful assessment of the children’s cognitive, linguistic skills and motoric skills. As indicated earlier, left-handedness in autism does not appear to predict either language competence or cerebral dominance, suggesting that left-handed autistic children are not a neurologically homogeneous group. However, the paucity of data renders interpretation difficult. It is possible, for example, that the more capable left-handed autistic children (i.e., those with better language) are left hemisphere dominant for some (Dawson, et a]., 1982), but not all language and related functions. A dissociation in the lateralization of language skills might be consisten with reports of difficulties in the autistic with co-ordinated motor acts such as writing (see, e.g., Healy et a]., 1982; Wing, 1981a). In any event, well-controlled studies (see, Fein, et al., 1984, for a critical discussion) might benefit from adopting Dawson, et al’s strategy of relating performance to the pattern of lateralization. To date, relatively few lateralized functions have been examined in autistic children, and no data exist on those which appear to distinguish the autistic most from other developmentally disabled populations

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(e.g., the processing of emotion). Such studies might also help identify the cerebral dysfunction necessary and sufficient for the autistic syndrome. Finally, research on handedness in autistic children has generally failed to include adequate control groups (but see Bryson, et al., in preparation). It bears emphasizing that most autistic children are also mentally retarded and/or language impaired, and severely retarded autistic individuals often present with associated medical conditions such as epilepsy. By the same token, exceptional skills in children of otherwise subnormal intelligence are common in, but not restricted to, autistic populations. Thus, it is possible that handedness (and other lateralized functions) in autistic children do not differ at all from those observed in children of comparable intelligence, linguistic competence, "savant" skills or neurological status. Assuming, on the other hand, that differences do exist (see Dawson, Finley, Phillips, & Lewy, in press, for preliminary evidence), one possibility alluded to earlier is that handedness in autistic children may reflect subcortical rather than cortical dysfunction. Dawson and Lewy (1989) suggest that this alternate possibility might be examined in longitudinal studies of handedness and other lateralized functions. On the assumption that anomalous lateralization in autistic children results from dysfunctional arousal rather than structural differences, they predict that dominance will change over time. Longitudinal studies would address the question of whether cerebral dominance in autistic children is particularly labile, thus implicationg subcortical involvement, or whether it is stable, and thus consistent with the notion of distinguishable subgroups (Soper & Satz, 1984). The critical remaining question is whether anomalous handedness marks anything specific to, or fundamentally important about, the biological conditions that produce autism. I have argued that future research might profitably address a number of outstanding questions by focusing on high-functioning autistic individuals. Males overwhelmingly predominate in this relatively "pure" subgroup. A program of research is recommended in which handedness is evaluated relative to cognitive/linguistic and motor skills, patterns of cerebral lateralization, familial learning disorders and the presence of immune disorders. Such studies need to include appropriate control groups (see Fein, et al., 1984, for a thoughtful discussion), and might examine within autistic and matched control children the stability of lateralized functions over time. It is hoped that this kind of research will contribute to our current understanding of autism and related developmental learning disorders.

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Acknowledgement I thank Isabel Smith for her thoughtful comments. This work was supported by a grant (No. 6603-1202-42) form the National Health Research and Development Program of Health and Welfare Canada.

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Dominance: The Biological Foundations (pp. 147-166). Cambridge, MA: Harvard University Press. Hardyck, C., & Petrinovich, L.F. (1977). Left-handedness. Psychological Bulletin, 84, 385-404. Healy, J.M., Aram, D.M., Horwitz, S.J., & Kessler, J.W. (1982). A study of hyperlexia. Brain and Language, 17, 1-23. Hermelin, B., & O’Connor, N. (1986). Idiot savant calendrical calculators: Rules and regularities. Psychological Medicine, 16, 885-893. Hetzler, B.E., & Griffin, J.L. (1981). Infantile autism and the temporal lobe of the brain. Journal of Autism and Developmental Disorders, 11, 317-330. Hobson, R. P. (1986). The autistic child’s appraisal of expressions of emotion: A further study. Journal of Child Psychology and Psychiatry, 27(5), 671-680. Kinsbourne, M. (1987). Cerebral-brainstem relations in infantile autism. In E. Schopler & G. Mesibov (Eds.), Neurobiological Issues in Autism (pp. 107125). New York: Plenum Press. Leboyer, M., Osherson, D.N., Nosten, M., & Roubertoux, P. (1988). Is autism associated with anomalous dominance? Journal ofAutism and Developmental Disorders, 18, 539-551. Lotter, V. (1966). Epidemiology of autistic conditions in young children. I. Prevalence. Social Psychiatry, I , 124-137. Lotter, V. (1974). Social adjustment and placement of autistic children in Middlesex: A Follow-up Study. Journal of Autism and Childhood Schizophrenia, 4, 11-32. Lucas, JA., Rosenstein, L.D., & Bigler, E.D. (1989). Handedness and language among the mentally retarded: Implications for the model of pathological left-handedness and gender differences in hemispheric specialization. Neuropsychologia, 27, 713-723. McCann, B.S. (1981). Hemispheric asymmetries and early infantile autism. Journal of Autism and Developmental Disorders, 11, 401-411. McGee, M.G., & Cozad, T. (1980). Population genetic analysis of human hand preference: Evidence for generation differences, familial resemblance and maternal effects. Behavior Genetics, 10, 263-275. McGlone, J. (1980). Sex differences in human brain asymmetry: A critical survey. Behavioral and Brain Sciences, 3, 215-263. McLaren, J., & Bryson, S.E. (1987). Review of recent epidemiological studies of mental retardation: Prevalence, associated disorders and etiology. American Journal of Mental Retardation, 92, 243-254. Menyuk, P., & Quill, K. (1985). Semantic problems in autistic children. In E. Schopler & G.B. Mesibov (Eds.), Communication Problems in Autism (pp. 127-144). New York: Flenum Press. Pipe, M-E. (1988). Atypical laterality and retardation. Psychological Bulletin, 104, 343-347. Pipe, M-E. (1987). Pathological left-handedness: Is it familial? Neuropsychologia, 25, 571-577.

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Rimland, B. (1978). Savant capabilities of autistic children and their cognitive implications. In G. Serban (Ed.), Cognitive defects in the development of mental illness (pp. 43-65). New York: Brunner/Mazel. Rutter, M.,Greenfeld, D., & Lockyer, L. (1967). A five to fifteen year followup study of infantile psychosis: 11. Social and behavioural outcome. British Journal of Psychiatry, 113, 1183-1199. Satz, P., Soper, H.V., Orsini, D.L., Henry, R.R., & Zvi, J.C. (1985). Handedness subtypes in autism. Psychiatric Annals, 15, 447-450. Siegel, B., Anders, T.F., Ciaranello, R.D., Bienenstock, B., & Kraemer, H.C. (1986). Empirically derived subclassification of the autistic syndrome. Journal of Autism and Developmental Disorders, 16, 275-293. Silva, PA. (1980). The prevalence, stability and significance of developmental language delay in preschool children. Developmental Medicine and Child Neurology, 22, 768-777. Smith, I.M., & Bryson, S.E. (1988). Monozygotic twins concordant for autism and hyperlexia. Developmental Medicine and Child Neurology, 30, 527-535. Soper, H.V., & Satz, P. (1984). Pathological left-handedness and ambiguous handedness. A new explanatory model. Neuropsychologia, 22, 511-515. Soper, H.V., Satz, P., Orsini, D.L., Henry, R.R., Zvi, J.C., & Schulman, M. (1986). Handedness patterns in autism suggest subtypes. Journal of Autism and Developmental Disorders, 16, 155-167. Sugiyama, T., & Abe, T. (1989). The prevalence of autism in Nagoya, Japan: A total population study. Journal of Autism and Developmental Disorders, 19, 87-98. Tager-Flusberg, H. (1986). The semantic deficit hypothesis of autistic children’s language. Australian Journal of Human Communication, 14, 51-58. Tsai, L.Y. (1982). A brief report: Handedness in autistic children and their families. Journal of Autism and Developmental Disorders, 12, 421-423. Tsai, L.Y. (1983). The relationship of handedness to the cognitive language and visuo-spatial skills of autistic patients. British Journal of Psychiatty, 142, 156162. Werry, J. (1968). Studies on the hyperactive child. IV. An empirical analysis of the minimal brain dysfuction syndrome. Archives of General Psychiatry, 19, 9-16. Wing, L. (1981a). Asperger’s syndrome: A clinical account. Psychological Medicine, 11, 115-129. Wing, L. (1981b). Language, social, and cognitive impairments in autism and severe mental retardation. Journal of Autism and Developmental Disorders, 11, 31-44. Wing, L. (1981~). Sex ratios in early childhood autism and related conditions. Psychiatry Research, 5, 129-137. Wing, L., & Could, J. (1979). Severe impairments of social interaction and associated abnormalities in children: Epidemiology and classification. Journal of Autism and Developmental Disorders, 9, 11-29. Wolf, L., & Goldberg, B. (1986). Autistic children grow up: An eight to twentyfour year follow-up study. Canadian Journal of Psychiatry, 31, 550-556.

LEFT-HANDEDNESS Behavioral Implications and Anomalies, S. Coren (Editor) 0 Elsevier Science Publishers B.V. (North-Holland), 1990

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Left-Handedness and Alcoholism Wayne P. London Dartmouth Medical School

Alcoholism is generally regarded as one of the most serious public health problems. At least one in ten men develops a problem with alcohol during his life. Alcoholism has a destructive effect on family members, particularly children, and estimates are that one in four people in society are affected by an alcoholic family member (Harwood, Kristiansen & Zachal, 1985). Given the seriousness of alcohol problems in society, that our understanding and treatment of alcoholism are less than adequate, and that a hypothesis concerning left-handedness is easy to establish or reject, serious scientific attention should be devoted to an association of left-handedness and alcoholism. This chapter reviews that evidence that links left-handedness and cerebral laterality to alcoholism. This association includes an increased frequency of lefthandedness in alcoholic men, that left-handedness is associated with a less favourable treatment outcome, with having an alcoholic father and with the more serious type 2 (male-limited)form of alcoholism. Also, several factors associated with alcoholism or with being at increased risk of developing alcoholism are related to left-handedness or to left-hemisphere dysfunction. In addition to left-handedness itself, alcoholism is also associated with phenomena that are themselves linked to left-handedness,such as atypical season of birth patterns, season sensitivity as an adult and thyroid disorder. Further, the association of left-handedness and alcoholism suggests that the current strategy of comparing, for example, the sons of alcoholic fathers with the sons of non-alcoholic fathers might not control for handedness or for patterns of cerebral laterality.

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Finally, the Geschwind theory (Geschwind & Galaburda, 1987) that prenatal sex hormones, particularly testosterone, affect the development of lefthandedness means that one form of alcoholism might also be the result of these prenatal events and that prenatal environmental factors such as season of conception, birth order and the spacing of children could affect the development of alcoholism as an adult. Thus, alcoholism might be a special case of more general biological principles involving cerebral laterality and handedness. The underlying theme of this chapter it that many features of alcoholism can be explained by its association to left-handedness and to issues of cerebral laterality.

Increased Frequency of Left-Handedness in Alcoholic Men Several studies have shown an increased frequency of left-handedness in alcoholic men. In the general population about 10% of men are left-handed (Karpinos & Grossman, 1953; Oldfield, 1971). Bakan (1973) reported that 15% (7/47) of male alcoholics wrote with the left hand and a total of 25% (12/47) were not strongly right-handed. Chyatte and Smith (1981) found that 30% (31/104) of Navy alcohol abusers who made a suicide attempt were left-handed. In a sample of 64 alcoholic individuals (51 men and 13 women) Smith and Chyatte (1983) found that 39% (25/64) were left-handed. In a sample of 99 consecutively hospitalized male alcoholics Nasrallah, Keelor, & McCalley-Whitters (1983) found no excess of strong left-handedness in the alcoholics versus the control group, but they did find a statistically significant excess of mixed-handedness (49% versus 23%) and of left eye dominance (27% versus 14%).In my sample of alcoholic men (London, 1986b), of 235 men hospitalized consecutively, 32 or 14% were left-handed (by a modification of the Edinburgh handedness inventory). Of the 85 women hospitalized consecutively, 10 of 12 % were left-handed. I know of no other data concerning lefthandedness in alcoholic women. In addition, I obtained family history information about the fathers of 143 alcoholic men hospitalized consecutively (London, 1986a). Of the fathers described as alcoholic, 10 of 54 or 19% were described as left-handed; of the fathers described as nonalcoholic, 7 or 89 or 8% were described as left-handed (p = .05). These data replicated further the finding of increased left-handedness in alcoholic men.

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Stated another way, 10 of 17 or 59% of the left-handed fathers were alcoholic versus 35% (44/126) of the right handed fathers (p = .05, as above). If generalized to men in the general population, these data indicate that left-handed men are 1.6 times (59%/35%) more likely to develop alcoholism than righthanded men. Thus, six samples of alcoholic men, totalling nearly 700 individuals, have shown a frequency of left-handedness of 14-39%, whereas less than 10% of men in the general population are left-handed. I could find no recent study that has attempted to replicate this finding nor any study concerning left-handedness in individuals who abuse primarily drugs. (As discussed in the section on correlations with latitude, the proportion of alcoholic men who are left-handed could vary with geographic latitude.) That alcoholic men are more frequently left-handed implies an increased frequency of alcoholism in left-handed men. This means that in men, lefthandedness is a risk factor for developing alcoholism, probably at the 1.5-2 fold level. In comparison, a son of an alcoholic father had a three to four-fold risk of developing alcoholism. (Monteiro & Schuckit, 1988). Finally, left-handedness also relates to drinking and to smoking. In a community survey of about 1,100 people in Michigan, proportionately more people who described themselves as left-handed smoked cigarettes, smoked more than ten cigarettes per day, both smoked and drank and abstained less in both uses than those who described themselves as right-handed (Harburg, 1981; Harburg, Feldstein, & Papsdorf, 1978). Perhaps the increased tendency of lefthanded people to smoke or drink relates to the finding of Irwin (1985) that lefthanded people showed larger brain wave changes in response to a variety of drugs than did right-handed people.

Left-Handedness and Treatment Outcome Two studies have shown that left-handed alcoholic individuals have a less favourable treatment outcome than right-handed individuals. Smith and Chyatte (1983) studies 51 alcoholic men and 13 alcoholic women and determined the number of relapses they experienced before achieving six months of continued abstinence. (The subjects reported whether they were right-handed or lefthanded.) The mean number of relapses for the right-handers was 0.9 versus 2.5 for the left-handers (p < .Ol). Among those who had the most trouble avoiding alcohol continuously for 6 months, 86% (12/14) were left-handed but among

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those who hand the least trouble avoiding alcohol, only 12% (4/33) were lefthanded. In my sample of alcoholic men (London 1985), 56% (45/81) of right-handed men versus 29% (7/24) of the left-handed men were "improved" during the year following hospitalization for alcoholism (p < . 03). These two studies totalling 169 individuals, appear to be the only data concerning left-handedness and treatment outcome. The question is certainly simple to study.

Left-Handedness and Having an Alcoholic Father An important observation is that alcoholic men who are left-handed or who have a left-handed first-degree relative have an increased frequency of having an alcoholic father (London, 1986a). This links being left-handed or having a lefthanded first-degree relative to the most serious, male-limited type 2 form of alcoholism. Of the largest sample of left-handed alcoholic men available for study (28 hospitalized consecutively to my treatment team plus, all known left-handed men (n = 17) admitted to the two other treatment teams on the Adult Alcohol and Substance Abuse Unit at the Brattleboro Retreat from October 1981 through May 1985) 24 of 45 or 53% had an alcoholic father. Of 128 right-handed alcoholic men hospitalized consecutively, 30 of 54 or 56% of those having a left-handed first-degree relative had an alcoholic father, versus 19 of 74 or 26% of the right-handed men not having a left-handed first-degree relative (p < .001). Thus, the alcoholic men who are left-handed or right-handed but having a left-handed first-degree relative had about twice the rate of having an alcoholic father as compared with those with no left-handed first-degree relative. These data extend the association of cerebral laterality and alcoholism to alcoholic men having a left-handed first-degree relative, which is a much larger group than the left-handed alcoholic men. For example, of 143 alcoholic men hospitalized consecutively, only 15 or 10% were left-handed, but 69 or 48% were either left-handed or had a left-handed first-degree relative; of these 69 alcoholic men, 35 or 51% had an alcoholic father versus 26% (19/74) of the right-handed men not having a left-handed first-degree relative (p < .003; London, 1986a). (Whereas for men in my sample, being left-handed or having a left-handed first-degree relative related to having an alcoholic father, being right-handed

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appeared related to having an alcoholic mother or maternal grandfather. Fifteen percent [28/187] of the right-handed men versus 8% [2/26] of the left-handed men had an alcoholic mother, and 23% [36/155] of the right-handed men versus 4% [1/25] of the left-handed men had an alcoholic maternal grandfather [p < .05; London, 1986al.) The association of being left-handed or having a left-handed first-degree relative and having an alcoholic father is important for at least two reasons. First, it means that the appropriate grouping is those left-handed and those righthanded but having a left-handed first-degree relative versus those right-handed but having no left-handed first-degree relative. As discussed in a later section, this has important methodological consequences. Second, it associates lefthandedness with the type 2 form of alcoholism, which is the more serious form. Many studies provide evidence for the classification of alcoholism into two forms, type 1 and type 2 (Cloninger, 1987). The type 1 form, which is less serious, is characterized by a later onset, a postnatal environmental provocation in addition to the genetic predisposition and less of a family history of alcoholism. Type 2 alcoholism is highly heritable, independent of environmental influences and is associated with severe alcohol abuse and criminal behaviour in the biological fathers. Type 2 alcoholics abuse alcohol early in life, are impulsive and risk-taking, and have a tendency to manifest antisocial behaviour such as fighting in bar? or reckless driving while intoxicated. That having an alcoholic father in an alcoholic man relates to his being lefthanded or to having a left-handed first-degree relative associates left-handedness and anomalous cerebral dominance with the more serious, type 2 form of alcoholism. Further evidence of this association would be that alcoholic men who are left-handed or who have a left-handed first-degree relative have an earlier onset of alcoholism, which is another important characteristic of type 2 alcoholism. Having an alcoholic father correlates with early onset (Buydens-Branchey, Branchey, & Noumair, 1989a). Of great interest is the finding that type 2 alcoholics manifest a low availability of tryptophan and a serotinergic deficit (Buydens-Branchey, Branchey, & Noumair, 1989b). Type 2 alcoholics show a decreased availability of the natural occurring amino acid tryptophan for conversion by the brain to the neurotransmitter serotonin. This finding suggests that these individuals would respond to tryptophan supplementation or to agents that modify the functional levels of serotonin. The association of left-handedness and type 2 alcoholism means that lefthandedness might also be related to tryptophan availability and the serotonin

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system. Geschwind and Galaburda (1987) cite several lines of evidence that implicate serotonin in cerebral laterality. As discussed in the section on seasonal sensitivity, serotonin is also involved in the seasonal affective disorder, which also appears related to left-handedness.

Cerebral Laterality and the Study of Alcoholism That alcoholic men who are left-handed or who have a left-handed first-degree relative more frequently have an alcoholic father suggests that the current strategy of comparing, for example, sons of alcoholic fathers with sons of nonalcoholic fathers, might not control for handedness or for patterns of cerebral laterality (London, 198%). In my sample of alcoholic men (London, 1986a), at least 50% of the men left-handed or having a left-handed first-degree relative had an alcoholic father versus only about 26% of the right-handed men not having a left-handed first-degree relative. For example, of 143 alcoholic men hospitalized consecutively, 69 or 48% were either left-handed or had a left-handed first-degree relative, and of these 69 alcoholic men, 35 or 51% had an alcoholic father. In contrast, only 26% (19/74) of the right-handed men not having a lefthanded first-degree relative had an alcoholic father (p < .003). Stated another way, in this sample of alcoholic men, 65% (35/54) of the sons of alcoholic fathers were left-handed or had left-handed first degree-relatives, versus 38% (34/89) of the sons of nonalcoholic fathers (p < .003, as above). If generalized to nonalcoholic individuals, these data (which require replication) suggest that a larger proportion of the sons of alcoholic fathers either would be left-handed or would have a left-handed first-degree relative, and might have anomalous cerebral dominance. Indeed, several findings associated with being at high risk for alcoholism have been associated with left-handedness or with left-hemisphere dysfunction. For example, boys reared away from their biological alcoholic fathers show depressed verbal but not performance IQ scores (Gabrielli & Mednick, 1983) and four-year old children of alcoholic fathers as compared with control children show delayed language development (Tarter, Alterman, & Edwards, 1985). In addition, sons of alcoholic fathers, as compared to sons of nonalcoholic fathers, show poorer verbal and reading abilities (Drejer, Theilgaard, & Teasdale, 1985; Knop, Teasdale, & Schulsinger, 1985; Schulsinger, Knop, & G o o d ~ n 1986). , These findings suggest left-hemisphere dysfunction.

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Other findings in individuals at high risk for alcoholism have been associated with left-handedness. For example, hyperactive children are more frequently lefthanded (Behan & Geschwind, 1985), and childhood hyperactivity has been strongly linked to alcoholism (Tarter, et al., 1985; Goodwin, 1985). Learning disorders occur more frequently in left-handed individuals (Geschwind & Galaburda, 1987), and sons of alcoholic fathers performed less well on the Halstead Category, WAIS vocabulary and Porteus Maze tests (Drejer et al., 1985), which suggests poor abstracting ability. In another study (Tarter & Hegedus, 1985), sons of alcoholics as compared with sons of nonalcoholics also perform less well on standardized tests of academic achievement and on tests of attention, sequencing and short-term memory; paternal alcoholism was the best predictor of the neuropsychological impairment. In addition, on neuropsychological tests measuring abstracting, problem solving and perceptual-motor capability, non-alcoholic adult first-degree relatives of alcoholics preformed less well than non-alcoholics who do not have a family history of alcoholism (Tarter et al., 1985). In terms of EEG findings, sons of alcoholic fathers as compared with sons of nonalcoholic fathers show greater increases of slow alpha activity, greater decreases of fast alpha activity, and greater decrease in alpha frequency after ingestion of alcohol. In addition, several laboratories have shown a reduced P300 component of the event related potential (ERP) in the visual and auditory mode in abstinent alcoholics and in males at risk for male-limited type 2 alcoholism (Begleiter & Porjesz, 1988). In terms of EEG changes and cerebral laterality, in one study, left-handed individuals showed a greater EEG changes to a variety of drugs than did right-handers (Irwin, 1985). A myriad of studies have reported low platelet MA0 levels in alcoholics and in their first-degree relatives versus controls and in type 2 (male-limited) alcoholics versus type 1 (milieu-limited) alcoholics (Begleiter & Porjesz, 1988). In one small study, males with low platelet MA0 levels were more frequently left-handed (Coursey, Buchsbaum, & Murphy, 1979). Finally, as discussed in the next section, thyroid disorder is associated with both left-handedness and alcoholism. As mentioned in the previous section, the serotonin neurotransmitter pathway is implicated in both cerebral laterality and in type 2 alcoholism. These considerations suggest that in the study of individuals who are either alcoholic or at high risk to develop alcoholism, handedness should be determined and information concerning a family history of left-handedness should be obtained.

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Having discussed the numerous associations between left-handedness and alcoholism, we turn our attention to the "other things" that are associated with left-handedness, The idea is, quite simply, that if alcoholism relates to lefthandedness, and left-handedness, in turn, relates to "other things," then alcoholism should also relate to these "other things." These include thyroid disorder, prenatal environmental effects, atypical season of birth patterns, seasonal sensitivity as an adult, light pigment, creativity, other neurological disorders, life expectancy, and modes of transmission.

Alcoholism and Thyroid Disorders As discussed in Habib and Galaburda (this volume) and Halpern and Coren (this volume), left-handed individuals are more prone to several immune disorders such as allergies, ulcerative colitis, regional enteritis and type 1diabetes mellitus. Of particular interest is the finding that left-handed individuals have shown a greater frequency of thyroid disorder and that individuals with immune disorder are frequently left-handed (Schachter & Galaburda, 1986). Given that alcoholism is liked to left-handedness,one might expect an association between alcoholism and thyroid disorder. Most, but not all, studies (Cicero, 1982) have found chronically alcoholic individuals have relatively normal thyroid function and that thyroid hormones are not useful in the treatment of alcoholism. Investigation of the hypothalamic-pituitary-thyroid axis in alcoholic men has generally found a blunted response of thyrotropin (TSH) to thyrotropin-releasing hormone (TRH) that may persist during several years of abstinence from alcohol (Loosen et al., 1983). A preliminary study of the offspring (ages 8-17 years) of alcoholic parents found that the sons but not the daughters had elevated basal TSH levels and an enhanced response to intravenous TRH (Moss, Guthrie, & Linnoila, 1986); this pattern of findings can occur in hypothyroidism. My sample of alcoholic individuals hospitalized consecutively at the Brattleboro Retreat in Vermont showed a high prevalence of personal and family history of thyroid disorders (London & Click, 1988): 23% (13/56) of the mothers of the women in the series and 13% (16/126) of the mothers of the men in the series were described as having thyroid disorder. (A brother or sister of an additional three women and five men in the series also had a thyroid disorder.) In contrast, the prevalence of thyroid disease in the general population, according to the ICD, is 1.4% for women and 0.22% for men. In addition, of the last 75

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women in the series, 13 or 17% were firmly diagnosed as having hypothyroidism (on the basis of low serum T3 or T4 level, elevated serum TSH level, or an enhanced response of TSH to intravenous TRH and a clinical response to thyroid supplement over a minimum six-month follow-up period). Only three of the last 150 men in the series were so diagnosed. As reviewed by MacGregor (1986), alcoholics suffer many dysfunctions of the immune system, and "alcohol should be considered as an immunosuppressive drug with far-reaching effects." The possibility that alcoholics might possess a vulnerability to immune disorders, independent of the toxic effects of alcohol, apparently has not been considered. My data, which require replication, that the mothers of alcoholics, most of whom were not themselves alcoholic, showed an increased frequency of thyroid disorder cannot be explained by toxic effects of alcohol. These considerations suggest that those who study or treat individuals who are alcoholic or at high risk to develop alcoholism should inquire about a personal and family history of thyroid disorder, and that alcoholic individuals, particularly women, should be evaluated thoroughly for hypothyroidism, which is a readily treatable disorder.

Prenatal Environmental Effects Geschwind and Galaburda (1987) hypothesize that one form of lefthandedness and anomalous cerebral dominance results from prenatal levels of sex hormones, particularly testosterone. The prenatal testosterone is thought to inhibit areas of the more slowly developing left side of the brain (producing lefthandedness and anomalous cerebral dominance) and to inhibit the developing thymus and immune system (producing vulnerabilities to immune disorder). Given an association between left-handedness and alcoholism, then according to this theory, alcoholism, particularly the more serious type 2 (male limited) form, would have prenatal origins. Three prenatal environmental effects--season of birth (or conception), birth order, and the spacing of children--could oe relevant to developing alcoholism as an adult. The atypical season of birth patterns of right-handed alcoholics (and the accompanying correlations with latitude) are discussed in the next two sections. Concerning birth order, several studies have shown that left-handedness is more frequent in first-born males (see Bakan, this volume for a review). As

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reviewed by Blane and Barry (1973), several studies have found an excess of last-born individuals in alcoholic men, but in my sample the birth order of alcoholic men related to handedness (London, Kibbee, & Holt, 1985). Indeed, significantly more of the left-handed alcoholic men were first-born (46%,12/26) and significantlymore of the right-handed alcoholic men were second born (37%, 33/90, p = .01). These data require replication. The findings of Maccoby, Doering, Jacklin, and Kraemer (1979) are relevant to the relation of left-handedness and being first-born and to the Geschwind theory of prenatal testosterone. These workers found that the umbilical cord level of testosterone and progesterone in the umbilical cord were highest in first-born male infants and declined in second and third-born males infants. They also found that if four or more years had elapsed between the birth of an older sibling and a later-born male infant, then the umbilical cord levels of testosterone and progesterone in the later-born male infant were as high as the first-born infant. This latter finding suggests that the spacing of children, that is, if the male child were born four years after their older sibling, would be relevant in the study of left-handedness and alcoholism. I know of no data on this question. These considerations suggest that certain women may be sensitive to these prenatal environmental factors and that their fetuses develop nervous and immune systems under different levels of sex hormones or perhaps other prenatal factors based on season of conception, birth order or the spacing of the children. A theory of the genetics or of the transmission of left-handedness or alcoholism should consider these three prenatal environmental effects. For example, if some individuals become left-handed or alcoholic because of a prenatal environmental factor (season of conception, birth order or spacing), then their offspring need not be at increased risk for left-handedness or alcoholism.

Season of Birth At least 20 biological phenomena, most of which are associated with an increased frequency of left-handedness, show an atypical season of birth pattern, that is, individuals showing the phenomena are not born uniformly throughout the year, as are individuals in the general population (Dalen, 1975; b n d o n , 1987a).

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The following phenomena show both an atypical season of birth pattern and an increased frequency of left-handedness: neurological phenomena such as left-handedness itself, schizophrenia, bipolar disorder, autism, and alcoholism; immune disorders such as allergies, diabetes and thyroid disorders; birth defects such as cleft palate and neural tube defects; chromosomal abnormalities such as Down’s, Klinefelter’s and Turner’s syndromes and twinning (Dalen, 1975; London, 1987a). In addition, in the Prader-Willi syndrome (which has not been related to left-handedness), 50% of those affected show a deletion of paternal chromosome 15 in the q l l region, and those showing the chromosomal deletion were born more frequently in the summer and fall (Butler, Ledbetter, & Mascarello, 1985). A lower life expectancy also relates to being left-handed and to season of birth. Left-handed men show a lower life expectancy (Halpern & Coren, 1988: London, 1989), and in two studies totalling 130,000 individuals (Jansson & Malahy, 1981), those born during summer months also had a lower life expectancy. In terms of breast cancer, Kramer, Albrecht and Miller, (1985) studied a sample of 1,027 women from the Pittsburgh, Pa. area and found that lefthandedness implied nearly a two-fold risk for developing breast cancer prior to age 45, and nearly a three-fold risk for developing left-sided breast cancer prior to age 45. (Left-sided breast cancer differs from right-sided breast cancer in several important ways.) These findings, which are of great concern, require replication. Studies of women with breast cancer in Japan, Greece and the United States have shown atypical season of birth patterns. In addition, of the women in the Pittsburgh sample who developed breast cancer prior to age 45 (the age group where the excess left-handedness was found), those with left-sided breast cancer were born more frequently during the fall and winter, and those with right-sided disease were born more frequently during the spring and summer (Albrecht & London, 1989). No one season is associated with excess births in the above phenomena and, indeed, autism and Down’s syndrome show excess births in the spring and fall (Bartlik, 1981; Harlap, 1974). The reasons for atypical season of birth patterns are not known. The association of season of birth patterns with left-handedness is not understood, but the relation is consistent with the Geschwind theory that left-handedness relates to prenatal environmental factors such as high level of fetal testosterone (Geschwind & Galaburda, 1987). Although not understood, atypical season of birth patterns are associated with such important biological phenomena as birth defects, chromosomal abnormalities, DNA functioning,

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cancer, handedness, neurological and immune phenomena, twinning and life expectancy. In terms of season of birth and alcoholism, a small study of alcoholic men and women showed excess births during August through January (Woodruff, Guze, & Clayton, 1974), but other studies have not shown atypical seasonal birth patterns (Dalen, 1975). My sample of 326 alcoholic individuals showed an atypical season of birth pattern (London 1987a). The most striking finding was that an excess of right-handed men were born during June through November, the six months showing the most births of righthanded men. Sixty-one percent (103/170) of the right-handed men were born during these six months versus 50% (61/122) of the women and the left-handed men (p = .07), or versus at most 52% of the men in the general population (p < -03). An unexpected finding, which might be a sampling artifact, was that the men of maternal Italian or German background were born more frequently during the spring. Sixty-one percent (17/28) of the Italian or German men (9 of 16 Italian and 8 of 12 German men) were born during the four months February, March, April and May, and 75% (21/28) of the Italian and German men were born during the six months December through May. If the individuals of Italian or German background are excluded (because the represent 13.5% [23/170] of the sample of right-handed men, but 25% [17/67] of the births during December through May), then 66% (97/147) of the righthanded men were born during June through November, versus 52% (57/110) of the women and the left-handed men (p c .03), or versus at most 52% of men in the general population (p c .001). In addition, in our sample of alcoholic men, having an alcoholic mother or maternal grandfather was associated with right-handedness, and being first-born, having an alcoholic father or having a left-handed first-degree relative was associated with left-handedness (London, et al. 1985, London, 1986a). The following subgroup of right-handed alcoholic men were born more frequently during June through November: those having an alcoholic mother (71%, 15/21, versus 63%, 75/119, of those having an alcoholic mother); those having an alcoholic maternal grandfather (72%, 21/29, versus 63%, 57/91, of those not having an alcoholic maternal grandfather); those not first-born (66%, 58/88 versus 58%, 25/43, of those first-born); those not having an alcoholic father (68%, 60/88,versus 63%, 31/49, of those having an alcoholic father); those not having a left-handed first-degree relative (65%, 33/51, versus 52%, 25/48, Of those having a left-handed first-degree relative). None of the above differences

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are significantly significant, but the trend suggests that the excess of births during June through November is associated with being right-handed. The foregoing data are not intended to be conclusive or convincing. In studying the season of birth of alcoholic individuals, a larger systematic study is required, and the above data suggests that gender, handedness, birth order and ethnic background may be relevant. (In terms of the ethnic finding [that those of Italian or German background were born much more frequently during the winter and spring], ethnic findings are well known in alcoholism; for example, the Irish, English and Native Americans have high rates of alcoholism and Orientals and Jews relatively low rates. Further, in my sample, those of Italian or German background had a lower rate of having an alcoholic father as compared with the Irish, Enghsh or French. Vaillant [1983] also reported that having an alcoholic father is less common in Italian men. Thus, the alcoholism in Italian and German men might be associated with right-handedness, with not having an alcoholic father and with a winter-spring season of birth pattern.

Correlations with Latitude Seasonal phenomena suggest correlations with latitude because seasonal phenomena usually intensify with increasing latitude. The apparent atypical season of birth pattern of right-handed alcoholic men, which presumably would be more intense at high latitude, suggests that at high latitude more right-handed men would become alcoholic. On this basis, at high latitude, (I) alcoholism would be more frequent, and (11) a smaller proportion of alcoholic men would be left-handed. Since left-handedness in alcoholic men is associated with being first born (London, et al., 1985) or with having an alcoholic father (London, 1986a), at high latitude a small proportion of alcoholic men also would be (111) first-born, or (IV) have an alcoholic father. In addition, (V) the alcoholism at high latitude would appear less severe, because the alcoholism associated with left-handedness would be more severe (see section on alcoholic father). (I and V imply a paradox; at high latitude alcoholism would be more frequent but less severe.) The available literature supports I through IV. With regard to I, in addition to anecdotal impressions, the consumption of beer, spirits, wine and total alcohol in the United States correlates positively with latitude (London & Teague, 1985). With regard to 11, the frequency of left-handedness in alcoholic men from

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relatively high latitude (Canada and New England) was 16% (Bakan, 1973; London, 1986a; n = 271, range 14-25%) versus 36% in Illinois; (Chyatte & Smith, 1981; Smith & Chyatte, 1983; n = 144, range 36-37%, p c .oooOl). With regard to 111, in studies with at least 100 subjects, four of five studies from north of the United States (e.g. Norway, Austria, and Canada), but only two of eight studies from the United States showed a significant excess of last-born alcoholics (Blane & Barry, 1973). With regard to IV, the frequency of alcoholic men having an alcoholic father in such northern countries as Sweden, Denmark, England and Canada was 26% (six studies; Cotton, 1979; n = 700; range 23-33%) versus 38% of alcoholic men in the United States (4 studies: London, 1986a; Schuckit, 1984; Tarter, McBride, Buonpane, 1977; Winokur, Rimmer, & Reich, 1971; n = 821, range 35-42%, p = .oooOl). The foregoing supporting evidence from the literature is not intended to be conclusive or convincing. For example, in the studies of having an alcoholic father, different methodologies and definitions of paternal alcoholism were used. The study of the proposed correlations with latitude requires a sophisticated analysis of the literature. The proposed correlations with latitude do not depend on the data of an apparent atypical season of birth pattern of right-handed alcoholic men. There are three independent findings: (a) the association in alcoholic men of being lefthanded and being first-born or having an alcoholic father (London, et al., 1985; London, 1986a), (b) a seasonal finding in right-handed alcoholic men, of which season of birth is one example, and (c) the proposed correlation with latitude. Assuming that seasonal changes in, for example, temperature, light or the earth’s magnetic field are involved, than any two of the above statements imply the third. For example, in the earlier discussion, (a) and (b) were used to deduce (c); similarly, (a) and (c) imply (b). Thus, the proposed correlations with latitude are an independent confirmation of the apparent atypical season of birth pattern in the alcoholic men who are right-handed. A consequence of many features of alcoholism correlating with latitude is that findings relevant to alcoholics at low latitude (e.g. in southern United States) might not apply to alcoholics at high latitude (e.g. in northern Europe), and vice versa.

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Left-Handedness and Seasonal Sensitivity As discussed in an earlier section, left-handedness is associated with atypical season of birth patterns, perhaps because of the seasonal sensitivity of some mothers. Several lines of evidence suggest that left-handed individuals may be more sensitive to seasonal changes and perhaps more sensitive to light. Since 1985, I have treated 125 women and 45 men in my private practice who met criteria for mild seasonal affective disorder (depression, lethargy, weight gain, carbohydrate craving, and increased sleeping during the winter). Seventeen percent (21/125) of these women and 22% of these men were left-handed (defined as writing, throwing, using a spoon, hammer or toothbrush lefthandedly); these rates of left-handedness are about twice the expected rates ( p < .01; W.P. London, unpublished observation). These individuals also showed chronic immune problems such as allergies, thyroid disorders and chronic candida infections, which is consistent with the finding that left-handed individuals are more vulnerable to immune disorders (see Habib & Galaburda, this volume; Halpern & Coren, this volume). If left-handed individuals are more sensitive to seasonal changes, and if lefthandedness is linked to alcoholism, then one might expect a seasonal pattern of admissions for alcoholism. Eastwood and Stiasny (1978) found admissions for alcoholism in Ontario, Canada, were higher than expected in the spring and lower than expected in the fall. In addition, I studied the month of admission to the Adult Alcohol and Substance Abuse Program at the Brattleboro Retreat in southern Vermont of more than a thousand individuals from 1980 through 1984. Admissions were most frequent in the spring and fall and least frequent in the summer. The month with the most admissions (and with the longest waiting list) was October (W.P. London, unpublished observation). The left-handed substance abusers (mainly alcoholics) admitted to one treatment team at the Adult Alcohol and Substance Abuse Program of the Brattleboro Retreat from October 1981 through May 1985 showed the most seasonal pattern (London, 1986a). Twenty-four percent (10/24) of the lefthanded individuals (7 or 32 men and 3 of 10 women) were hospitalized during October, versus 7% (19/278) of the right-handed individuals (p < .005). In this sample, the right-handed admissions were distributed equally throughout the year (7-10% per month; expected value, 8%). Stated another way, 13% of the sample were left-handed and so the expected percent of left-handed admissions during any month is also 13%. During October, however, 34% (10/29) of the admissions were left-handed, versus 11% (32/291) during the rest of the year (p

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< .005, as above). The only other month with an excess of left-handed admissions was May, during which 23% (6/26) of the admissions were lefthanded (p < -10). Parenthetically, these data suggest that the frequency of left-handedness in substance abusing (and perhaps other) populations merits investigation over a twelve month period. Seasonal changes suggest correlations with latitude because seasonal changes usually intensify with increased latitude. As expected, the prevalence of seasonal affective disorder in the United States increases with latitude (Wurtman & Wurtman, 1989). As discussed in the previous section, alcohol consumption (London & Teague, 1985) and several features of alcoholism correlate with latitude (London, 1987a). Thus, both alcoholism and seasonal affective disorder show an increased frequency in left-handedness, seasonal patterns and correlations with latitude. A possible connection between alcoholism, seasonal affective disorder and lefthandedness might involve the serotonin neurotransmitter pathway, which is implicated in type 2 alcoholism (Buydens-Branchey, et al., 1989b), in seasonal affective disorder (Blehar & Rosenthal, 1989) and in cerebral laterality (Geschwind & Galaburda, 1987). 1hese considerations suggest an association between seasonal affective disorder and alcoholism. I know of no data on this point, but a recent report linked seasonal cocaine abuse to seasonal affective disorder (Satel & Gawin, 1989). In my private practice, several former alcoholics met criteria for seasonal affective disorder and benefitted from supplemental full spectrum light. A connection between seasonal affective disorder and alcoholism, in addition to being of theoretical interest, suggests that phototherapy with supplemental full spectrum light might benefit individuals with alcoholism. In this connection, the P300 event-related potential is reduced in type 2 alcoholism (Begleiter & Porjesz, 1988) and is enhanced in patients with seasonal affective disorder following phototherapy (Blehar & Rosenthal, 1989). (I know of no data that link event-related potentials to cerebral laterality.)

Left-Handedness and Light Pigment Left-handedness correlates with light pigment. In one study, individuals with blond hair, particularly men, were more frequently left-handed (Schachter, Ransil, & Geschwind, 1987). Males with the attention deficit disorder ( or

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childhood hyperactivity) show both an increased frequency of left-handedness and of light pigment (Behan & Geschwind, 1985). In addition, I studied the eye colour of all 111 men and 103 women in southern Vermont who were lefthanded (defined as writing left-handedly) and found an excess of blue eyes in left-handed men, 52% (58/111) versus 33% (74/223) in the control group of men (p < .001). Of the left-handed women, 42% (43/103) had blue eyes, which did not differ statistically from the 33% (80/243) of blue eyes in the control group of women. (The control group -- individuals selected randomly from the general population -- undoubtedly included at least 10% left-handed people; thus actual differences in the frequency of blue eyes would be underestimated [W.P. London, unpublished data].) The relationship of light pigment and left-handedness is important for at least two reasons. First, it fits in with the notion that left-handed people may be more sensitive to light, because individuals who are hypopigmented (fair skin, blond hair or blue eyes) are more sensitive to light (Lerman, 1988). Second, it is relevant to the prenatal migration of cells from the neural crest region, which is implicated in disorders associated with left-handedness (Geschwind & Galaburda, 1987). Lack of Melanin pigment slows the migration (Schachter, et al., 1987) and can result in abnormal migration patterns, for example, in the visual and optic pathways (Creel, Bendel, Wiesner, et al., 1986; Drager, 1985). I know of no data concerning eye colour and alcoholism, but these considerations suggest that alcoholics who are blue or hazel-eyed may have a more severe form of alcoholism, again because of the link to left-handedness and cerebral laterality. Concerning left-handedness and life expectancy, these considerations also suggest that blue or hazel-eyed people may not live as long as brown-eyed people, a question easily studied from information on driver’s licenses. One also wonders if eye colour correlates with the three prenatal environmental effects of season of conception, birth order and spacing.

Left-Handedness and Life Expectancy Several lines of evidence indicate that left-handed individuals have a lower life expectancy than right-handed individuals (see Halpern & Coren, this volume). The increased tendency of left-handed individuals to have accidents (Coren, 1989) could account in part for the decreased life expectancy in left-handed people, but certainly the association of alcoholism and left-handedness would be

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relevant (London, 1989). In our society, at least one in ten men develops alcoholism, and alcoholics die prematurely from trauma, cancer, infection and liver disease. In addition, as already noted, a community survey in Michigan showed that proportionatelymore left-handed people smoked cigarettes, smoked more than ten cigarettes per day, both smoked and drank, and abstained less from both uses than right-handed people (Harburg, 1981; Harburg, Feldstein, & Papsdorf, 1978). Since the data of Halpern and Coren (1988) involved baseball players, I note that three of the game’s greatest players, Ty Cobb, “Shoeless”Joe Jackson and Babe Ruth, were left-handed and had alcohol problems. Cobb and Jackson threw right handed and batted left; Cobb died of prostrate cancer at the age of 74 and Jackson died of heart attack at 64. Ruth, who threw and batted left and who smoked and drank heavily, died prematurely at age 53 of throat cancer, which is associated with smoking and alcoholism. With regard to baseball players and other athletes, proportionately more athletes are left-handed. In sports such as baseball or tennis, being left-handed carries an advantage,but this apparently cannot explain the excess of left-handed athletes. Perhaps left-handedness or anomalous cerebral dominance produces superior motor or hand-eye coordination. Also one has the impression from the media that alcohol and drug problems are perhaps more common in professional athletes than in the general population, I know of no systematic data on this point, and clearly professional athletes are highly visible and their difficulties make news, whereas ours do not. Regardless, one must consider the possibility that left-handedness or anomalous cerebral dominance produces both superior athletic ability and an increased vulnerability to alcohol or drug abuse. If so, professional athletes would be more prone to substance abuse difficulties.

Alcoholism and Creativity An anecdotal impression is that many creative people suffer from alcoholism. For example, several American writers were alcoholic, including five who have won the Nobel Prize for Literature--Sinclair Lewis, John Steinbeck, William Faulkner, Ernest Hemingway and Eugene O’Neil. Other alcoholic American writers include F. Scott Fitzgerald, Hart Crane, Edna St. Vincent Millay, Thomas Wolfe, Dorothy Parker, Tennessee Williams and John Chewer (Dardis, 1989). Perhaps the association of alcoholism and cerebral laterality can shed somk light on the question of alcoholism and creativity.

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Left-handed people are over-represented in samples of musicians, artists, architects and mathematicians (Geschwind & Galaburda, 1987). In addition, individuals with dyslexia (which is linked to left-handedness) and their relatives show superior right hemisphere talents (Gordon, 1980). Thus the link of alcoholism to cerebral laterality might account for a putative association of alcoholism and creativity. The alcoholic American writers need not have been left-handed, because as discussed throughout this chapter, some right-handed people can exhibit the traits of left-handers. Recent neurological evidence regarding the size of the corpus callosum--the part of the brain that connects the left with the right hemisphere--is relevant. Integrative, creative thinking is thought to involve both hemispheres, and in terms of the corpus callosum, "bigger is better" because the bigger the corpus callosum (actually, the splenium and isthmus portions), the better the integrative thinking. Women, who generally have larger splenium and isthmus portions of the corpus callosum than men, preform better on tasks that involve both sides of the brain, whereas men generally preform better on tasks that use only one side of the brain. Left-handed men also have a larger isthmus portion of the corpus callosum than do right-handed men, and left-handers are known to have greater bihemispheric representation of cognition. In women, the size of the corpus callosum does not vary with handedness. Also, the front portion of the corpus callosum ages differently in men versus women. (Findings presented at the March 1989 meeting of the Neuropsychology Group of the New York Academy of Sciences and S. Witelson, to appear in Bruin.) Thus, a putative association of alcoholism and creativity may be because male alcoholics are more frequently left-handed or have anomalous cerebral dominance, which in turn correlates with a larger corpus callosum and the ability to do integrative, creative thinking.

Alcoholism and Other Neurological Phenomena Alcoholism shares several epidemiological characteristics with the following neurological phenomena of left-handedness, dyslexia, stuttering and the attention deficit disorder (or childhood hyperactivity): 1.

The phenomena are more prevalent in men than women, generally by a factor of two to one.

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2.

Depending on the strictness of the definition, the phenomena occur in about 1-10% of men.

3.

They are associated with an increased frequency of lefthandedness.

4.

They are associated with immune disorders. Left-handers show a variety of immune disorders (Habib & Galaburda, this volume; Halpern & Coren, this volume). In my sample, alcoholic women and the mothers of male and female alcoholics showed increased rates of thyroid disorder (see section on thyroid disorder). Mothers of boys with dyslexia showed increased rates of Lupus erythematosis and of thyroid and other antibodies. Individuals with attention deficit disorder or stuttering have increased rates of allergy (Geschwind & Galaburda, 1987).

To this list of shared characteristics we can add light pigment (blond hair, blue eyes or fair skin). As mentioned in the section in the section on light pigment, males who have learning disorders, attention deficit disorder or who are left-handed are more frequently light pigmented. I know of no data concerning light pigment and alcoholism or stuttering. If these neurological phenomena are related, then one would expect the phenomena to occur together. As is well known, attention deficit disorder often occurs in conjunction with learning disabilities. Goodwin (1985) and Tarter, et al. (1985) have reviewed evidence that alcoholism is related to attention deficit disorder or childhood hyperactivity. In this connection, in my sample of alcoholic individuals (London, et al., 1985), 42% (20/47) of the women and 28% (37/130) of the men had a first-degree relative who evidenced either attention deficit disorder, learning disabilities or stuttering. These data require replication, and in studying one of the phenomena, a personal or family history of the other phenomena should be sought. These considerations suggest that the neurological phenomena of left-handedness,alcoholism, dyslexia, attention deficit disorder and stuttering can be manifestations of more general biological principles involving cerebral laterality and prenatal neurological development. This means that research from one phenomena might apply to the other phenomena. Perhaps clues come from dyslexia, which is the best studied neuroanatomically.

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In dyslexia, the migration of cells form the neural crest area in one area of the left frontal cortex is abnormal and as mentioned previously, lack of melanin pigment slows the migration of cells (Schachter, et al., 1987). In addition, in dyslexia, portions of the right hemisphere show more nerve cell pathways (Geschwind & Galaburda, 1987; Galaburda, 1988). Perhaps similar prenatal abnormalities occur in these other disorder. A third neuroanatomical finding is that left-handed men have a larger isthmus portion of the corpus callosum (see section on alcoholism and creativity).

Issues of Methodology Several independent lines of evidence indicate that some traits associated with left-handedness, left hemisphere dysfunction or with anomalous cerebral dominance can occur in individuals who are themselves left-handed, but who are the close relative of someone left-handed (or of someone possessing the traits associated with left-handedness): 1.

Individuals left-handed or those having left-handed relatives show superior recovery from aphasia (Hardyck & Petrinovich, 1977).

2.

In my sample of alcoholic men, about 50% of those left-handed or who had a left-handed first-degree relative had an alcoholic father, versus 26% of those right-handed with no left-handed first-degree relative (see section on alcoholic father).

3.

Nonalcoholic adult first-degree relatives of alcoholics perform less well on neuropsychological tests measuring abstraction, problem solving and perceptual motor capacity than do nonalcoholics who do not have a family history of alcoholism (Tarter, et al., 1985).

4.

Alcoholic men and their first-degree relatives, show lower levels of platelet M A 0 activity (Begleiter & Porjesz, 1988).

5.

In my sample, alcoholic women and the mothers of alcoholic men and wonten showed an increased frequency of thyroid disorder. (See section on thyroid disorder.)

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6.

Individuals with dyslexia and their relatives show superior right hemisphere talents (Gordon, 1980).

That some traits associated with left-handedness occur in people who are not themselves left-handed, suggests that left-handedness itself may be an epiphenomenon of more basis biological phenomena. Being left-handed, which is more common in males, may be the result of a high level of, for example, prenatal testosterone, possibly from inheriting certain genes or by the prenatal environmental factors of season of conception, birth order or spacing. The foregoing data also indicate that studies that compare left-handed and right-handed people will underestimate the true difference (for example, in life expectancy or in eye colour) because the right-handed group will contain individuals who have left-handed relatives and who should be included in the left-handed group. The appropriate grouping would be individuals who are lefthanded or have first-degree relatives versus those right-handed with no first-degree left-handed relatives. (A consequence of this is that refined measurements of left-handedness are probably not critical, because information about the handedness of relatives is usually obtained by merely asking is a relative is left-handed, which generally means writing left-handedly.) As noted by Geschwind and Galaburda (1987) at least 30% of the population would have anomalous cerebral dominance, that is either be left-handed or have left-handed relatives or would have a personal or family history of traits associated with left-handedness such as alcoholism, dyslexia, attention deficit disorder stuttering, thyroid disorders, etc. If at least 30% of the population had the traits of anomalous cerebral dominance, then no single gene could account for anomalous cerebral dominance, because no single gene exists at a frequency of 30%. This suggests multiple genes. Transmission of anomalous cerebral dominance traits from father to son (the most common inheritance of type 2 male-limited alcoholism) means that the X chromosome is not essential; likewise, transmission from father to daughter or from mother to son means that the Y chromosome is not essential. This leaves 22 autosomal or nonsex chromosomes. That the traits are inherited so frequently (for example, at least 25% of the sons of alcoholic fathers become alcoholic) suggests a dominant form of inheritance (that is, the trait is acquired if inherited from either the mother or the father, as opposed to recessive where inheritance must come from both mother and father.) Thus, a simple genetic model of the inheritance of anomalous cerebral dominance would be multiple autosomal dominant genes. (In breast cancer,

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which relates to left-handedness and which shows an atypical season of birth pattern (see season of birth section), the familial pattern, which accounts for less than 10% of breast cancer cases, follows an autosomal dominant pattern, and includes even rare male to male transmission (Lynch, Albano, & Heieck, 1934). Suppose there were 10 such autosomal dominant genes, which, for simplicity, were inherited independently. Inheriting all ten anomalous cerebral dominance genes would result in a heavy dose of anomalous cerebral dominance traits, inheriting five anomalous cerebral dominance genes, less of a dose, and inheriting only one anomalous cerebral dominance gene, very few anomalous cerebral dominance traits. According to this simple model, the anomalous cerebral dominance traits in society would be distributed according to the binomial or normal distribution, that is, described by the familiar bell shaped curve. (The "pegs and falling ball" analogy may help clarify these ideas: think of each of the ten genes for anomalous cerebral dominance as a horizontal row of pegs. A ball dropped onto the middle peg of the top row will make either a lefthand or a right-hand turn; as the ball falls, it will again either make a left-hand or right-hand turn as it hits the peg in the next lower row. Very few balls will make all ten left-hand turns and end up in the left-most bin at the bottom; likewise, very few balls will make all ten right-hand turns and end up in the right-most bin at the bottom. Most balls will make five left-hand and five righthand turns (in any sequence) and will end up in the middle bin at bottom. The resulting pattern of balls in the bins at the bottom is the familiar bell shaped curve.) Common traits in society such as height, weight, intelligence -- and two talents associated with left-handedness -- musical and mathematical ability follow the bell shaped curve; these traits are undoubtedly the result of multiple dominant genes on the autosomal (and perhaps sex) chromosomes. The genetic aspects of anomalous cerebral dominance -- and the genetic tendency to become alcoholic -- could follow this same pattern. That the traits of anomalous cerebral dominance would be distributed normally, would affect the selection of a control group on the study of anomalous cerebral dominance straits. In summary, the traits of anomalous cerebral dominance may be thought of as secondary sex characteristics (affected by testosterone and perhaps other sex characteristics), that are influenced by prenatal environmental factors (such as season of conception, birth order and perhaps the spacing of children) and, (if determined by multiple autosomal dominant genes), distributed in society according to the familiar bell shaped curve.

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Conclusions This chapter asserts that alcoholism is linked to left-handedness, and that alcoholism may be a special case of more general biological principles involving cerebral laterality. These assertions are based on more than 50 independent bits of data or evidence. In evaluating this material, the student of left-handedness or of alcoholism may wish to think of himself as a judge or a member or a jury and decide, in a legal sense, if the standard of evidence is merely circumstantial, a preponderance of the evidence, clear and convincing or beyond a reasonable doubt. Perhaps there is a reluctance to think that one underlying notion such as cerebral laterality could account for so many of the biological features of alcoholism, in the same way that a single organism could produce the multiple symptoms of syphilis, leprosy or tuberculosis. In addition, left-handedness is often thought of as cute, frivolous or superficial and without biological significance. The Buddha said, "We are what we think; all that we are arises with our thoughts; with our thoughts we make the world." Perhaps some of the difficulty in associating left-handedness with alcoholism is not so much "out there" as in our minds. When I was in medical school, an often-heard phrase was, "you see what you look for, and you look for what you know." In trying to understand and treat alcoholism--one of the most serious public health problems in modern society--one should look for left-handedness and the phenomena related to left-handedness.

Acknowledgement I thank the Asa Keyes Medical Library at the Brattleboro Retreat.

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Bartlik, B.D. (1981). Monthly variation in births of autistic children in North Carolina. Journal of the American Medical Association, 36, 363-368. Begleiter, H. & Porjesz, B. (1988). Potential biological markers in individuals at high risk for developing alcoholism. Alcohol: Clinical Ehperimental Research, 12, 488-493. Behan, P. & Geschwind, N. (1985). Dyslexia, congenital anomalies and immune disorders: the role of the fetal environment.Annals of the New York Academy of Science, 457, 13-18. Blane, H.T. & Barry, H. (1973). Birth order and alcoholism: A review. Quarterly Journal of Studies in Alcoholism, 34, 837-852. Blehar, M.C. & Rosenthal, N.E. (1989). Seasonal affective disorders and phototherapy. Archives of Genetic Psychiatry, 46, 469-474. Butler, M.G., Ledbetter, D.H. & Mascarello, J.T. (1985). Birth seasonality in Prader-Willi syndrome. Lancet, 2, 828-829. Buydens-Branchey, L. Branchey M.H., & Noumair, D. (1989a). Age of alcoholism onset I. Relationship to psychopathology. Archives of Genetic Psychiatry, 46, 255-230. Buydens-Branchey, L., Branchey M.H. & Noumair, D. et al. (1989b). Age of alcoholism onset 11. Relationship to susceptibility to serotonin precursor availability. Archives of Genetic Psychiatry, 46, 231-235. Chyatte, C. & Smith V. (1981). Brain asymmetry predicts suicide among navy alcohol abusers. Military Medicine, 146, 227-278. Cicero, T. (1982). Alcohol effects on the endocrine system. US Department of Health and Human Services, Biomedical Processes and Consequences of Alcohol Use: Alcohol and Health Monograph 2. Rockville, MD. Cloninger, C.R. (1987). Neurogenetic adaptive mechanisms in alcoholism. Science, 236, 410-416. Coren, S . (1989). Left-Handedness and accident-related injury risk. American Journal of Public Health, 79, 1040-1041. Cotton, N.S. (1979). The familiar incidence if alcoholism. Journal of the Study of Alcohol, 40, 89-116. Coursey, R.D., Buchsbaum M.S. & Murphy D.L. (1979). Platelet MOA activity and evoked potentials in the identification of subjects biologically at risk for psychiatric disorder. British Journal of Psychiatry, 134, 374-381. Creel, D.J., Bendel, C.M., Wiesner G.L. et al. (1986). Abnormalities of the central visual pathways in Prader-Willi syndrome associated with hypopigmentation. New England Journal of Medicine, 314, 1606-1609. Dalen, P. (1975). Season of birth: A study of schizophrenia and other mental disorders. Amsterdam: North Holland Press. Dardis, T. (1989). The thirsty muse: Alcohol and the American writer. Ticknor and Fields Drager, U.C. (1986). Albinism and visual pathways. New England Journal of Medicine, 314, 1636-1638. Drejer, K., Theilgaard A., Teasdale, T.W., et al. (1985). A prospective study of young men at high risk for alcoholism: neurophysiological assessment. Alcoholism (W),9, 498-502.

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Eastwood, M.R. & Stiasny S. (1978). Psychiatric disorder, hospital admission and season. Archives of Genetic Psychiatty, 35, 769-771. Gabrielli, W.F., & Mednick, S.A. (1983). Intellectual performance in children of alcoholics. Journal of Nervous and Mental Disorders, 171, 444-447. Galaburda, A.M. (1988). From Reading to Neurons. Cambridge MA: MIT Press. Geschwind, N. & Galaburda A.M. (1987). Cerebral lateralization, biological mechanisms, associations, and pathology. Cambridge MA: MIT Press. Goodwin, D. (1985). Alcoholism and genetics.Archives of Genetic Psychiatry, 42, 1717-174. Gordon, H.W. (1980). Cognitive asymmetry in dyslexic families. Neuropsychologia, 18, 645-656. Halpern, D.F. & Coren, S. (1978). Do right-handers live longer? Nature, 333, 213. Harburg, E., Feldstein, A., & Papsdorf, J. (1978). Handedness and smoking. Perceptual and Motor Skills, 47, 1171-1174. Harburg. E. (1981). Handedness and drinking-smoking types. Perceptual and Motor Skills, 52, 279-282. Hardyck, C , & Petrinovich L.F. (1977). Left-handedness. Psychological Bulletin, 84, 385-404. Harlap, S. (1974). A time-series analysis of the incidence of Down’s syndrome in West Jerusalem. American Journal of Epidemiology, 99, 210-217. Harwood, H.J., Kristiansen, P. & Zachal J.V. (1985). Social and economic costs of alcohol abuse and alcoholism. (Issue report no 2) Research Triangle Park, NC.: Research Triangle Institute. Irwin, P. (1985). Greater brain response of left-handers to drugs. Neuropsychologia, 23, 61-67. Jansson, B. & Malahy, MA. (1981) Cancer risk, age at diagnosis and age at death as functions of season of birth. Cancer Detection and Prevention, 4, 291-294. Karpinos, B.D. & Grossman, HA. (1953). Prevalence of left-handedness among selective service registrants. Human Eiology, 25, 36-50. b o p , J. Teasdale, T.W. Schulsinger, F., et al. (1985). A prospective study of young men at high risk for alcoholism: school behavior and achievement. Journal of the Study of Alcohol, 46, 273-278. Kramer, MA., Albrecht, S. & Miller, RA. (1985). Handedness and the laterality of breast cancer in women. Nursina Research. 34. 333-337. Lerman, S. (1988). Ocular phototoxich. New Ehglakd Journal of Medicine, 319, 1475-1477. London, W.P. & Teague, G.B. (1985). Alcohol consumption and latitude in the United States. American Journal of Psychiatty, 142, 656-657. London, W.P., Kibbee, P., Holt, L., (1985). Handedness and alcoholism.Journal of Nervous and Mental Disorders, 173, 570-572. London, W.P. (1985). Treatment outcome of left-handed versus right-handed alcoholic men. Alcohol: Clinical Experimental Research, 10,357. London, W.P. (1986a). Handedness and Alcoholism: A family history of left-handedness.Alcohol: Clinical Experimental Research, 10, 357.

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London, W.P. (1986b). Month of hospitalization of left-handed substance abusers. Neuropsychologia, 24, 455-456. London, W.P. (1987a) Alcoholism: Theoretical consideration of the season of birth and geographic latitude. Alcohol, 4, 127-129. London, W.P. (198%). Cerebral laterality and the study of alcoholism. Alcohol, 4, 207-208. London, W.P., & Click, J.L. (1988).Alcoholism, thyroid disorders and left-handedness. American Journal of Psychiatry, 145, 270. Loosen, P.T., Wilson, I.C. Dew, B.W., et al. (1983). Thyrotropin-releasing hormone (TRH) in abstinent alcoholic men. American Journal of Psychiatry, 140, 1145-1149. Lynch, H.T., Albano, WA., Heieck, J.J. et al. (1984).Genetics biomarkers, and control of breast cancer: A review. Cancer Genetics and Cytogenetics, 13, 43-92. Maccoby, E.E., Doering, C.H., Jacklin, C.N., & Kraemer, H. (1979). Concentrations of sex hormones in umbilical-cord blood: Their relation to sex and birth order of infants. Child Development, 50, 632-642. MacGregor, R.R. (1986).Alcohol and immune defense. Journal of the American Medical Association, 256, 1474-1479. Monteiro, M.G., & Schuckit, MA. (1988). Populations of high alcoholism risk: recent findings. Journal of Clinical Psychiatry, 49(supplement 9), 3-7. Moss, H.B., Guthrie, S., & Linnoila, M. (1986).Enhanced thyrotropin response to thyrotropin releasing hormone in boys at risk for development of alcoholism. Archives of Genetic Psychiatry, 43, 1137-1142. Nasrallah, N.A., Keelor, K., & McCalley-Whitters, M. (1983).Laterality shift in alcoholic males. Biological Psychiatry, 18, 1056-1067. Oldfield, R.C. (1971). The assessment and analysis of handedness: The Edinburgh Inventory. Neuropsychologia, 9, 19-113. Satel, S.L., & Gawin, F.H. (1989).Seasonal cocaine abuse. American Journal of Psychiatry, 146, 534-535. Schachter, S.C., & Galaburda A.M. (1986). Development and biological associations of cerebral dominance: Review and possible mechanisms. Jountal of the Amencan Academy of Child Psychiatry, 25, 741-750. Schachter, S.C., Ransil, B.J. & Geschwind N. (1987).Associations of handedness with hair color and learning disabilities. Neuropsychologia, 25, 269-276. Schuckit, M.A. (1984).Relationship between the course of primary alcoholism in men and family history. Joiintal of the Study of Alcohol, 45, 334-338. Schulsinger, F., Knop, J., Goodwin, D.W., et al. (1986).A prospective study of young men at high risk for alcoholism: social and psychological characteristics. Archives of Genetic Psychiatry, 43, 755-760. Smith, V., & Chyatte, C. (1983).Left-handed versus right-handed alcoholics: An examination of relapse patterns. Journal of the Study of Alcohol, 44,553-555. Tarter, R.E., McBride, H., Buonpane, N., et al. (1977). Differentiation of alcoholics: Childhood history of minimal brain dysfunction, family history and drinking pattern. Archives of Genetic Psychiatry, U34, 761-768.

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Tarter, R.E. Alterman, A.I., & Edwards, K.L. (1985). Vulnerability to alcoholism in men: A behavioral-genetic perspective. Journal of the Study of Alcohol, 46, 329-356. Tarter, R.E. & Hegedus, A.M. (1985). Neurological mechanisms underlying inheritance of alcoholism vulnerability. International Journal of Neuroscience, 28, 1-10. Vaillant, G.E. (1983). The natural history of alcoholism: Causes, patterns and path of recovery. Cambridge MA: Harvard University Press. Winokur, G., Rimmer, J., & Reich, T. (1971). Alcoholism IV: Is there more than one type of alcoholism? British Journal of Psychiatry, 118, 525-531. Woodruff, RA., Guze, S.B.& Clayton, P.J. (1974). Psychiatric illness and season of birth. American Journal of Psychiatry, 131,925-926. Wurtman, R.J. & Wurtman, J.J. (1989). Carbohydrates and depression. Scientific American, 260(1), 68-75.

LEFT-HANDEDNESS Behavioral Implications and Anomalies, S. Coren (Editor) 0 Elsevier Science Publishers B.V. (North-Holland), 1990

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Chapter 17

Left- and Mixed-Handedness and Criminality: Explanations for a Probable Relationship Lee Ellis Minot State University

For countless centuries, left-handers have been looked upon in most societies with suspicion and mistrust (Barsley, 1966; Hardyck & Petrinovich, 1977). Are such attitudes justified, or do they reflect unwarranted prejudice against a convenient minority group? This chapter will review the literature having to do with handedness and criminality. After showing that there is general support for the hypothesis that left- and mixed-handers are somewhat more prone toward criminality than right-handers, various theoretical explanations for why such a relationship should exist are considered. The most plausible explanations for a handedness-criminality relationship seem to be based upon knowledge about how the two hemispheres differentially respond to stimuli both cognitively and emotionally. As to why people would vary in the relative involvement of one hemisphere over the other -- thereby making for co-variation in handedness and criminality -- prenatal exposure to sex hormones appears to play a major role.

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A Review of Evidence for an Association between Criminality and Sidedness At least seven studies have reported higher incidences of delinquency and criminality among left- and mixed-handers than among right-handers (Lombroso, 1903; H. Ellis, 1910, p. 118; Fitzhugh, 1973; Virkkunen, Nuutila & Huusko, 1976; Krynicki, 1978; Gabrielli & Mednick, 1980; Ellis & Ames, 1989; also see Standage 1983). Nevertheless, four studies have reported insignificant differences (West & Farrington, 1977; Yeudall, Fromm-Auch, & Davies, 1982; Hare & Forth, 1985; Denno, 1985, p. 733; Hare & Connolly, 1987, p. 225), and one study actually found higher rates of criminality among right-handers than among lefthanders (Nachshon & Denno, 1987a). Given this somewhat mixed picture, it is necessary to review the nature of the nonsupportive evidence before considering any possible theoretical explanations for why handedness and criminality might be related. The study reported by West and Farrington (1977) was conducted in England based on a sample of 101 official delinquent males and 288 males who had no official delinquency record, Their finding of no significant relationship between handedness and delinquency was reported without presenting any details (p. 72). Since this was actually the first study which failed find left- (and/or mixed-) handers more criminal or delinquent than right-handers, I wrote to West for details soon after their book appeared. In a response dated July 10, 1978, West kindly provided me with the following figures: Among their sample of official male delinquents, 16.8% were left-handed, whereas among the nondelinquent males, 13.2% were left-handed. This is what they determined to be statistically insignificant. However, in his letter, West went on to mention some additional figures that was not alluded to the book that he and Farrington authored. Specifically, 62 of their 101 delinquents were considered recidivists, and, among this group, 21% were left-handed. This supplemental information suggests that, in fact, a significant relationship did exist between handedness and persistent delinquency; thus, their study actually should be counted on the confirming side of the ledger. The study by Yeudall, Fromm-Auch, and Davies (1982) compared 64 male and 35 female delinquents recommended for neurological testing by court authorities in Canada. The comparison group of 29 males and 18 females were student volunteers from a local high school. Yeudall and associates failed to give specific figures on the percentage of delinquents (of either sex) who were left-

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handed, but did report that the proportions involved were not sigdicantly different from the nondelinquent comparison group (p. 257). Hare and Forth’s (1985) study compared Canadian samples of 258 male prisoners with 1,211 nonprisoners (apparently of both sexes). No significant group differences were found with regard to handedness. Among the offenders 23.2% were mixed-handed and 6.6% were left-handed, whereas among the nonoffenders, 25.9% were mixed-handed and 6.3% were left-handed. In a refined analysis, the prison sample was divided into those with high, medium, and low psychopathic symptoms. This yielded one significant difference: Inmates who scored intermediate in Hare’s scale of psychopathy contained 10.6% who were considered left-handed, a percentage that was significantly greater than for the prisoners with both the highest psychopathy scores and for the nonprison comparison group. The failure of this study to find significantly higher rate of left- and mixed-handers among the offenders as a whole compared to a general population is certainly contrary to any hypothesis that criminality is more common among nonright-handers than among right-handers. Another study which failed to find left-handers more disposed toward crime than right-handers was carried out by Denno (1985). Her study was based upon a sample of 410 black males and 390 black females who had been born in Philadelphia and were all at least 17 years of age at the time police records were consulted. Without giving details concerning the procedures used to select these 800 youth, Denno concluded that no significant relationship existed between delinquency and left-handedness. Regarding the one study which actually reported that right-handers were significantly more prone toward criminality than left-handers, it was apparently based upon many of the same subjects as utilized by Denno (1985). Without mentioning this earlier study, Nachshon and Denno (1987a) reported that their sample consisted of 1,066 black males from Philadelphia. Based upon police records, subjects in this study were classified into one of six categories of delinquency: very violent, violent, property destruction, theft, status, and nonoffender. The results revealed that a significantly higher proportion of the subjects without an arrest record were left-handed than was true of any of the five offender categories (p. 198). Before leaving this issue, let me briefly review the most recent study bearing on this issue, since I was directly involved in its design and it contained several fairly unique characteristics (Ellis & Ames, 1989). For example, this was the first study to used a self report delinquency/criminality scale, instead of one or more official measures. Second, only one other study had sampled both sexes

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(i.e., Denno's, 1985), instead of simply males. Third, handedness was measured as a continuum using two separate scales, one for general handedness and the other for visually guided handedness (rather than being simply dichotomized or tricotomized). In addition, we included continuous scales for earedness, eyedness, footedness, and sidedness in upper body strength. Basically, what we found was that, among males, both handedness measures were significantly related to delinquency, with those scoring on the left-handed end of both handedness scales being more delinquent/criminal than those scoring on the right-handed end of both scales. Few of the other measures of sidedness, however, showed any real relationship to delinquency/criminality. The only real exception had to do with drug offenses, where no significant differences in handedness were found. Among females, on the other hand, almost no si@icant correlations were found, even though most of the relationships were in the direction of delinquency/criminality being more common among those who were left as opposed to right-handed. Overall, it seems fair to say that the bulk of evidence supports the hypothesis that left- and mixed-handers are more prone toward criminality than rightbanders, especially when the West and Farrington study is added to the "support column." Nevertheless, the fact that some studies with large samples and valid methodologies have failed to support the hypothesis means that the issue can not be considered closed.

Possible Explanations for Handedness-Criminality Associations Apart from the possibility that most of the studies are incorrect, various explanations can be offered for why a relationship between handedness and criminality should be found. As is the case with so many relationships discovered by social scientists, the explanations can be categorized into those that only consider social environmental variables, and those that focus upon biological variables (albeit usually in conjunction with environmental variables). A Purely Social Environmental Explanation

Evidence clearly indicates that criminology is still dominated by social environmental explanations of criminal behaviour (Ellis & Hoffman, 1990). It is surprising therefore that no one seems to have ever proposed a social explanation for the tendency for left- and mixed-handers to be more criminally

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disposed than right-handers. A quite reasonable social explanation could be constructed in the tradition of labelling theory. It could begin by noting that leftand mixed-handers are, and always have been, a conspicuous minority group in all human societies, at least of any major size (Laponse, 1976; Marx, 1983, p. 439). As is true of so many other minority groups, left- and mixed-handers have been subjected to centuries of discrimination and sometimes even persecution (von Hentig, 1948, p. 62; Barsley, 1966; Corballis, 1980). For example, in many societies it has been historically a common practice to strongly discouraged people from using their left hand in performing most "civilized tasks, especially writing (Hertz, 1973, p. 11; Wieschhoff, 1973,p. 62; Hardyck & Petrinovich, 1977, p. 397). As a result of this prejudice and discrimination, many left- and mixedhanders may have come to view themselves in negative terms. Their greater involvement in crime could simply be a result of this negative self image and/or the result of further discrimination by the right-handed majority. Along similar lines, one could argue that left- and mixed-handedness fundamentally reflects a conscious or unconscious decision to disobey authority and to defy societal conventions, and that engaging in criminality is simply another form of disobedience. Unfortunately, such an explanation would be hard pressed to account for why left- and mixed-handedness have been and remain a minority group, or to explain why such a seemingly trivial characteristic as handedness would be singled out for discrimination. Another problem is that, unlike nearly all other minority groups in which crime rates are high, left- and mixed-handers do not appear disadvantaged from a socioeconomicstandpoint. At least one study has indicated that they do not differ to any noteworthy degree from right-handers in terms of family social status background (Hardyck, Petrinovich, & Goldman, 1976). And, in terms of their own achievements in income and occupational prestige, left- and mixed-handers have been at least as successful as right-handers, and possibly slightly more so (Fritsch, 1968;Thompson & Marsh, 1976; Laponce, 1976, p. 49). Nevertheless, as will be discussed more later, left- and mixed-handersare similar to other minority groups with high probabilities of crime in tending to score below normal on standardized tests of intelligence(Briggs, Nebes, & Kinsbourne, 1976; Hicks & Pellegrini, 1978; Bradshaw, Nettleton, & Taylor, 1981; Searleman, Herrmann, & Coventry, 1984; Searleman, Cunningham, & Goodwin, 1988), although one study found mixed-handers scoring higher than either left or righthanders (Newcombe, et al., 1975). Left- and mixed-handers also exhibit learning disabilities at higher than normal rates (Sabatino & Becker, 1971; Blai, 1972;

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Bradshaw & Taylor, 1979; Begley, 1982;Schachter, Ransil, & Geschwind, 1987), especially in reading throughout the elementary grades (Annett, 1981, p. 117; Geschwind & Behan, 1982; Duane, 1983, p. 6). Neurologically-Based Explanations

At least four distinguishable attempts to explain why left- and mixed-handers might be more crime-prone than right-handers have been offered. They are all biological in the sense that they specificallymention neurological variables having to do with the neocortex as being involved. Before specifically describing these explanations, the following four points about how the two hemispheres of the neocortex influence thoughts, emotions, and behaviour need to be mentioned: 1. The movement of each hand tends to be controlled by the hemisphere on the opposite side of the body. The two hemispheres of the neocortex are differentially involved in the control of hand movement (Hardyck & Petrinovich, 1977; Lassen, Ingvar, & Skinhoj, 1978, p. 67). The left hemisphere tends to control movement of the right side, and the right hemisphere tends to control movement of the left side of the body. This phenomenon is called contra-lateral control. 2. For most people, the left hemisphere attends to and understands complex linguistic commands better than the right hemisphere. Using a wide variety of methods, numerous studies have shown that the left hemisphere is more adept at processing, and especially at uttering and writing linguistic communication than is the right hemisphere (Kohn, 1980; Schwartz & Tallal, 1980; Kinsbourne, 1981; Moore, 1986).In contrast, the right hemisphere appears to be more specialized for processing visual-spatial stimuli, and for doing so in a holistic-integrative ways, compared to the left hemisphere (Otto, Yeo, & Dougher, 1987, p. 1205). 3. The right hemisphere is more involved in emotional responding to environmentalinput than the lejl hemisphere, especially when it comes to "negative" emotions. Using various methodologies, studies have indicated that the right hemisphere is quicker than the left hemisphere to respond to stimuli in emotional terms (Schwartz, Davidson, & Maer, 1975; Tucker, Watson & Heilman, 1977). The evidence is especially strong for what can be called "negative-rejecting" emotions such as hate, jealousy, and cynicism (Alford & Alford, 1981; Borod, Koff, & White, 1983; Hirskowitz, Karacan, Thornby, & Ware, 1984; Miller, 1988). Indicating that these hemispheric differencfs involve

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little learning, Fox and Davidson (1988) recently reported that these sorts of hemispheric differences were generally apparent in ten-month old human infants. 4. The right hemisphere plays a more fundamental role in attention and independent self-motivation than the ieji hemisphere, especially in response to aversive stimuli, although the right hemisphere appears more responsive to goals set by others. While the evidence is not nearly as strong as in the case of verbal reasoning and emotionality, various studies suggest that the right hemisphere is more involved in independently maintaining attention than the left hemisphere (Heilman & VanDenAbell, 1980). A greater tendency by the right hemisphere to independently maintain vigilance appears to be especially pronounced in regard to unpleasant stimuli (Otto, Yeo, & Dougher, 1987). However, when it comes to commitments to goals set by others, the left hemisphere seems to dominate (Tucker &. Williamson, 1984). The four neurologically-based explanations for why left- and mixed-handers may be more criminally prone than right-handers are as follows: First, Luria (1961) and Andrew (1974, 1977) have proposed that language comprehension could be fairly important for law abiding behaviour. Their arguments are based on evidence that, because (a) the left hemisphere usually exercised greater control over the comprehension of language than the right hemisphere, and (b) left- and mixed-handers are less exclusively left hemispheric dominant than right-handers, they simply take longer than right-handers do to comprehend language at a given level of efficiency (also see Hare & McPherson, 1984, p. 148). Inasmuch as language comprehension is a prerequisite for understanding and then obeying criminal statutes, Luria and Andrew infer that left- and mixed-handers may be more criminal than right-handers. Besides explaining their apparently greater criminality, the reasoning of Luria and Andrew coincides with evidence already reviewed that left- and mixed-handers have higher rates of learning disabilities than right-handers, especially with reference to reading. Their proposals are also appealing because of the well established positive correlation between learning disabilities and criminality (reviewed by Ellis, 1987, p. 910). A second neurologically-based explanation has been offered by Gabrielli and Mednick (1980, p. 654,1983, p. 68). They have suggested that, because left- and mixed-handers are less exclusively left hemispheric dominant than right-handers, such persons tend to be less analytic and more impulsive than right-handers. While there is only a minimal amount of evidence linking left- or mixedhandedness with impulsiveness (Gabrielli & Mednick, 1980; Bowers & Heilman,

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1980 p. m), such a proposals is certainly consistent with evidence that criminality is associated with impulsiveness (Ellis, 1987, p. 911). Third, both Yeudall (1980) and Nachshon and Denno (1987a; Nachshon, 1990) have proposed that associations between criminality and handedness may reflect disfunctions of the right hemisphere, and, in Yeudall's case also possibly the left hemisphere. According to this perspective, dysfunctions in the left hemisphere may not only serve to decrease tendencies toward right-handedness, but may disrupt language comprehension and positive affect, thereby increasing the probability of criminal law violations. Yeudall, Fromm-Auch, and Davies (1982) have presented evidence suggesting that dysfunctions in the right hemisphere may trigger strong negative emotional outbursts, and, presumably, these outbursts could sometimes manifest themselves in criminal ways. As to the causes of hemispheric dysfunctions, various abnormalities in prenatal and perinatal environmental factors have been hypothesized (Nachshon & Denno, 1987a). Such proposals are given credence by studies which have suggested that left- and mixed-handers are more likely than right-handers to have traumatic birthing experiences (Bakan, Dibb, & Reed, 1973; Coren & Porac, 1980; Smart, Jeffrey, & Richards, 1980; Coren, Searleman, & Porac, 1982). Nevertheless, some studies have failed to confirm the existence of a significant link between traumatic birthing and left-handedness (Searleman, Tsao, & Balzer, 1980; Nachshon & Denno, 1987b). Also, while the concept of hemispheric dysfunction is intuitively appealing for explaining unusual/undesirable behaviour such as persistent and serious criminality, defining a "dysfunction" seems inherently subjective. If such a term is going to be employed, careful studies need to be conducted in which dysfunctions of either hemisphere are described in strictly neurological (or neurochemical) terms quite apart from any knowledge of the behaviour to be explained (rather than being inferred from the behaviour). The fourth neurologically-based explanation for why there might be a relationship between handedness and criminality is one that I have recently advocated (Ellis, 1987; Ellis & Coontz, 1990). The proposal actually compliments and extends the three explanations already reviewed more than it competes with them. The proposal is that exposure to various sex hormone regimens (especially regimens that are fairly typical of males) prior to birth causes the neocortex to shift at least partially away from its normal left hemispheric dominance in cognitive and emotional functioning. This rightward shift affects both thoughts and emotions in ways that increase the probability of criminal behaviour. Evidence supporting this "rightward shift" hypothesis includes the following:

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1. Males tend to perform better in spatial reasoning than females, and sex honnones appear to account for fhe average difference. Male superiority in spatial reasoning is not only true of humans (McKeever, Seitz, Hoff, Marino, & Diehl, 1983, p. 665; Smail & Kelly, 1984), but also true of various species of rodents, as evidenced by more rapid maze learning among males than among females (Begley & Carey, 1979, p. 104; Goldman, 1976; Goy & McEwen, 1980, p. 31). As noted earlier, spatial reasoning appears to be performed better by the right hemisphere than by the left. Experiments with laboratory animals have shown that the typical male superiority in maze learning can be eliminated by exposing genetic female to high male-typical levels of sex hormones perinatally, and again at puberty (Beatty, 1979; McGivern, Claney, Hill, & Noble, 1984; Rhawn, Hess, & Birecree, 1978). 2. Males have slightly thicker right hemispheres than females do, and sex honnones appear to be largely responsible for the difference. This sex difference in the relative thickness of the right hemisphere has been observed in humans (Diamond, 1988, p. 21) and in rats (Diamond, Johnson, & Ingham, 1975). In addition, experiments with rats have shown that the differences can be largely eliminated by removing the testes of male rats while their brains are still being sexually differentiated during the first week or so after birth (Diamond, 1984). (Incidentally, such a procedure would probably not be effective with humans because our,brains are much more fully differentiated prior to birth than is the case with the brains of rodents.)

3. Males are more prone toward lefi- and mired-handedness than females, and sex hormones may be responsible for the difference. Many studies have reported sex differences in handedness, and the vast majority of these studies have found males somewhat more likely to be left- or mixed-handed than females (e.g., Bakan, 1971; Bryden, 1977; Hicks & Kinsbourne, 1976; Thompson & Marsh, 1976, p. 220; Spiegler & Yeni-Komshian, 1983, p. 653; Sakano & Pickenhain, 1985, p. 109; Saunders & Campbell, 1985; Benbow, 1986; Schachter, Rand, & Geschwind, 1987; Lansky, Feinstein, & Peterson, 1988). While there have been several failures to find significant sex differences in handedness among adults (Belmont & Birch, 1963; Suchenwirth, 1969; Porac, Coren, & Duncan, 1980, p. 719; Roszkowski, Snelbecker, & Sacks, 1981, p. 204), most of these studies have been in the same direction as the majority of studies and merely fell slightly short of statistical significance. Inasmuch as the sex differences in fact appear to be small, some failures to replicate are to be expected (Porac & Coren, 1981, p. 36; Hines & Ciorski, 1985, p. 79).

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It is also worth mentioning that, in addition to the majority of studies of human adolescents and adults, three recent studies of infants less than a year old have all reported that females exhibit a slightly, but significantly,greater tendency than males to consistently reach for objects presented to their midline with their right hands rather than their left hands (Rice, Plomin, & de Fries, 1984; Michel, Ovrut, & Harkins, 1986; Humphrey & Humphrey, 1987). Outside the human species, further evidence has been found of sex differences in sidedness measures analogous to handedness. Paw preferences have been found to be more consistently right sided in female mice than in male mice (Glick, Zimmerberg, & Jerussi, 1977; Collins, 1977; Glick, Schonfeld, & Strumpf, 1980). Also, in a T-maze prior to the establishment of any reinforcement schedule, female rats choose to turn right more frequently than male rats did (Hines & Gorski, 1985, p. 85). All in all, the evidence has fairly clearly established that males are less prone to be right sided (at least in upper body movements) than females, and that this slight bias is the result of male-typical sex hormone regimens causing at least a slight rightward shift in neocortical functioning. Additional support for this view comes from two additional lines of research, although they are less definitive, especially when considered in isolation. 4. Males tend to exhibit negative emotions to a greater degree than females. Evidence that a negative emotional tone is more characteristic of males than of females comes, first, from studies of smiling among human adolescents and adults. In all cultures yet studied, males are less likely to exhibit smiling in response to social stimuli than males (Cohen, 1977; Freedman, 1979, p. 55; Morse, 1982; for a slight qualification see Denmark, 1977). Speculation that this sex difference is simply due to sex role training is diminished by evidence that the sex differences appear even among neonates (Korner, 1969; Kagan, 1971; Freedman, 1979, p. 54); and, at least one study found comparable sex differences among baboons (DeVore, 1%5). This sort of sex difference, of course, would be predictable from the evidence reviewed earlier that the right hemisphere is less "prosocial" in emotional tone than the left hemisphere. 5. Males appear to rely less than females do on external social standards and authority in making ethical and legal decisions. Possibly reflecting their lower reliance on the left hemisphere relative to females, males appear to be a bit less motivated by external social conditions and more motivated by internal drives in making decisions about their behaviour (Douvan, 1960; Turk, 1969; Hindelang, 1974). Perhaps, also for this reason, human males have been more often regarded as tenacious and stubborn than females (Garai & Scheinfeld, 1%8,223;

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Valentich & Gripton, 1977; Stringer, et al., 1978, p. 228; Laszlo, et al, 1980; for a similar description of sex differences in rats see Joseph, 1979, p. 42). A final issue I have attempted to address in regard to male’s exhibiting a rightward shift in hemispheric functioning relative to females is how exposing the brain to androgens could actually induced such a shift. Studies have indicated that exposing the brain to fairly high androgen levels during the time the brain is being sexually differentiated serves to cause a slight retardation in neurological development, especially in the left hemisphere (Toran-Allerand, 1978; Diamond, Dowling, & Johnson, 1981; Marx, 1982, p. 144; Geschwind, 1984, p. 203). In the long run, this retardation of left hemispheric development appears to have the effect of allowing the right hemisphere to assert itself a bit more in males than in females. On the basis of such evidence, I have postulated that the tendency for males to exhibit a slightly greater average rightward shift in hemispheric functioning than females is not only responsible for their greater tendencies toward left- and mixed-handedness, but also partially responsible for their greater involvement in criminal activity. To more fully account for the differences in criminality (irrespective of handedness), subcortical regions of the brain need to be considered along with the cortical regions, for the functioning of subcortical regions also appears to be affected by exposure to sex hormones (Ellis, 1987a). One of the subcortical regions to a considerable extent has to do with the reticular formation. Various lines of evidence indicate that prenatal exposure of the brain to high (male-typical) androgen regimens causes the reticular formation (and its support structures extending into the prefrontal region of the brain) to become less sensitive to environmental input (Ellis, 1987a, 1987b). As a result, affected individuals tend to seek more intense and varied stimulation and tolerate more adverse consequences in the course of doing so than unaffected individuals. The other subcortical region involves the limbic system. Prenatal exposure to high (male-typical) androgen regimens appears to increase the risk of seizuring, especially in and around the limbic system (Ellis & Coontz, 1990). Some seizuring is convulsive and thus diagnosed as epilepsy, but many seizures have few if any convulsive symptoms, especially when the seizures are relegated to the limbic system (which has little direct influence on basic motor control). The results of subconvulsive seizures in and around the limbic system are particularly hard to predict, but are likely to alter basic drives and moods.

(1)

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P e r ina t a 1 Neurological Organization

Genetic h Perinatal Factors

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proaoclal

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f S t i m u l u s Seeking and r i s k taking behavior

- - _ _ - _- -f childhood hyperactivity

e p i l e p s y 6 other

Figure 1: A theoretical model of the main variables postulated to account for higher rates of criminality among left- and mixed-handers than among right-handers.

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A Model of How Sex Hormones Influence Hemispheric Functioning in Ways that May Influence Handedness and Criminality Based upon the above evidence, Figure 1 offers a theoretical model that predicts the existence of a modest relationship between handedness and criminality. The model depicts a "neurochemical scenario" which would increase the probability both of left or mixed-handedness and of criminality (including delinquency). Beginning in the first column, Figure 1postulates that there are genetic and in utero (prenatal) factors which combine to bring about varying degrees of exposure of the fetus' brain to sex hormones (Ellis & Ames, 1987). If the exposure to sex hormones (especially androgens) is in the high (male-typical) range, especially during key critical periods of fetal development (that still remain to be fully specified), three neurological events are postulated to occur. As shown in the second column of Figure 1, one of these events is a decrease in the size and functioning of the left hemisphere relative to the right hemisphere. Then, as shown in the third column, this leads to what is being called a rightward shift in hemispheric functioning following birth, and especially following puberty (when androgen levels are again elevated). As a result of the rightward shift in hemispheric functioning, a diminution of three cognitive and emotional tendencies occur: (a) attention given to linguistic stimuli, (b) socially-dependent motivation, and (c) prosocial emotions. Theoretically, such a rightward shift in the neocortex should also increase the probability of the left hand being preferred over the right hand in performing find motor movements, especially in societies which do not routinely punish such preferences. The final column of Figure 1 shows what is postulated with reference to the main behavioural manifestations of the three cognitive and emotional tendencies resulting from a rightward shift in neocortical functioning. However, since the neocortex and the subcortical regions combine to control complex behaviour (unlike handedness), it is impossible to separate the neocortical effects from the subcortical effects upon most of the behaviour patterns shown in the last column. Basically, Figure 1 proposes that the multiple behaviour patterns depicted in the large square in Column 5 are all manifestations of the brain being exposed to high levels of androgens when all three parts of the brain are being sexually differentiated (albeit at slightly different critical periods of gestation).

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Tentatively, I would identify the behaviour patterns near the top of the box as being more exclusively a function of neocortical functioning, whereas the behaviour depicted near the bottom reflect mainly subcortical functioning.

Discussion and Conclusions Studies have repeatedly found that the brains of left- and mixed-handers, on average, are different from those of right-handers, both structurally (Galaburda, LeMay, Kemper, & Geschwind, 1978; LeMay & Kido, 1978; Andreassen, Dennert, Olsen, & Damasio, 1982; Falzi, Perrone, & Vignolo, 1982; Witelson, 1985) and functionally (Warrington & Pratt, 1973; Rasmussen & Milner, 1977; Hecaen, DeAgostini, & Monzon-Montes, 1981). Therefore, it is reasonable to believe that, on average, left- and mixed-handers will exhibit somewhat different thought processes than right-handers, and that these differences in thought could produce differences in their probabilities of criminal behaviour. This chapter has shown that there is considerable support for the hypothesis that left- and mixed-handers are slightly more likely to engage in crime than right-handers, at least among males. This is despite the fact that there are some failures-to-replicate that need to be checked and ultimately accounted for. Such verification efforts may eventually determine that there are some types of offenses for which this generalization does not hold, or that there are certain narrow ranges within the broad spectrum of left-right-handedness which do not tit the overall pattern. The research reviewed in this chapter suggests that the most reasonable explanations for why even a modest relationship should exist between handedness and criminality are neurologically-based. Such proposed have focused on differences in how the two hemispheres respond to incoming stimuli. Basically, compared to the left hemisphere, the right hemisphere appears to be (a) less capable of handling language and other forms of logical-serial thought, (b) more emotional, especially with reference to negative emotions, and (c) may be less motivated by social influences than by self-interest. There arc reasons to believe that, in fact, all three of these cognitive-emotional tendencies may promoted criminal behaviour under various social conditions. The above arguments raise questions about why people would exhibit varying degrees of left-right dominance in their neocortical functioning. I have offered a theoretical model which suggests that three identifiable parts of the brain are altered by being exposed to sex hormones, one of which is the neocortex. While

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the timing of a complex combination of sex hormones are involved, several studies indicate that exposing the brain to sex hormone regimens that are more typical of males than of females tend to bring about a rightward shift in hemispheric functioning. According to the theoretical model herein advocated, a rightward shift in neocortical functioning slightly increases the probability of left- or mixedhandedness. In addition, because the right hemisphere tends to be less responsive to verbal commands, more negative in emotional tone, and less socially oriented than the left hemisphere, affected individuals eventually exhibit an increased probability of criminality. According to the theory I have outlined, exposing the brain to male sex hormones also affect two subcortical regions -- the limbic system and the reticular formation -- in ways that further increase the probability of criminal behaviour. Specifically, the limbic system seems to be made somewhat more seizure-prone, and the reticular formation more insensitive to environmental stimuli, by high exposure to male-typical sex hormone regimens. The model contends that the combined effects of sex hormones during fetal development on these three regions of the brain not only helps to explain why, at least among males, left- and mixed-handedness are somewhat more criminal than righthanders, but also suggests that they will tend to exhibit higher rates of learning disabilities, poorer academic performance (at least in elementary grades), childhood hyperactivity, alcoholism and drug abuse, and epilepsy. Finally, returning to the issue with which this chapter began, the question of prejudice and discrimination against left- and mixed-handers deserves attention in light of the evidence reviewed. As a left-hander myself, I would be disappointed to find any of the information brought together in this chapter used to justify returning to the practice of discouraging people from the dominant use of their left hand. At least three points should dissuade resumption of any such practices. First, the association between handedness and crime probabilities is at most a modest one detectable only with large samples, and it is still an indisputable fact that the majority of criminals are right-handed. Second, there is no evidence that forcing someone to switch hands in performing fine motor movements (such as writing) causes them to switch hemispheres as far as their thought patterns or emotional responses are concerned. Nevertheless, this is a possibility worthy of research attention (perhaps with animal models).

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Third, because of their rightward shift in hemispheric functioning, lefthanders may well make more than their share of contributions to human knowledge and artistic creativity. Evidence suggesting this comes from studies which have found left-handers to be either over-represented, or unusually talented, in the fields of architecture, mathematics, music, art, athletics, and possibly law and science (Peterson & Lansky, 1974,1977; Peterson, 1979;Mebert & Michel, 1980; Annett & Kilshaw, 1982, p. 547; McLean & Ciurczaj, 1982; Smith & Chyatte, 1983; Geschwind & Galaburda, 1987; Hassler & Birbaumer, 1988; for qualifications and failures to replicate see Oldfield, 1969; Byrne, 1974; Shettel-Neuber & OReilly, 1983). Despite their tendencies to have greater difficulties with school (at least in the elementarygrades), and apparently greater legal problems, left- and mixed-handers may be among the most productive members of human society once they reach full adulthood. If so, there is irony in the fact that despite their learning difficulties, left- and mixed-handers contribute more than their share to what is passed on to future generations in the way of significant knowledge and creative expression. For that reason alone, one should think twice about acting in any simple-minded way on evidence of their greater crime probabilities.

Acknowledgement I thank Israel Nachshon for useful criticisms of a draft of this chapter.

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Chapter 18

Laterality and Longevity: Is Left-Handedness Associated with a Younger Age at Death? Diane F. Halpern California State University, San Bernardino and Stanley Coren University of British Columbia

It is a notion current among the Maori that the right is the "side of life" (and of strength) while the left is the "side of death (and of weakness). Fortunate and lifegiving influences enter us from the right and through our right side; and, inversely, death and misery penetrate to the core of our being from the left. (Hertz, 1909, translated by Needham, 1973) Anthropologists, psychologists and popular writers (e.g., Barsley, 1976; Fincher, 1977; Hertz, 1909; Wile, 1934) have often commented about the association of the left hand and death in rituals and superstition. Thus, one circles a corpse widdershins, with the left hand in toward the body. One casts a death curse with the left hand. One buries the body of a warrior with his weapons in his left hand, as though he were left-handed. Such superstitions seem to be quite divorced from the extensive research literature that has grown up

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attempting to discern the correlates, causes, and consequences of hand preference, which has manifested itself in the form of hundreds of journal articles and other publications within the last ten years. Surprisingly, however, a picture has begun to emerge which suggests that handedness may have biological implications that result in a younger age at death for left-handers than for their right-handed counterparts, giving a somewhat macabre validity to these older beliefs. Perhaps the most intriguing indirect evidence that sinistrality is associated with reduced longevity comes from a series of life span and population surveys of handedness. Such studies have produced the surprising result that, regardless of the index used to measure handedness, with increasing age, the number of left-handers diminishes markedly (Ellis, Ellis & Marshall, 1988; Fleminger, Dalton & Standage 1977; Lansky, Feinstein & Peterson, 1988; Porac & Coren, 1981; Porac, Coren & Duncan, 1980). For example Lansky, Feinstein and Peterson, (1988) report that in a sample of 18 to 39 year olds, there were 11.3% left-handed. This diminished by more than half to 4.7% in a sample of 40 to 80 year olds. Porac and Corm (1981) find that the proportion of left-handers diminishes from 13% in 20 year olds to less than 1% in 80 year olds. They suggest that this age trend could occur because of an number of factors including cultural pressures on left-handers, covert environmental pressures toward dextrality, and maturational factors that differentially affect mortality. In this chapter, we will concentrate on this latter possibility, although we will discuss the other alternatives in turn. Ultimately we hope to clarify the association between handedness and life span, and place it into a sound neuropsychological and behavioural framework, by reviewing some of the relevant literature that has emerged in the last decade in conjunction with some new, as yet unpublished, data.

Right-Handedness as the Human Norm All of the available evidence suggests that left-handers have been in the minority ,since prehistoric times. Data in support of this claim come from a variety of sources. For example, Dart (1949) studied the fossilized skulls of baboons that had been hunted by australopithecine hunters some two million years ago. Most of these showed wound patterns consistent with the hypothesis that the baboons died from blows caused by clubs held in the right hands. Hand tracings which appeared on the walls of CroMagnon caves show that over 80%

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of the depicted hands are the left hand. Since it seems reasonable to infer that these patterns were made by each individual placing his nondominant hand against the wall and using his dominant hand to make the tracing, it follows that right-handers were in the majority in this group (Springer & Deutsch, 1989). Finally, several studies that have looked at the microscopic wear and flake patterns of stone tools in paleolithic groups have supported the notion of a population that was predominantly right-handed. (See Corbaliis, 1989, for a review). The most extensive study demonstrating that the left-handers have formed a relatively constant minority, at least from the stone age to the present time, was conducted by Coren and Porac (1977). They estimated the percentage of left-handers by scoring artistic representations of tool or weapon use in a sample of 1,180 works of art spanning a 5,000 year period. The percentage of lefthanders depicted in these works of art was relatively constant at about 8% across these 50 centuries. Furthermore, when a breakdown was done by geographic regions, to assess whether there were manifest cultural influences on handedness, there were no apparent differences among regions of the world in terms of the distribution of handedness. Furthermore, as demonstrated in reviews by Harris (this volume), Porac and Coren (1981), and Porac et al. (this volume), it appears that there never has been a culture or society in which lefthanders have, been in the majority. Thus, it seems reasonable to conclude, the pattern of right hand dominance can be taken as the contemporary and historical norm for human beings.

Why Left-Handedness? Many researchers and others have wondered why the percentage of lefthanders has remained constant over thousands of years, especially in light of subtle and overt prejudice against left hand use and pressures to convert lefthanders to dextral usage. Given the fact that right-handedness is the norm for human beings, and it seems to be such a stable population characteristic, there seems to be little reason to doubt that we are looking at some form of genetically controlled behavioural predisposition (much like the presence of two eyes is genetically programmed and relatively invariant across the species). Numerous investigators, however, have raised the issue of what controls the emergence of left-handedness (as a deviation from the dextral norm). Let us try

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to answer this question before we consider the consequences of left-handedness for other aspects of behaviour and survival.

Is Lea-Handedness Genetic? Several genetic models have been proposed to explain sinistrality as an inherited trait (see Coren, this volume for a further discussion). Simple single gene theories in which sinistrality is viewed as a recessive trait that is transmitted much like blue eyes or blonde hair cannot account for the fact that a majority of the children of two left-handed parents are right-handed. All of the children of two left-handed parents would have to be left-handed, if this theory were true, because the recessive gene for left-handedness could not be masked by a dominant gene for dextrality. A more complex two-gene model was proposed by Levy and Nagylaki (1972). According to these authors, laterality is determined by the joint action of two genes--one that determines which cerebral hemisphere is dominant for speech and one that determines whether motor control will be dominant in the contralateral or ipsilateral hemisphere. A third genetic model known as the "right shift" theory was proposed by Annett (1974, 1985). Annett has hypothesized that the normal development of language in the left hemisphere, and right-side motor control is genetically programmed. When this genotype is absent, right or left dominance for speech and motor control is equally likely. Another two allele model is proposed by McManus (1985) where one (D) produces a dextral pressure on the phenotype, while the other (C) operates on chance fluctuations (which if the genotype were CC would produce a 50/50 split in handedness). Although there is a current debate in the literature as to which of these genetic models provides the best fit to the actual incidence of left-handers in the population, all of the genetic theories fail to account for the fact that monozygotic twins are no more similar in handedness than dizygotic twins. As anyone familiar with the nature-nurture debate knows, if a trait is under genetic control, then monozygotic twins who share a common gene pool would have to be more similar with regard to that trait than dizygotic twins who share, on the average, only about half of their genes. The actual data on this issue is quite clear. McManus (1985) reviewed 13 twin studies of handedness. If the appearance of left-handednesswere genetically controlled then we would expect the proportion of twin pairs in which handedness differs (eg. one right-hander

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and one left-hander) would be markedly less for the monozygotic twins than for their dizygotic counterparts. When one computes the mean percentage of pairs with discrepant handedness we find that 24.1% of the dizygotic twin pairs fall into this category, while 23.4% of the monozygotic twins also do. The absence of any difference between groups with similar and different zygosity suggests that the emergence of left-handedness in twins does not appear to be directly under genetic control. The failure of all genetic theories to explain handedness patterns in twins has led several researchers to consider other types of explanations.

Is Left-Handedness Learned? When genetic theories cannot be used to describe a phenomenon, psychologists typically look for environmental explanations such as pressures imposed on young children to use one hand or the other (e.g., Ashton, 1982; Harris, this volume; Porac et al., this volume). Here it is important to first consider whether handedness (in any form) can be learned, and hence altered by environmental factors. It is clear that natural left-handers are subjected to environmental pressures toward dextral hand usage simply due to the fact that the world is engineered for right-hand use. Porac and Coren (1981) have dubbed societal preference for right-handers the Right-sided World Hypothesis. They note that virtually all power tools and manual equipment are designed for right hand and right foot use. All motor vehicles, for example, have gears and gas pedals on the right. Everyday implements such as carrot peelers, scissors, ice cream scoops, can openers, playing cards, and even the winding stem on wrist watches, are biased toward ease of use by right-handers. Musical instruments such as violins, guitars, and banjos, common machinery such as voting machines, soda and candy dispensers, time punch cards, and sporting gear such as rifles, archery sets, and fishing reels, are all designed with a bias that facilitates right-handed operation. In addition to the general environmental design that favours dextral usage, left-handers also report overt pressures to switch hand use. Observation of these pressures led John B. Watson (1925) to offer an explanation of the prevalence of right-handedness that was completely based upon social pressure and learning processes. He says: Our whole group of results in handedness leads us to believe that there is no futed differentiation of response in either hand until social usage

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Halpern and Coren begins to establish handedness. Society soon thereafter steps in and says "Thou shalt use thy right hand, Willy." We hold the infant so that it will wave "by-bye'' with the right hand. We force it to eat with the right hand. This in itserf is a potent enough conditioning factor to account for handedness. (p. 101)

In some cases, the social pressure placed on left-handers is extremely overt and also cruel and drastic. Thus Kidd (1906) reported that there are a group of African tribes who "cure" left-handedness by pouring boiling water into a hole in the ground and then forcing the child's left hand into the hole and packing the earth around it. This results in severe scalding that forces the child to use the right hand. It appears that the centuries old practice of requiring left-handers to use their right hand for writing and eating has been diminishing in contemporary society. Thus, if handedness does have a significant learned component, and this is reinforced or maintained by societal pressures, then the decrease in societal coercion to switch left-handers should result an increasing percentage of lefthanders in the population in comparison to that observed around the turn of the century. Porac, Coren, and Duncan (1980) examined this possibility in a sample of 34 published studies which looked at the distribution of right- and lefthandedness in adult populations. These surveys were taken over the period 1913 to 1976, and all were based upon North American and Western European samples. A scatter plot of the percentage of right-handed subjects plotted as a function of the date each survey was published is shown in Figure 1. Although there is a slight downward trend in the data suggesting a greater percentage of left-handers in more recent surveys, the obtained correlation (r = -2) is not statistically significant, suggesting that social pressure is not playing much of a role in determining handedness. In addition to the small change in reported incidence of left-handedness over the last 70 years, research has shown that it is extremely difficult to successfully change preferred hand use. Porac, Coren, and Searleman (1986) found that a high proportion of initially left-handed individuals had been subjected to direct pressures from parents, teachers, and others to shift to right-handedness. Their subjects reported a successful shift rate of only 29% for males and 62% for females. To place these numbers in terms of the population shift that they would represent, this means that the successfully shifted females account for 2.1% of the population of right-handers, while the successfully shifted males account for 1.6% of the population. In total then, only 3.7% of the fotal

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Figure 1: The percentage of the population who were determined to be righthanded as a function of the date of publication for 34 studies surveyed by Porac, Coren and Duncan (1980).

population represents individuals that have been shifted from left- to righthanded. While the data reviewed above suggests that environmental pressures and learning have little influence on overt handedness, we have, in some respects been looking at the wrong side of the coin in any event. The issue that we must address is the appearance of left-handedness. All of the research that we have presented above is oriented toward the suppression of natural left-handedness with a learned overlay of dextral performance. This learning, if it occurs (and as we have seen there is some doubt about its efficacy) is supported by social pressure and implicit pressures from environmental objects such as tools etc., toward dextral use. The actual questions which we have been asking, of course, is "Why are some people left-handed?," and "Can learning account for lefthandedness?" Yet, as we have seen, society and the technological world are biased toward dextrality. There seems to be no evidence that any society has ever

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made a virtue out of left-handedness, thus resulting in systematic pressure toward dextrals to become sinistrals. No one has ever set up any technological environment in which the bias is toward sinistrality. In other words, there does not seem to be any evidence suggesting a context in which a fixed segment of the population should become left-handed. Abram Blau (1946) attempted to deal with this issue. H e suggested that it is not that left-handedness is learned, but rather that right-handedness should be learned, and when it is not, left-handedness then emerges. Specifically he maintains: I will show that the various origins of sinistrality can be explained by this one general principle: they are due to some deviations in the normal learning process which should have led to dextrality. . . . The origin of dextrality may be summed up as due to an encouragement of dextral tendencies. When this encouragement is absent, we have educational conditions for sinistrality, whether these are deliberate and intentional or accidental and unconscious. As a rule, it is mostly due to accidental factors. (Blau, 1946, pages 87 and 89) There is some evidence that in the absence of a consistent set of righthanded models the percentage of left-handedness may be somewhat higher. Thus Harkins and Michel(1988) looked at infants averaging ten months of age. When the infant’s mother was left-handed infants tended to do more reaching with their left hand. While one can not eliminate a genetic component in these results, there is a suggestion that the increased left hand usage may have an environmental component. This may be due to a tendency of infants, during their first year, to match the hand use of their mothers during social interactions or when playing with toys (Harkins, 1987). Of course, this would suggest that in families where both parents are right-handed, left-handers ought to be relatively rare. If we reanalyze the data presented by McManus (1985) based upon a review of twelve family studies of handedness, we find that in families in which both parents are right-handed, the average incidence of left-handedness is still 10.5%. Blau does allow for one other factor to influence sinistrality, and whether dextral behaviours emerge. This involves, if not a direct form of learning, or at least suggests direct volitional control over handedness by the individual. He says:

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This theory, stated simply, is that sinistrality is the product of a contrary attitude on the part of the infant and young child. In other words, sinistrality is thus a symptom or manifestation of an attitude of opposition or negativism along with such other signs as disobedience, refusal to eat, temper tantrums, rebelliousness, etc. In place of a wish to comply with the social and cultural pressures toward the use of the right hand, there exists an active attitude of opposition which manifests itself in the development of sinistrality. It is as though the child says: "Since you want me to use my right hand, I won't! I'll spite you by using my left!" (Blau, 1946, page 91) Such speculations, however, have never been supported by any systematic evidence. There is no evidence that suggests that individuals who grow up in permissive surroundings, which would tolerate such rebelliousness, have any higher proportions of left-handedness. Nor is there any evidence that intolerant and authoritarian societies, which would not be expected to tolerate such "rebelliousness" have any lower proportions of left-handedness (cf. Harris, this volume; Porac and Coren, 1981; Porac et al., this volume). To summarize the material that we have discussed thus far, it seems reasonable to suggest that, although left-handers seem to make up a stable minority of the population, genetic theories have generally failed to fully account for this phenomenon, and covert and overt pressures to change hand use seem to have done little to account for the proportion of left-handers in the population. Given the failure to find any theory to adequately explain the etiology of left-handedness, researchers have begun to examine correlates of sinistrality as a means of discovering why approximately 10% of all people show a left-sided lateral preference.

Is Left-Handedness the Result of Pathology? There is an unusually high incidence of left-handedness in selected clinical populations which has led researchers to suspect that left-handedness is sometimes pathological in origin (Coren & Searleman, this volume; Harris & Carlson, 1988; Satz, Orsini, Saslow, & Henry, 1985). The notion of "pathological left-handedness" explains the relationship between sinistrality and clinical syndromes by suggesting that both result from a single common cause. We shall

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see below that the most likely candidate for this cause is early neurological insult. Although left-handedness may be considered as abnormal in the statistical sense (suggesting that it is rare and deviates from the norm or more common dextral pattern) it can only be considered pathological if we can make a case that there is a valid relationship between certain clinical syndromes and sinistrality. Recent research suggests that left-handedness is more prevalent in certain clinical populations than in the general public. For example, reviews of the literature by Porac and Coren (1981), Harris and Carlson (1988), Coren and Searlernan (1987) and several other authors in this volume, have listed increased proportions of left-handers in groups suffering from brain damage, epilepsy, reading disability, neuroticism, alcoholism, drug abuse, homosexuality, mental retardation, allergies, autoimmune disorders, migraine headaches, chromosomal damage, deficits in spatial and verbal ability, autism, psychosis and insomnia, to give only a partial listing. It would make no sense to suggest that any of these particular clinical problems would, by itself, be a cause of left-handedness. While one might imagine such a relationship having some validity for brain damage, it is difficult to see how left-handedness could be caused by insomnia or migraine headaches. Rather, let us suggest that both sinistrality and these various disorders are sop signs of some underlying pathological factor, or some otherwise hidden lesion or neurological insult. Let us consider how such an insult might come into existence.

Prenatal and Perinatal Stressors and Sinistrality A spate of research during the past two decades has suggested that lefthandedness may be a marker for the presence of some form of neuropathological insult as a result of prenatal or birth-related complications. Coren and Searleman’s (this volume) Rare Trait Marker Model suggests that the reason for this relationship is based upon the distribution of traits in the population, and hence, although a valid association, is simply due to the operation of statistical mechanisms. Harris and Carlson (1988) suggest that lefthandedness is associated with pathology because the control system for handedness is so diffuse and complex that any of number of different types of lesion might produce effects on manual dominance. Both, however, agree that left-handedness is over-represented in certain clinical groups. There have been a number of recent studies that have observed that lefthandedness is more prevalent in samples of individuals who have experienced

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stressful births. A number of specific birth stressors have been identified as being associated with elevated proportions of left-handedness. These stressors include premature birth, prolonged labour, low birth weight, RH incompatibility, breech delivery, multiple birth and several others (Ashton, 1982; Badian, 1983; Bakan, Dibb & Reed, 1973; Bakan, 1977; Coren & Porac, 1980a; Coren, Searleman & Porac, 1982; Leviton & Kilty, 1976; Smart, Jeffery & Richards, 1980). This relationship is not limited to handedness, but seems to generalize over most of the lateralized behaviours, including foot, eye, and ear preferences as well (Coren & Porac, 1980a; Coren, Searleman & Porac, 1982; Porac & Coren, 1981). For example, in a recent study of children with extremely low birth weights, 54% were left-handed, while among those with greater birth weight, only 8% were left-handed (O’Callaghan et al., 1987). Similarly, Ross, Lipper, and Auld (1987) found that at age four years, 80% of their normal term children showed right-handedness while only 63% of their sample who had been born prematurely did. Bakan, Dibb and Reed (1973) and van Strien, Bouma and Bakker (1987) show that left-handers in their sample are twice as likely to have been born with a history of birth stress than their right-handed counterparts. As these figures indicate, the size of the effect is often quite marked. There is now a fairly large literature on the issue of handedness and birth stress. In an attempt to summarize the current knowledge in this area, Searleman,. Porac and Coren (1988) conducted a review and meta-analysis. Their literature search emphasized studies that had been published since 1971. They focused on ten specific indexes of birth stress, plus a composite index (subjects being classified as having had a stressful birth if any of several different birth stressors were present at birth). Separate analyses were performed for males only, females only, and collapsed across sex. It was found that 30 of the 33 possible comparisons were in the direction that suggested that a history of birth stress is associated with increases in the incidence of left-handedness. Up to now we have viewed handedness as a discrete and dichotomous variable in which an individual was either left- or right-handed. There is, in contradistinction to this view point, a notion that handedness may be viewed as a continuum, containing not only a direction, but also a strength or consistency factor (cf. Coren & Porac, 1980b; Porac & Coren, 1981). The advantage of this point of view is that it allows one to look for more subtle shifts in handedness pattern as a function of stressors. Thus, one can suggest right-sidedness is the normal pattern of lateral preference and that deviations from dextrality which may result from physiological interventions cause a shift away from normal dextrality as a matter of degree, rather than an abrupt change to left-sidedness.

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This was the conceptualization used by Hicks, Dusek, Larsen, Williams and Pellegrini (1978) and Coren, Searleman and Porac (1982) who proposed that birth stress causes only a shift in the continuum from right to left sidedness, not a discrete or abrupt change to full sinistrality. Their data confirmed such a continuous leftward shift as a function of birth stress. Coren et al. (1982) then extended this notion to the other three indexes of sidedness, namely: eye, foot and ear preference. The importance of the relationship between birth stress and left-handedness lies, not in the fact that handedness itself has shifted as a function of prenatal and birth-related complications, but rather in the fact that the shift in handedness may serve as a marker for some form of neurological insult that may in turn reduce survival fitness in the sinistral group. For this reason we should attempt to be as specific as possible and to ask the question “What forms of neuropathy that might be incurred during a stressful birth are also apt to result in a shift in handedness?” One approach to answering the question asked in the last paragraph begins its speculation by using handedness and other measures of lateral preference (foot, eye, and ear) as an index of brain organization. It is well known that motor control is mediated by mechanisms in the contralateral cerebral hemisphere (Springer & Deutsch, 1989). Thus, for the right-hander, motor control of the dominant hand resides in the left-hemisphere. Bakan, Dibb, and Reed (1973) and Bakan (1977) offered the hypothesis that left-sidedness results from physiological trauma as the consequence of a stressful birth. The specific linkage between lateral preference and stress is based upon earlier suggestions that the left cerebral hemisphere is more subject to damage than is the right hemisphere of the brain (Bingley, 1958; Gordon, 1921; Hecaen & De Ajuriaguerra, 1964; Redlich, 1908). Because of the contralateral control of the limbs, such damage to the left hemisphere would be expected to result in hypofunction of the right hand and this, in turn, can cause a naturally right-handed individual to develop a left-handed preference. Bakan suggested that the actual causal factor is oxygen deficiency induced by the perinatal stress. This anoxia then results in pyramidal motor dysfunction of the left hemisphere. If the extent of the neuropathy is not too great, left-handedness, without any other visible accompanying problems, would be the most prevalent and benign sequela of the stressor. Experimental work with animals has borne out Bakan’s contention that anoxia can have dramatic effects on the neonatal brain of mammals. Dickerson, Merat, and Yusuf (1982) found that even short periods

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of anoxia reduce the rate of cell division in the neonatal rat brain. They conclude that: Moreover, anoxia at birth in the human full-term baby occurs at a time when brain growth and cell replication is taking place at a rapid rate and it clearly may have an effect on the process (p. 93). Support for the role of anoxia in the determination of human left-handedness was reported by Fox (1985) who found a significant increase in left-handedness among two-year olds who underwent birth asphyxia and among healthy pre-term infants relative to a control group with no known birth-related complications. In a recent review of the literature, Harris and Carlson (1988) cite numerous studies that report that epilepsy is approximately twice as likely to occur in lefthanders than in right-handers. Epilepsy represents one clinical syndrome in which central nervous system (CNS) damage can reliably be inferred. The association of sinistrality with epilepsy provides additional evidence that at least some left-handedness is related to CNS dysfunction. Seltzer, Burris, and Sherwin (1984) reported a higher prevalence of sinistrality than expected in early onset dementia (before the age of 65). They attribute this finding to the selective vulnerability of the left hemisphere relative to the right. Support for this hypothesized relationship is found in the fact that there are more language disorders in the early onset group than in later onset dementia. Several chapters in this volume also deal with related associations between left-handedness and disrupted neural organization of the cerebral hemispheres. Thus, left-handedness can be viewed as a marker for mild or otherwise difficult to detect instances of neuropathy.

Variability in Cognitive Functioning If left-handedness is (sometimes) the result of damage to the left hemisphere, then we would expect differences between right- and left-handers in cognitive abilities that are known to be lateralized in the right or left hemisphere. There are at least two possibilities: 1.

overall poor performance by left-handers on cognitive tasks that are usually associated with the left hemisphere, and/or

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exceptionally high performance on cognitive tasks that arc believed to be primarily under right hemisphere control.

This is a highly controversial area of research that has been marked with contradictory findings (e.g., Benbow, this volume). On the other hand, several reliable results have begun to emerge that support both of these possible outcomes that we outlined above. To deal with the first question, let us note that there is a large body of research that demonstrates an increased incidence of certain mental retardation disorders among left-handers. For example, Pipe (1987, and this volume) recently reported that sinistrality is more prevalent in retarded individuals with developmental disorders and Down’s Syndrome than among control groups. Pipe also found that individuals in these retarded populations are more likely to have a first-degree relative (parent or sibling) who is left-handed than comparable controls. In Pipe’s (1988) review of the literature, she concluded that atypical patterns of hand, foot, eye, and ear preference are associated with various types of retardation. More direct support for a relationship between left hemisphere cerebral damage and retardation was provided by BradshawMcAnulty, Hicks, and Kinsbourne (1984) who found that right-handedness varied inversely with the severity of mental retardation. Thus, the proportion of left-handers increases with decreasing levels of mental ability. Other measures of left-sided laterality are also associated with mental retardation. For example, Soper, Satz, Orsini, and Van Gorp (1987) noticed a large incidence of mixed-handedness subtypes in a sample of nonautistic severely retarded adults (IQs 11 - 31). They also found that profoundly retarded adults are likely to use either the right or left hand for the same activity (e.g., throw a ball with either hand). Several investigators have searched for a way of differentiating between natural left-handers (is., genetically determined sinistrality) and pathological left-handers (is., sinistrality caused by neural damage to the left hemisphere). One possible variable that may be useful in this regard is the incidence of familial sinistrality. In an attempt to determine if familial sinistrality may serve as such a marker, Searleman, Cunningham, and Goodwin (1988) examined the occurrence of familial sinistrality in mentally retarded young adults and a nonretarded control group. They found that the left-handed retarded subjects, in general, were more likely to have an immediate family member who is lefthanded, with familial sinistrality significantly more likely to be found among the mildly retarded. These results suggest the possibility that left-handedn’ess, as a

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family trait, is more likely associated with milder forms of retardation; whereas, profound retardation is more likely to occur in left-handers without a family history of sinistrality. This possibility supports the notion that the most severe forms of retardation are pathological in origin and reflect damage to the left hemisphere rather than a familial tendency to be left-handed. A more positive note is struck, however, when we turn to the second expected outcome that we noted earlier. There is some strong support for the contention that left-handedness is identified with precocious mental abilities that are under right hemisphere control. It is a generally accepted finding that lefthanders are overrepresented among architects and engineers--two prestigious professions that rely heavily on the use of visual-spatial skills, a cognitive ability that is associated with right hemisphere functioning (Hardyck & Petrinovich, 1977). For instance, although left-handers were reported as being less efficient in a variety of verbal tests, Porac and Coren (1981) found that there were superior to their dextral counterparts in a mental rotation task. (For a review of the literature on the relationship between sinistrality and cognitive abilities such as mathematical and visual-spatial, see Halpern, 1986a, 1986b, 1988, in press). Additional evidence for superior mental abilities among left-handers is provided in a series of studies by Benbow (1986, 1987, 1988). She has studied thousands of adolescents who have been identified as precocious in their mathematical reasoning ability by scoring extremely high on the mathematical portion of the Scholastic Aptitude Test (SAT-M). A pattern of biological correlates are found in this group of elite young people including an increased incidence of left and mixed hand use which supports the notion that lefthandedness can also be an indicator of enhanced right hemisphere functioning. Benbow also found that the mathematically precocious youth were overwhelming male and reported a high incidence of allergies. These findings suggest that prenatal sex hormones and immune disorders may be correlated with lefthandedness, a hypothesis that we will now consider.

Immune System Disorders In what has quickly become one of the most influential theories of laterality ever proposed, Geschwind and his colleagues (e.g., Geschwind, 1983, 1984; Geschwind & Galaburda, 1987) submitted an alternate to the suggestion that left-handedness is the result of neurological insult. Their theoretical position still, however, contains a pathological component in that they maintain that sinistrality

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and immune system disorders both result from a common cause. The proposed cause is hormonal factors found in the intrauterine environment. This theory is based upon the presumption that the prenatal sex hormones (the same ones that both direct and reflect the sexual differentiation of a fetus) also exert powerful influences on the central nervous system of developing organisms. Specifically, high levels of testosterone during fetal development and/or heightened sensitivity to these prenatal sex hormones will disrupt normal neural development, causing a number of physiological changes, and also resulting in an increased likelihood of sinistrality (Geschwind & Behan, 1982; Geschwind & Galaburda, 1987). The biological rationale for this relationship is based on the effect of these hormones on neuronal growth within each hemisphere. Specifically, it is hypothesized that the left hemisphere matures later than the right hemisphere in normal individuals. Because of this differential rate of development, the left hemisphere is at risk for a longer period of time than the right, and therefore is more likely to be affected by an adverse intrauterine environment. This is the same underlying assumption that was made in hypothesizing that the left hemisphere is more likely to be damaged by perinatal anoxia than the right hemisphere because the left hemisphere is more vulnerable to all risk factors. Proponents of this theory assert that high levels of prenatal tes!osterone will be associated with sinistrality because these hormones slow neuronal growth in the left hemisphere, hence weakening its relative control. The result is right hemisphere dominance which is manifested in lefthandedness. There are two sources of prenatal testosterone; maternally produced testosterone which comes from the maternal ovaries, adrenal glands, and other structures such as fat and, for male fetuses, testosterone produced by their own developing testes. Thus, males are exposed to higher levels of prenatal testosterone. As would be predicted by this theory, numerous studies have found a higher proportion of sinistrality in males than in females (Bryden, 1977; Hardyck, Goldman, & Petrinovich, 1975; Porac & Coren, 1981) with females more strongly right-handed and more consistently right sided (Butler, 1984; Porac & Coren, 1981). Lelong, Thelliaz, and Thelliaz (1986), for example, found twice the frequency of left and mixed hand use among boys (28.5%) than among girls (14.2%). It is interesting, in this context, to also note that in the literature review and meta-analysis of Searleman, Porac and Coren (1989), it is reported that the association between left-handedness and birth stress was much stronger in males than in females, which would be consistent with the notion that the

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higher testosterone levels of the male increase the size of the temporal window of vulnerability to neuropathy. In terms of the survival fitness of left- versus right-handers, the presumption of Geschwind and his associates that lateral dominance may also be associated with deficiencies of the immune system becomes quite important. There is evidence, from a number of different sources, that this may be the case. For instance, individual animals, other than humans, do show an analog to handedness. This form of limb or paw preference, however, tends to evenly distributed through the population. Thus, although any one animal will be leftor right-pawed, there is no population bias toward dextrality, as found in humans. In such animal models, however, there is ample evidence that intrauterine sex hormones also affect brain structures and functions, as required by the Geschwind model. In Gorski’s (1985) review of the experimental literature, he concluded that: The existence of functional and structural sex differences in the rat brain is firmly established, as is the dependence of these sex differences on the hormonal environment during development. (p. 590) A recent study using animal subjects demonstrated a direct association between handedness (here, of course, paw preference) and lymphocyte reactivity (Neveu, Barneoud, Vitiello, & Le Moal, 1988). Left-handed mice exhibited higher mitogen-induced T bntphocyte proliferation than right-handed mice. This finding is strong support for the hypothesized relationship between laterality and immune system functioning in mammals. An important aspect of the sex hormone hypothesis is that there are a number of organ systems in the developing fetus that appear to be susceptible to the influence of high intrauterine testosterone levels. One such organ is the thymus gland which is also an essential component of the developing immune system. Geschwind and Galaburda cite several studies that document the finding that testosterone diminishes the size of the thymus gland during development (e.g., Dougherty, 1952; Frey-Wettstein, & Craddock, 1970). The simultaneous effect of testosterone on the development of the left hemisphere and the thymus should result in a greater incidence of immune disorders among left-handed individuals. As predicted, Geschwind and Galaburda reported a strong positive relationship among left-handedness, high levels of prenatal testosterone (both chemically induced and secondary to maternal stress), allergies (asthma, eczema,

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and hay fever), and other immune disorders (particularly those involving the bowel and thyroid). (For a dissenting critique, see Satz and Soper, 1986.) Data in support of the relationship between sinistrality and immune disorders come from a variety of sources. There are well documented sex differences in immune system activity, as predicted by this theory. For example, both animal and human studies show that females (who, of course are less exposed to testosterone) have stronger immune responses than males including more tumour resistance and higher antibody responses (Holden, 1987). Male-female differences in immune system activity could explain a portion of the gender gap in longevity which favours women by an average of four to seven years. In humans, immune disorders can be expressed in a number of ways. One manifestation involves the lymphocytes mounting an attack on the body’s own cells, resulting in autoallergy or autoimmunity and such diseases as ulcerative colitis, ileitis, myasthenia gravis and Hashimoto’s thyroiditis. In a series of studies, Geschwind and his associates were able to demonstrate that individuals suffering from such diseases had an elevated incidence of left-handedness, thus supporting the hypothesized relationship between sinistrality and immune disorders. (See Geschwind and Galaburda, 1987 for a review.) Searleman and Fugagli (1987) confirm this in a recent study of individuals suffering from Crohn’s disease or ulcerative colitis, which are similar inflammatory bowel diseases that are often assumed to be of autoimmune origin. They report while only about 12% of their control sample was left-handed, among the Crohn’s disease and colitis sufferers the percentage of sinistrality was more than doubled, rising to about 27%. Searleman and Fugagli (1987) also found that early onset insulin-dependent diabetes (Type I ) is more prevalent among left-handed males than late-onset noninsulin-dependent (Type 11) diabetes. It is generally believed that Type I diabetes is caused by the autoimmune system, thus linking another serious clinical syndrome with sinistrality. Immune disorders can also manifest themselves when defensive reactions occur to harmless substances. Acting as allergens, these substances elicit the inappropriate formation of specific antibodies such as immunoglobulin E antibodies, that, upon interaction with the mast cells, trigger the release of the histamines. These, in turn, produce the symptoms of allergy, including skin rash, itching, sneezing and breathing difficulties. Given this second form of immune disorder, if the theory is correct, we should predict an association between lefthandedness and allergic diseases. Smith (1987) was able to confirm this, showing a higher incidence of left-handedness in eczema, asthma, rhinitis and urticaria sufferers. Atopic individuals (those who show positive skin reilctions to

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allergens) also demonstrated a higher percentage of left-handedness. Recall that in Benbow's study of mathematically precocious youth, she and her colleagues (Benbow, 1986, 1988; Benbow & Benbow, 1984) report that twice as many mathematically precocious Caucasian students are left-handed than in the general population and that 54% of this highly gifted sample suffers from allergies (symptomatic atopic disease). By contrast, only 10% of the general population of the United States suffer with this affliction. Since certain infections can trigger immune disorders, especially in susceptible individuals, this may result in greater risk for left-handers, such that increasing age and assaults on the immune system could be associated with reduced physiological endurance, lowered resistance to physiological assault, and hence a younger age at death. Breast cancer, which purportedly involves immune system dysfunction, is one of the leading causes of death in women. It is estimated that one in every ten American women will develop breast cancer and that 1/3 of these women will die from this disease. Recent research shows that early onset breast cancer (before the age of 45) is statistically associated with sinistrality in women. Kramer, Albrecht, and Miller (1985) report that "almost twice as high a percentage of left-handers as right-handers developed breast cancer before the age of 45" (p. 335). This finding, along with the others add strength to the hypothesis that left-handedness would be associated with reduced longevity.

Alcoholism and Smoking Over fifteen years ago, Bakan (1973) reported a high incidence of mixed- and left-hand use among male alcoholics. At that time, he hypothesized that brain pathology resulting from birth-related complications may be a precursor of alcoholism. Numerous studies since then have confirmed the association between alcoholism and sinistrality (London, this volume), although the cause of this relationship is still speculative and may also result from the action of fetal testosterone on the immune system. London (1985,1986) and London Kibbee, and Holt (1985), for example, found an increased incidence of left-handedness among alcoholics hospitalized for alcoholic abuse and report that left-handed males were more likely to have alcoholic fathers than right-handed males. In his review of five studies on sinistrality among alcoholics involving a total of 450 individuals, London reported that 17 - 39% of alcoholics are left-handed as compared to approximately 10% in the general population. He (London, 1985)

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also reported that left-handed alcoholic men have less favourable treatment outcomes than right-handed alcoholic men. It has also been reported that more left-handers smoke than right-handers, especially among those who smoke more than ten cigarettes a day (Harburg, Feldstein, & Papsdorf, 1978). Further support for the tendency to smoke and drink among left-handers was provided by Harburg (1981). In a survey of 1,153 white adults, Harburg found that a greater percentage of sinistrals both smoke and drink and abstain less from cigarettes and alcohol than right-handers. This relationship was stronger for men than for women, although it was confirmed in both sexes. It is, thus, reasonable to predict that left-handers are more likely to die from alcohol and cigarette related deaths (e.g., heart disease, lung cancer, liver disease) at a younger age than right-handers.

The Alinormal Syndrome In general, despite evidence of the sort that is presented above, there has been some hesitancy among many researchers to accept the idea that left-handed may be the result of pathological factors. The most important reason for this seems to be the fact that the majority of left-handed individuals in the general population do not show any manifest evidence of neuropathy. Most left-handers perform competently, and many left-sided individuals achieve prominence in their respective fields. For example in the fine arts we find the left-handers: Leonard0 da Vinci, Pablo Picasso and Paul Klee; in music we find Paul McCartney, Jimi Hendrix and Cole Porter; in theatre and cinema we find Charlie Chaplin, Keenen Wynn, Harpo Marx, and Robert Redford; in politics we find Julius Caesar, Alexander the Great, Charlemagne, Queen Victoria and several presidents of the United States, namely, Carter, Ford, Reagan and Bush, and similar lists could be generated for other fields. Most left-handers do not show any manifest physical or cognitive abnormalities which would immediately single them out from their dextral counterparts, other than their sinistral hand preference, hence it is difficult for many researchers to accept the possibility of extant pathology in a large percentage of such cases. Despite this reluctance to accept the presence of possible pathology in sinistrals, to the extent that we believe the data that we summarized above, there seems to be a mounting body of evidence which does suggest some pathological component to left-handedness. To try to set left-handedness into a clearer context, and to show that it is reasonable to still speak of possible pathological factors, even in nonclinical

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groups, Coren and Searleman (1985, 1987) and Coren, Searleman and Porac (1986), introduced the concept of a syndrome, in which left-handedness is only one of several possible markers. Their line of reasoning begins with an acceptance of a continuous conceptualization of lateral preference (where there can be slight shifts away from dextrality, as we discussed in a previous section) and also a continuous conceptualization of pathology (where it can range from quite slight to profound). Within this framework it is possible to suggest that if the pathology is mild enough, the resultant pathological sequelae could be quite subtle. Left-handedness, or more correctly, deviation from the dextral norm, might simply be only one readily visible behavioural marker which is probabilistically related to a syndrome caused by a set of minor abnormalities in neurological development. Thus, left-handed individuals might well be suffering from some covert trauma which does not render them manifestly abnormal, but rather alirtonital (where the Latin suffm ali conveys the meaning of elsewhere or otherwise), suggesting that except for certain deviations from the statistically observed norms, gross inspection does not reveal any readily manifest abnormality. Thus Coren and Searleman (1985,1987) and Coren, Searleman and Porac (1986) suggested that the consequences of a difficult birth or an unusual intrauterine environment may create a fairly predictable syndrome in which sinistrality is one of several possible behavioural outcroppings. The nature of the deviations from the population norms in behaviour or physiological characteristics that seem to be associated with left-handedness, in manifestly normal individuals, seem cover a wide gamut. For example, Coren and Searleman (1987) investigated sleep disturbances (difficulty falling asleep and frequent night wakenings) in a normal sample of 1,274 university students. They found with an increasing degree of left-handedness, students were more likely to report sleep difficulties. Furthermore, individuals with sleep disruption were more apt to also show sinistral tendencies for the other indexes of laterality, namely: footedness, eyedness and earedness. To investigate the notion that sinistrality may be associated with delayed physical maturation, Coren, Searleman, and Porac (1986) assessed the time at which sexual maturity was achieved. For a sample of 713 females, the onset of puberty was noted by the age of menarche and relative body size. For a sample of 467 males, the index of sexual maturity was the onset of secondary sexual characteristics (body hair) as well as relative body size. They report that lefthandedness is associated with delayed maturation in both genders. Coren (1989b), followed up upon these data, which suggested, among other things, that left-handers might have slighter body builds, as a function of their

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delayed maturation pattern. As a convenience sample, Coren scored data from The Baseball Encyclopedia (Reichler, 1979) on all 3,707 pitchers who had ever played in major league baseball up to 1975, in order to determine the relative body size of left- versus right-handers. The data analysis was limited to pitchers because there appear to be some systematic differences in size a function of position played. It also seemed likely that the throwing hand of a pitcher would be a good index of his handedness, and probably would reflect native handedness fairly well. The height, weight and handedness (throwing hand) was coded for 2,745 right-handers and 962 left-handers. The mean height of the right-handers was 72.45 inches, while left-handers were shorter, with a mean of 72.02 inches. This difference of nearly one half inch was statistically significant with t(3705)=5.19, p < 0.001. A similar pattern was found for weight. The mean weight of the right-handed pitchers was 181.85 pounds, while the left-handed pitchers were lighter with a mean of 178.70 pounds. Here, the difference amounts to a bit over three pounds, and is again statistically significant with t(3705)=5.46, p < 0.001. Thus the left-handers, even in this highly selected sample, tend to be somewhat smaller in stature, consistent with the notion of a delayed maturation. One likely inference from these findings is that sleep disturbances and maturational delays are among a host of possible "soft" signs of underlying neurological insult or other pathology associated with sinistrality. It is important to note that the concept of the alinormal syndrome does not necessarily brand individuals with nonright-sided lateral preference patterns as pathological. It does, however, suggest that left-handedness may be one expression of covert neurological differences which could result from some subtle trauma. Sinistrality may be probabilistically linked to a complex of minor deviations from the physical norm for dextrality, and these deviations may result from perinatal stressors, hormonal effects or developmental aberrations. In this context, a deviation from dextrality becomes a marker or symptom of the implicit or covert insult, rather than the primary sequela of pathology. The unseen neuropathy or immune deficiency subsequently is the mechanism by which the survival fitness of the individual is reduced.

Left-Handedness and Mortality Risk Up to now we have focused on the left-handedness as a marker for some form of neuropathy, and perhaps the direct consequence of sohe form of

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pathological condition. Specifically, we have seen that left-handedness is associated with prenatal and perinatal complications which include: premature birth, prolonged labour, low birth weight, RH incompatibility, breech delivery, and multiple birth, and birth-related anoxia. We have also seen that sinistrality is also found to be associated with a variety of measurable abnormal conditions which have included: epilepsy, early onset dementia, allergies, such as asthma, eczema, and hay fever, deviations in cognitive functioning, Crohn’s disease (ulcerative colitis) insulin-dependent diabetes, early-onset breast cancer, alcoholism, smoking, sleep disturbances, smaller body stature and delayed maturational processes. Some of these pathological situations are obviously lifethreatening in and of themselves. Others do not appear to be too serious, however it should be clear that this complex of problems are bound to have some implications for survival fitness for left-handers taken as a group. Even if we adopt the mildest formulations of the association between sinistrality and pathology that we have documented above, we must deal with the possibility of immune system malfunction (which could lead to life-threatening complications in the presence of infectious attack) or covert neuropathy that may be associated with reduced bodily efficiency in unspecified realms. All of this suggests that, for any given age, gender or genetic subgroup, left-handers may be at greater risk of mortality than their right-handed contemporaries. In an actuarial sense, this should, cumulatively, lead to a reduced life span for sinistrals relative to dektrals.

Left-Handedness and Environmental Risk Factors In addition to the host of pathological factors that are related to lefthandedness, and hence may decrease their survival fitness, sinistrals face additional environmental hazards. Earlier in this chapter we discussed the notion of Right-sided World Hypothesis, suggesting that the world has a strong bias toward dextral activities and usage. While we considered this bias as a factor that may affect learned handedness patterns, let us now suggest that this state of affairs may also provide an increased mortality risk for left-handers. You might feel that the notion that there is something inherent in how a left-hander behaves or interacts with the environment that might place the sinistral individual at risk relative to dextral members of the population is, at first glance, preposterous. However, there are problems set to the left-hander by the physical manifestation

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of the cultural and social environment which can provide problems and may elevate risk. First, we may look at some anecdotal evidence that suggests that left-handers do not seem to fare as well in the constructed environment as do right-handers. This probably occurs because most tools, equipment and patterns of activity are predicated on right-handed operation or predispositions. This state of affairs is often reflected in the numerous reports that left-handed individuals are more "clumsy," with suggestions that they may be more accident prone (Porac & Coren, 1981). An unflattering and somewhat extreme portrayal of this comes from Burt (1937) who noted that:

Not infrequently the left-handed child shows widespread difficulties in almost every form of finer muscular coordination . , . they shuffle and shamble, they flounder about like seals out of water. Awkward in the house, and clumsy in their games, they are fumblers and bunglers at whatever they do. (p. 287) It is possible that this clumsiness is simply due to motor incoordination, perhaps itself evidence of the sort of covert neurological insult that we have reviewed above. This conclusion, however, seems unlikely given since numerous great athletes, such as baseball's Babe Ruth, Lou Gehrig, Ted Williams, and Lefty Grove or tennis stars Rod Laver and John McEnroe, or golfs Arnold Palmer and Ben Hogan were all left-handed. Furthermore, few people would doubt the fine motor coordination of artists Leonard da Vinci or Pablo Picasso, or musicians Ringo Star or Paul Williams yet all were left-handed. This suggests that reputation for clumsiness on the part of left-handers is not due to motoric inabilities but rather may be an outcropping of the difficulties that left-handers have living in a right-sided world. An interesting example of how the dextral bias built into the world can wreck havoc upon sinistrals comes from the history of former United States President Gerald Ford. Ford was left-handed and became quite infamous for his clumsiness. He walked into doors which had been opened for him, tripped over equipment and collided with his honour guards so often that his staff eventually resorted to putting an election year embargo on official press releases and photos of such events. Ford's problems seemed to arise from the fact that the environment and the standard protocols were set up for the convenience of righthanders.

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This right-handed bias to the constructed and behavioural environment is more than just a source of annoyance for left-handers, but it also has certain survival implications. Left-handers are placing themselves at some risk when they are forced to use their non-preferred hand when working with equipment designed for right hand use, or when they must use their dominant left hand on equipment shaped for dextral manipulation. Safety levers are apt to be on the wrong side for the left-hander in an emergency. Awkward positions must be adopted to conform to the operation of some machines. It would seem likely, that such a state of affairs would be conducive to an increased number of accidents and mishaps. Collisions with other individuals, accidental spills and even the occasional encounter with implements held by others are likely consequences of the left-handed male’s anti-clockwise predispositions in a world where his right-handed contemporary displays a clockwise bias (e.g., Bracha, Seitz, Otemaa & Glick, 1987). Surprisingly, there has been very little research on this question. In the only study we know of that investigated the relationship between hand preference and accident proneness, Coren (1989a) surveyed 1,896 college students about their preferred hand and the incidence of any accidents associated with tool use, sports activities, traffic or around home and the work place. Only accidents that occurred within the last two years and were sufficientlysevere to require medical attention were scored. Coren found that both male and female left-handers reported more accidents than their right-handed peers. While this relationship was fairly generalized across the five accident categories surveyed, the association was most pronounced for accidents that occurred while driving an automobile. This tendency toward accident susceptibility places the left-hander at increased risk. Certainly, in industrial settings or in traffic, any one accident might be fatal, or lead to complications that are eventually fatal. Alternatively, the cumulative effect of many small injuries, might contribute to an accumulation of damage and disfunction which would also increase the risk of early mortality in left-handers.

Sinistrality and Longevity: Archival Data We have presented a case which is consistent with the prediction that sinistrality will be associated with reduced life span. This prediction is based upon the reduced survival fitness of left-handers due to covert neuropathies, decreased immune response and increased accident susceptibility. Let us now see

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if this prediction can be empirically supported. Unfortunately, the data that exists to date is still quite scanty, however, as shall be seen below, what there is of it certainly supports this hypothesis. Obviously, the primary requirement to conduct a study of the relative mortality of right- and left-handers is a large sample of deceased individuals for whom reliable measures of hand preference are available. However, there are also several methodological considerations that have to be taken into account. To begin with, sex is a variable that interacts with both lateral preferences and longevity. A number of studies have demonstrated that left-handedness is more likely in males than in females (see Halpern, 1986a; Porac & Coren, 1981). There is also between-sex variability in death age due to war deaths and sexrole related deaths such as differential occupational hazards, smoking practices, and alcohol consumption, and also biological biases which show that males have less immune activity (Holden, 1987). For this reason, we (Halpern & Coren, 1988) decided to limit our initial study to male subjects. It also seemed important to begin with a nonclinical sample of individuals who, at the outset of the study, were nominally in good health. Fortunately, we were able to locate a sample that met these requirements. Reliable hand use statistics and date of birth and date of death records are available in archival records for professional baseball players. Thus, the subjects were all of the baseball players listed in The Baseball Encyclopedia (Reichler, 1979) for whom dates of birth and death, and also throwing and batting hand were reported (N = 2,271). Subjects were divided into handedness groups such that strong right-handedness was coded when both throwing and batting hand were right and there was no indication of change in hand use. Similar criteria were used for strong left-handedness, Individuals who changed handedness or who had mixed hand use patterns were not included in this sample. While it might seem that the simplest means of assessing the effects of handedness upon life span is to look at the mean age of death for right versus left-handers, this approach may not be the most effective means of describing the differences between the groups. Mean age at death for strong right-handers was 64.64 years (s.d. = 15.5, N = 1472); for strong left-handers 63.97 years (s.d. = 15.4, N = 236). Although this difference is in direction that we predicted, and shows a difference of slightly over eight months in favour of right-handers, this value is probably not particularly meaningful due to three factors. First, the range of life spans in the sample is very large (with age of death varying from 20 to 109 years); second, the distribution of age of death has a marked deviation from the normal, with a pronounced positive skew; and third, these is a large

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disparity in the two sample sizes (1,472 right-handers versus 236 left-handers). Consequently, a nonparametric test of group differences was selected, namely the Wald-Wolfowitz Runs test (Siegel, 1956) which was conducted on the age of death rounded to the nearest year for the two handedness groups. This analysis showed that the difference between the groups is in fact significant with Z=6.63, p < 0.001, suggesting greater longevity in the right-handers. A much better picture of the relative survival as a function of lateral preference can be obtained by examining the age at death data for the two handedness groups in terms of their actual distributional properties. When we do so we find that, at the far end of the age spectrum, differences in the right and left-handedness groups become most marked. The oldest surviving lefthanded subject was 91 years of age, while the oldest right-hander was 109. More than 2.5% of the right-handers, as compared to less than 0.5% of the lefthanders, survived to the age of 90. The Moses Test of Extreme Reactions (Siegel, 1956) was conducted in order to assess the significance of these effects at the far end of the distribution. It confirmed that the groups differed in survival, with the right-handers more likely to survive into old age, p < 0.001. This result is not simply due to a few extreme outliers in the sample, since the group differences are still significant (p < 0.001) even when the oldest and youngest 5% of each sample are trimmed prior to analysis. These results may be interpreted as indicating that if we look at any sample of very elderly individuals we 'should find a greater predominance of right-handers than among younger subjects, and is consistent with the observations of Porac and Coren (1981) that the number of left-handers that they found in their normative sample was less than 1% for individuals who were 80 years of age or older. One final way to look at the data, which makes the nature of the mortality differences clearest, is to consider the cumulative proportion of individuals surviving from each group, at the various ages. If we set a threshold level of 0.5% as indicating a measurable difference in survival between the groups, and then look at the relative survival of left and right-handers for each year, we find that the groups are virtually identical in mortality until age 33. From that age onward, the percentage of right-handers who survive averages around 2% higher than the corresponding percentage of surviving left-handers at each age. In 52 of the 58 instances in which the difference between the groups exceeds 0.5%, the right-handers have a higher survival rate, which is a highly significant difference (p < 0.001). This relationship is shown graphically in Figure 2, which depicts the relative difference in survival as a function of handedness. Points above the line indicate that right-handers have higher cumulative survival by the plotted

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AGE AT DEATH The relative survival of right and left-handed baseball players plotted as the difference between the cumulative percentage survival of the two groups. Points above the line represent higher survival rates for right-handers and points below the line indicate higher survival rates for left-handers. No left-handers survived beyond age 91 (Halpern & Coren, 1988).

amount for that age, while points below the line indicate that left-handers have higher survival. The overall pattern of results from our sample of deceased baseball players suggests that left-handednessis associated with a somewhat earlier age at death. The difference in risk level between the handedness groups does not manifestitself until individuals are in their mid-thirties, and from that age onward there seems to be a survival advantage to being right-handed. Our controversial conclusion that left-handers are more likely to die at a younger age than their right-handed peers has not gone unchallenged. Wood (1988) repeated our mortality study with a different sample of baseball players. She found a nonsignificant mean difference favouring right-handers of approximately 1.2 months. Unfortunately, she did not choose to use the same

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statistical procedures that we used, and resorted to parametric tests, hence it is difficult to make a meaningful comparison of the two studies. Given the large variability in the data and highly disproportionate sample sizes, we believe that Wood's failure to reject the null hypothesis does not weaken our conclusion. In fact, we believe that her finding of nonsignificance would not have been published if the topic were not as controversial. It is important to note several limitations of our study of the longevity of baseball players. First, the sample is all male, and therefore, it is not possible to generalize the findings to females. More importantly, however, professional baseball players are an atypical sample because they were all exceptionally athletic and presumably healthy as young adults. Third, there is an advantage to batting left-handed in baseball because left-handed batters are several feet closer to first base and are oriented toward first base when they bat. It is interesting to baseball enthusiasts to note that three of baseball's most famous players were mixed- or left-handers with alcohol problems--Babe Ruth, "Shoeless" Joe Jackson, and Ty Cobb providing anecdotal support for the types of relationships we propose.

Left-Handedness and Mortality: Next of Kin Data Because we were concerned by some of the factors that limited generalizability of conclusions based on a sample of professional athletes, we decided to replicate these results on an unselected and more general population. To this end, we are currently conducting a study using a random sample of recently deceased subjects. As part of an ongoing project (Halpern & Coren, in preparation), we sent handedness questionnaires to the next of kin listed on public death certificate records in two counties in southern California. The respondents were asked to complete a postage-paid card concerning the deceased family member. Three hand-use questions were included on the card-hand used by the deceased subject for writing, drawing, and throwing a ball. All cards were coded prior to mailing for age at death, sex, and cause of death listed on the death certificate. An enclosed letter explained that the results may be "useful for scientific and medical purposes." To eliminate any potentials sources of response bias, there was no indication as to the focus of the experiment, or as to our hypotheses and all responses were anonymous. Letters were not sent to the next of kin of any person whose age at death was under six years because handedness cannot be determined reliably before

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this age. In addition, midway into data collection, we stopped requesting data from the next of kin of homicide and suicide victims because such cases raised certain ethical and humane considerations. Approximately 2,100 letters have been sent. Seven hundred sixty responses have been received for a response rate of 36%. Of this number, there were 745 usable responses. Responses were not included for analysis if the code for age at death or sex had been obliterated, or if the respondent indicated that the deceased did not have use of both arms (e.g., missing arm), or if the respondent indicated that she or he didn’t know the deceased’s preferred writing hand. According to the reports of next of kin, 6% (N = 45) used their left hand to write, 6.4% (N = 46)used their left hand to draw, and 7.3% (N = 52) used their left hand to throw a ball. These figures are somewhat low compared to other estimates of the percentage of left-handers that range from 8% to 15% (Porac & Coren, 1981). These somewhat low statistics are not surprising in light of the finding that right-handed individuals have a tendency to simply assume that all other individuals are also right-handed. Thus, Porac and Coren (1979) found that high school aged children tended to underreport the incidence of left-handedness for their parents even though they were currently still living with them. Although the relationship between the respondent and the deceased is unknown, it is likely that many were adult children responding for their deceased parents. It also seems reasonable to expect that, in general, other family members would also tend to underreport the incidence of left-handedness as dextrality is the statistical norm. Two related analyses were computed on age at death as a function of sex and handedness. First, handedness was determined by preferred writing hand. In a second analysis, subjects were designated as right-handers if they wrote, drew, and threw a ball with the right hand. All other subjects (left-handers and mixed-handers) were assigned to a nonright-hand group. Because the results of both analyses are essentially identical (allp’s c.OOOl), the data are presented for the second analysis only. Mean age at death for right and left-handed males and females is shown in Figure 3. As the figure indicates, there is an overall advantage to being female, in terms of longevity, which amounts to 5.94 years (mean age at death for females = 77.55 years, N = 349; mean age at death for males = 71.61, N = 357), which is consistent with the usual actuarial predictions of life span as a function of gender. When we turn to the effect of handedness on life span, however, the results are striking in their magnitude. The mean age at death, for the right-handed

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sample was 75.34 years (N = 645). This is to be compared with a mean age at death of the left-handers of 66.20 years (N = 61). This results in an increased life span for the right-handers by 9.14 years! (Thirty-nine subjects were not included in this analysis because either their drawing hand or writing hand was unknown.) The age at death data were analyzed with an analysis of variance in which hand group and sex were between-subjects variables. A significant main effect was found for handedness, with F (1, 702 = 17.82),p < .OOOl, and for sex, F (1, 702 = 24-91), p <.0o01. The hand group by sex interaction failed to obtain significance [F (1, 702 = 2.33) p >.lo]. To our knowledge, this is the first random sample of deceased individuals in which age at death was studied as a function of hand use. The results are significant for two reasons. First, they are consistent with our predictions based upon the literature review which suggested that, due to implied pathological factors and environmental interactions, left-handers should be at greater risk of mortality. Secondly, these results are important because they confirm our earlier archival study, in which we concluded that left-handedness is associated with reduced longevity.

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Are Left-Handers at Increased Risk of Mortality? Although we now have a theoretical rationale, plus two studies that indicate that left-handers die at a somewhat earlier age than right-handers, this conclusion must still be interpreted cautiously. First, these data should not, of course, be used to predict the life span of any one individual since the results represent summaries that computed by collapsing data points of several thousand individuals into a single composite which do not take into account individual fitness factors. From the theoretical viewpoint, it is important to note that we are not implying that using one's left hand causes an earlier risk of death. It certainly seems likely that it is the correlates of sinistrality and not sinistrality per se that is responsible for the increased risk. These correlates include health-related biological factors that we have discussed above, such as: covert neuropathy, immune system dysfunction, prenatal and perinatal complications, increased incidence of alcoholism and smoking, and so forth. These are the factors that are most apt to contribute to reduced longevity. The behavioural aspect of lefthandedness only comes into play when one considers the interactions between the sinistral individual and the technological environment which may increase accident susceptibility, hence raising an individual's level of risk. It is also possible that there are other, as yet unstudied, behavioural factors that may play a role in survival fitness. One that springs to mind is the potential elevated stress level that may be associated with continued difficulty in adapting to the rightsided technological and behavioural bias to the world at large. Left-handedness, however, should not be taken to be a negative stigma, or, to use an older term, the "mark of the banshee that harbingers death." The concept of left-handedness as a handicap has already been legally tested in court and found to be invalid. This was the finding of a recent court case in which a left-handed letter carrier challenged his dismissal under the United State Rehabilitation Act of 1973 that prohibits discrimination against the handicapped (Torres v. Bolger, 781 F2a 1134 5th Cir., 1986). The court found that lefthandedness is not a handicap or impairment under the law. On the other hand, left-handedness is a behavioural trait that does seem to have actuarial significance. To the long list of factors that predict longevity, such as: gender, weight, fitness, genetic endowment, and race, we may now add handedness. It is, perhaps, with new meaning that we now interpret Fitzgerald's stanza in the Rubayal of Omar Khayyam:

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Dreaming when Dawn's Left Hand was in the Sky I heard a voice within the Tavern cry Awake, my little ones, and fill the Cup Before Life's Liquor in its Cup be dry.

Acknowledgements The "next of kin" data reported in this chapter were supported by a California State University Professional Development Research Grant to the first author. We thank the following students for their assistance with data collection: Layne Sizelove, Cathy Ibbs, Melanie Cass, Nora Gumayagay, LaDonna Cummings, Jennifer Stark, and Diwn Richardson.

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547

Name Index A Abe 441,456 Abel 41, 55, 64 Abern 58, 66 Aboitiz 123 Accardo 39, 64 Achenbach 350, 371 Achenback 82, 96 Adevai 55, 73, 322, 341 Aggleton 204, 258 Agostini 57, 65, 122 Ahrens 123 Ajersch 145, 150 Ajuriaguerra 77, 80, 153, 164,543 Akiyama 123 Albano 479,483 Albert 358, 366 Albrecht 57, 60, 69, 467, 480, 482, 526, 543 Alcock 198, 245 Alexander 416 Alford 490, 500 Allebeck 54, 64 Alley 322, 337, 350, 366 Alterman 462, 484. Althoff 58, 66 Alvarez 44, 64 Alvord 122 Amatruda 43, 67 Ambrosi 122 Ames 110, 118, 124, 421, 431,486,487,497,502 Anders 443,456 Andreasen 156, 163,421, 422, 435 Andreassen 498, 500 Andrew 40, 64,491,500 Annett 46, 51, 52, 59, 63, 64. 76. 77. 79. 81. 82. 92, 93, 112, 122, 133, 145, 150, 154, 164, 172, 185, 186, 188, 190, 222, 223, 226, 239, 240, 242, 244, 245, 260, 261, 286, 294, 301, 305, 310, 314, 324, 329, 336, 343, 350, 355, 357, 361, 364, 368, 396, 403, 407, 417, 422, 426, 427, 435, 490,

500, 511, 540 Apgar 86, 87 Aram 343, 370, 445,455 Archer 223, 245 Ardila 168, 190,222,230, 231, 236, 237, 245 Arensburg 220, 252 Arlitt 214, 216, 245 Ashton 76, 80, 93, 219, 222, 226, 246, 261, 290, 345, 364, 512, 518, 540 Athenes 355 Auld 49, 72, 126, 518, 544 Austin 200, 246 Axel 192 Axelrod 82, 95, 226, 252, 2l7,28a Azemar 356

B Bach 416 Bacon 200, 224, 246 Baden-Powell 270 Badian 81, 93,518, 540 Bairstrom 504 Bakan 17, 30, 33, 34, 46-49, 52, 55-57, 63, 64,79-83, 92, 93, 119, 122, 309, 310, 312, 314, 343, 364, 403, 407, 458, 465, 470, 480, 492, 493, 500, 501, 518, 519, 526, 540 Bakare 220, 230, 231, 246 Baker 58, 70, 125 Bakker 48, 74, 127, 150, 352, 371, 518, 545 Bakwin 133, 150, 216, 246 Baldwin 196, 246. 288 Baldy 438 Ball 504 Ballantyne 41, 64 Ballard 205, 207, 210, 213, 227, 233, 246, 295, 300,314 Balsam0 58, 70, 125 Baltaxe 442, 453

Balzer 81, 97, 492, SO6 Bancard 439 Bancroft 504 Banich 359, 369 Barban 438 Barbara 256 Barber 200, 246 Barker 60, 64 Barneoud 56, 64, 524, 543 Barnes 79, 93, 304,315 Barnsley 148, 150 Barr 417, 435 Barron 154,165,244,253 Barry 54, 65, 224, 246, 294, 315, 466, 470, 481 Barsley 203, 485, 489, 501, 508, 540 Bartak 442, 453 Bartels 343, 366 Bartlik 467, 481 Barton 52, 68, 19, 95, 133, 151, 295, 316 Bartzokis 153 Bascom 165 Bastian 210, 246 Batheja 294, 300, 307, 308, 315 Bathurst 349, 353, 366, 368

Bauman 447,454 Bayless 349, 364 Baymur 80, 96, 306,317 Beach 502 Beale 77, 94, 301, 304-306, 308, 310, 315, 317, 428, 435 Bear 57, 65, 113, 122 Beatty 493, 501 Beaumont 95, 252, 337, 339, 369 Becker 489, 506 Beckman 286 Begleiter 463, 412, 417, 481 Begley 490, 493, 501 Behan 50,56,65,67,116, 117, 122-124, 175, 190, 226, 249, 310312, 316, 358, 359, 366, 448, 454, 463, 473, 481, 490, 503, 523,542

548

Name Index

Beidelman 200, 203, 246 Bell 8, 30 Belmont 493,501 Ben-Chaim 324, 336 Benbow 75, 112, 122, 172, 176, 177, 190, 293, 323, 336, 342, 348,358-361, 365,369, 370, 493, 501, 521, 522,526, 540 Bendel 473, 481 Benjamins 135, 141, 150, 151 Bennett 230, 246 Benson 371, 504 Benton 82, 93, 148, 150, 350. 371 Berenbaum 230, 250, 325, 338, 351, 354, 359, 365, 367 Berg 164, 218, 256 Bergstrom 54,65 Berlucchi 393, 407 Berman 295, 303, 315 Berrebi 106, 122, 126 Berry 104, 127, 322, 336 Best 250, 303, 315, 324, 329,334,338,343,365 Betancur 543 Bethe 207, 246 Beukelaar 218, 239, 242, 246,260, 261,280-282, 286 Bever 357, 365, 368 Beveridge 80, 95, 323, 327, 338, 344,367 Biaxy 58, 65 Bichat 202, 215, 246 Bienenstock 443, 456 Biervliet 205, 246 Bigelow 54, 65, 248 Bigler 54, 59, 71, 295, 317, 438, 445, 455 Bille 54, 65 Bingley 343, 365, 519, 541 Birbaumer 356, 367,500, 503 Birch 44, 65, 154, 166, 423,424,440,493,501 Birecree 493, 506 Birkett 345, 365, 395, 396,407 Bishop 58, 65, 177, 178, 181, 190, 227, 246, 293, 305, 310, 315, 343, 352, 353, 365, 394, 403, 407

Bisiacchi 356 Bisschop 286 Black 336, 338 Blai 489, 501 Blane 466, 470, 481 Blau 4, 30, 211, 247, 295, 302, 304, 313, 315, 420,435,515,516,541 Bledin 153 Blehar 472, 481 Bleier 122 Blizard 287 Bloch 164 Blondel 38, 65 Bochnik 435 Bogdanowicz 228, 256 Boklage 58, 65,423425, 432, 433, 435 Boles 187, 190 Bolger 539,545 Boller 436 Boneparte 416 Bonfa 125 Bonis 439 Boone 82, 95 B o d 373,407, 490, 501 Botticelli 58, 66 Boucher 445,453 Bouma 48, 74, 127, 352, 371, 518, 545 Bourbon 328,337 Bowers 322, 337, 345, 366,389, 408, 491 Brabeck 322, 337, 348, 366

Bracha 532, 541 Brackenridge 53, 65, 76, 78, 93,218, 247, 286, 295, 304. 315 Bradshaw 105, 122, 309, 315, 319, 322, 336, 347, 365, 385, 398, 407, 415, 438, 489, 490,501 Bradshaw-McAnulty 133, 150, 295, 301, 308, 309, 315, 403, 407, 415, 435, 445, 453, 521.541 Brain .80, 93, 219, 226, 231, 247, 305, 315 Bramwell 209, 210,247 Branchey 461, 481 Brander 43, 65 Brann 49,60,65 Braukmann 502 Braun 49, 60,65 Brausam 233, 248

Braverman 442, 454 Bray 135, 150, 332, 336 Brewster 3, 30 Briggs 217,247, 286, 322, 329, 336, 345, 347, 365, 381, 384, 407, 489, 501 Brinton 247 Brito 222, 247, 286 Brizzolara 407 Broca 208, 210, 211, 247 Brockley 52, 67 Brodal 26 Broer 147, 150 Broman 19, 30 Broun 204247 Broverman 399, 409 Brown 72, 241, 247, 353, 368,439 Browne 198, 247 Bruce 54, 65,428,435 Bruder 54, 65 Brunn 58, 73 Brush 42, 72 B r u p 97 Bryden 24, 30, 82, 93, 105, 122, 150, 178, 187, 190, 192, 217, 222, 239, 245, 247, 256, 257, 260, 261, 286, 287, 320, 341, 350, 351, 368, 372, 373, 381, 383-385, 387-389, 392, 407, 409, 411, 412, 493, 501, 523, 541 Bryngelson 216, 227, 247 Bryson 300,441443,445, 446, 451456 Buchsbaum 58, 66, 427, 435, 463, 481 Buller 76, 259 Bulmer 246 Bulwer 198, 200-202, 247 Buonpane 470,483 Burd 445, 453 Bumett 323, 336, 345, 348, 350, 365, 398, 400, 407 Bums 126, 438, 543 Burres 47, 73, 439, 544 Bums 520 Burt 4, 30,43, 66, 211, 226, 247, 293, 295, 302, 315, 531, 541 Burton 359, 369 Bush 527 Butler 467, 481,523, 541

Name Index Buydens-Branchey 461, 472, 481 Byne 105, 122 Byme 356, 365,500,501

C Cacciari 58, 70, 125 Caesar 527 Calnan 222, 247 Cameron 39,66 Campbell 8, 30,122, 213, 223,245,289,493,506 Campos 223, 256 Caplan 71 Cappa 110, 122 Caracci 435 Caraka 37 Carey 493,501 Carlson 5, 12, 26, 31, 170, 177, 178, 181, 191, 196, 223, 227, 237, 247, 250, 321, 338, 343, 367, 516, 517, 520, 543 Carlyle 263, 287 Carpenter 324, 338 Carper 373, 407 Carrivick 80, 96, 505 Carter 344,346,365,368, 527 Carter-Saltzman 76, 77, 93 Carterette 316 Casey 322, 337, 348,366 cass 540 Caterette 366 Cattell 229, 247, 323, 337 Chall 152 Chalmers 44, 71 Chamberlain 81, 82, 85, 93, 301, 315 Chang 219, 257, 262, 290 Chaplin 416, 527 Chapman 47, 54,66,154, 164,260,287 Chard 60, 68 Charlemagne 527 Charlemaine 416 Chase 165 Chaugule 54, 66, 154, 164, 422, 435 Chayette 244, 247 Cheever 474 Chelhod 200, 247 Chen 440 Cheshire 364 Chess 443, 453

Chi 102, 122 Chiarello 357, 365 Child 224, 246 Chitkara 433, 435, 437 Christiansen 336, 339 Churchill 118, 122 Chyatte 55, 58, 66, 73, 458, 459, 470, 481, 483, 500, 507 Ciaranello 443, 456 Cicero 464, 481 Citterio 287 Ciurnaj 500,504 Claney 493, 504 Clark 24, 30, 216, 222, 224, 227, 229, 238, 247, 248,287,441,453 Clarke 127, 303, 318 Clarren 111, 122 Clausen 302, 303, 305, 315 Clayton 468,484 Cleary 247 Clement 197, 248 Cloninger 461, 481 Cobb 474,536 Coffman 164 Cohen 54, 59, 71, 115, 122, 204, 254, 347, 372, 398, 400, 412, 438,494,501 Colby 54, 66, 423, 424, 435 Cole 216, 248, 288 Coleman 443,453 Colendge 33 Collins 30, 77, 78, 93, 94, 263, 287, 304, 315, 494, 501 Colter 46, 54, 73 Conner 344,369 Connolly 486, 503 Cook 126,438,544 Coontz 492, 495 Cooper 157, 166, 222, 229, 233, 248 Corballis 10, 30, 31, 75, 77, 94, 262, 263, 287, 301,304-306,308-310, 315, 328, 337, 356, 366, 403, 407, 428, 435,489,501,510,541 Coren 6, 15, 18, 20, 21, 24, 26, 30, 32, 45, 48, 53, 56, 58, 61, 66, 68, 71, 73, 75-82, 84, 85, 94, 96, 97, 105, 126, 145, 152, 170, 175,

549

191, 192, 1 9 5 , 222-224, 232, 240, 244, 248, 255, 262, 263, 266, 280, 281, 287, 289, 293, 294, 304, 310, 317, 318, 342, 370, 395, 403, 411, 427, 429, 438, 464, 467, 471, 473, 474, 476, 481, 482, 492, 493, 501, 505, 509-514. 516-519.522. 523, . 5 2 8 , 5 2 9 ; 531-537. 541. 542. 544 Corner 252, 336, '338, 365

Correa 230, 245 Corsiglia 123, 223, 249, 359, 366 Corter 164, 196, 250, 258,543 Coryell 303, 304,315 Cosi 287 Cosmi 66 Cotman 123 Cotton 470, 481 Coursey 58, 66, 463, 481 C o u r t - B r c ~ 19, ~ ~ 31 Coventry 335, 341, 347, 371, 398, 411, 489, 506 Cowan 126 Cox 442,453 Cozad 225, 240, 253, 448, 455 Craddock 524,542 Cranberg 358, 366 Crane 474 Crayton 54, 66 Creel 473, 481 Crichton-Browne 259, 287 Crone 244, 253 Crovitz 82, 94, 150, 239, 248 Crowley 110, 124 Crown 507 Cuff 56, 66 Cummings 540 Cummins 57, 66, 223, 256 Cunningham 294, 318, 403, 411, 439, 489, 506,521,544 Czubakski 57, 66

550

Name Index

D D’Amico 324, 335, 338 D’Aquila 324, 340 Da Vinci 356, 416, 527, 531 Dagenbach 389, 390, 408 Dalby 124 Dalen 43, 66, 466-468, 481 Dalton 78, 94, 154, 164, 166, 218, 248, 287, 436,439,509,542 Damasio 156, 163, 435, 498,500 Damon 289 Dardis 481 Dareste 117 Dart 509,541 Davidson 322, 426, 435, 490,491,502,506 Davies 102,486,492,507 Dawes 51, 66 Dawson 219-222, 246, 248, 262, 282, 287, 447,449, 451454 DeAgostini 350, 367,375, 376, 392, 407, 408, 498, 503 Deiijuriaguerra 95, 519 Dean 52, 67, 220, 223, 248, 278, 287 DeBruin 252, 336, 338, 365 Decina 54, 72,426,438 DeFries 494,506 Degreef 435 Delacato 150, 294, 302, 303, 316 DeLacoste 105 Delaunay 205, 248 Deleni 255 Delilah 36 DeLong 447,454 Demisch 427, 435 Denckla 441, 454 Denenberg 108,122, 123, 126 Denkla 127, 343, 366 Denmark 494, 502 Dennert 156, 163, 435, 498,500 Dennis 75, 94, 222, 224, 248 Denno 288, 429, 438, 486488,492, 502,505 Deutch 342, 356 Deutsch 75, 97, 328, 337,

366,371,510,519,545 DeVore 494. 502 DeVries 127, %2, 336, 338, 365,540, 541 Dew 483 Deyo 344, 369 Diamond 109, 111, 123, 126, 287, 411, 493, 495, 502 Dibb 33, 34, 47, 64, 79, 81, 82, 93, 309, 310, 314, 343, 364, 492, 501, 518, 519, 540 Dickerson 519,542 Diehl 349, 369, 384,390, 399, 410, 493, 504 Dietrich 57, 72 Dilling-Ostrowska 228, 256 Dimond 95,252,339,369 Doering 466,483 Dolan 294, 295, 318 Doll 300,316 Domhoff 217, 248 Donchin 322, 338 Dooling 122 DBmer 113, 123 Donvart 127 Doty 94, 501 Dougher 490, 491,505 Dougherty 524,542 Douglas 222, 229, 233, 248 Douvan 494,502 Dowling 495, 502 Downey 261, 287 Drager 473, 481 Dratt 323, 336, 345, 365, 398, 407 Drejer 462, 463, 481 Driver 417, 418, 439 Druzin 49, 73, 125 Duane 490,502 Duffy 343, 366 Dugdale 126, 438,543 Dumcan 514 Dumic 58, 70, 125 Duncan 20, 30, 78, 82, 94, 96, 240, 255, 266, 287, 293, 295, 317, 493, 505, 509, 513, 541, 544 Dunn 432,435 Dusek 81, 94, 416, 437, 519, 543 Dvirskii 154,164 Dvirsky 419, 422, 435 Dvorak 115, 123

E Eastwood 443, 453, 471, 482 Edwards 462,484 Eggett 258 Eglon 36 Ehrman 263, 289 Ehud 36 Eisenberg 328, 337 Eklund 55, 68 Ekstrom 325,337 Eling 148, 151 Elkon 125 Ellis 287, 485488, 491, 492, 495, 497, 502, 505, 509, 542 Elston 286 Elyot 198, 248 Erne 322, 337, 353, 366 Endo 224, 235, 256, 290 Engle 133, 344,370 Enstrom 24, 31 Eskes 442, 454 Etaugh 233, 248 Evans 79,95 Everitt 107, 123

F Fagen-Dubin 322, 337, 344,366 Fairweather 326,337,507 Falek 226, 232, 233, 248, 267, 287 Falzi 498, 502 Fam’ngton 486, 488, 507 Faryna 55, 72 Faulkner 474 Fedora 56, 66 Feierabend yand Kuhn 167 Fein 156, 164, 366, 44247,449,452,454 Feindel 253 Feinstein 56, 60,69, 260, 288, 493, 504, 509, 543 Feit 123 Feldstein 55, 56, 68,459, 474,482,527, 542 Fennel 345, 350 Fennel1 82, 96, 322, 337, 366, 371, 389, 408 Ferguson 56, 67 Ferris 8, 30 Fetter 60,68 Feuerstein 171. 190

Name Index Feyerabend 49, 70 Fields 481 Filskov 504 Fincher 263, 287, 508, 542 Fink 244, 251 Finley 447, 452, 453 Finn 58, 67 Fischer 328, 337 Fisher 445, 453 Fitch 122 Fitzgerald 232, 257, 474, 539 Fitzhugh 486, 502 Flechsig 241, 248 Fleminger 78, 94, 154, 164, 166, 218,239-241, 248, 287, 419, 420, 422, 427, 436. 439, 509, 542 Fletcher 133, 151, 152, 258, 343, 345, 371 Flick 321, 337 Fliess 31, 42, 67 Flor-Henry 66, 72, 163, 164, 415, 427, 428, 431, 434437, 439, 440 Fohl 57 Folstein 443, 445, 450, 454 Ford 527, 531 Forth 428, 437,,486, 487, 503 Fox 126, 260, 289, 491, 502, 520, 542 Franklin 197, 201, 248 Freedman 366, 494, 502 Freedom 344 Freeman 57,65. 197, 249, 432, 438 Freides 138, 151 Freidman 366 Freizdrich 122 French 325, 337 Freud 39, 41, 42, 44,67 Frey-Wettstein 524, 542 Fride 51, 67 Friedrich 421, 438 Frith 443, 454 Fritsch 4, 42, 67, 489, 502 Froide 287 Fromm-Auch 486, 492, 507 Fugagli 57, 73, 289, 312, 318,352,371,525,544 Fukuoka 207, 219, 223, 239, 252 Fuller 447, 454

G Gabrielli 40, 56, 67, 429, 436, 462, 482, 486, 491, 502 Gaddes 127 Gaier 54, 67, 155, 163, 165 Gainotti 436 Gajkiewicz 209, 249 Galaburda 50, 56-58, 65, 67, 72,75, 77, 79-82, 92, 94, 98, 101-103, 105, 107,109,113-115, 117,120,122-127,153, 164, 168, 172, 186, 187, 190, 223, 226, 249,302,310-314,316, 343, 352, 359, 366, 448451, 454, 458, 462465,467,471473, 475478,482,483,498, 500,502,503,522-525, 542 Gall 123 Galpert 447, 453 Ganzell 54, 67, 153 Garai 494, 503 Garbo 416 Garfinkel 20, 32 Garman 258 Garruto 260, 289 Gawin 472, 483 Gazzaniga 126 Geffen 383, 384 Gehrig 531 Geller 8, 30 Georgi 435 Gerstman 349,367 Geschwind 46,50,52,56, 58, 63, 65, 67, 75, 77, 79-82, 92, 94, 98, 99, 101, 103, 113, 116, 117,120,122-125,127, 153, 164, 168, 172, 175, 186, 187, 190, 223, 226, 249, 43, 67, 110, 118, 124, 260, 288,289,302,310-314, 316, 343, 352, 358, 359,366,448451,454, 458,462,463,465467, 472, 473, 475478, 481483,490,493,495, 498, 500, 502, 503, 506,522-525, 542 Ghent 151, 152 Ghesquiere 250

551

Gibson 72, 80, %, 323, 337, 345, 366, 505 Giencke 57, 67 Gilbert 322, 337, 345, 366, 392, 394, 408 Gill 349, 369 Gillberg 222, 249 Gilles 122 Gilman 40, 41, 67 Glanville 303, 315 Glass 267, 287 Glick 57, 70, 107, 124, 454, 464, 483, 494, 503, 532, 541 Gloning 235, 236, 249 Godfrey 204, 249 Goldberg 441, 456 Golden 163, 164 Goldfinger 165 Golding 60,64 Goldman 24, 31, 80, 95, 133, 151, 220, 222, 229, 249, 287, 316, 322, 337, 344, 367, 489, 493, 503, 523, 543 Goldman-Rakic 102, 124 Goldstein 94, 436, 501 Goodglass 320, 337, 373, 407, 408 Goodman 54,59,67,385, 387, 392, 394, 411 Goodwin 119, 125, 294, 318, 403, 411, 439, 462, 463, 476, 482, 483, 489, 506, 521, 544 Gordon 5, 31, 79, 94, 124, 133, 151, 206, 221, 249, 256, 290, 293-295, 300, 302, 305, 306, 316, 343, 366, 475, 478, 482, 519, 542 Gorski 106, 124, 493, 494, 504, 524, 542 Gottesman 424, 425, 436 Gottfried 353, 366, 368 Gould 214, 249, 441, 456 Gowers 39, 67 Goy 493, 503 Graber 164 Grace 56 Grafman 436 Granet 203, 249 Grapin 117, 124 Gray 52, 67 Green 54, 67, 68, 126,

552

Name Index

153, 155, 156, 164, 16.5, 318,426,436 Greenberg 57, 65 Greenfeld 445, 456 Greenhalgh 243, 254 Gregory 322, 337, 350, 366

Grellong 57, 72 Griffin 447, 455 Gripton 495, 507 Groenendaal 60, 68 Grossman 53, 69, 458, 482 Grove 531 Grubben 127 Gruber 409 Grudzinskas 60, 68 Gruzelier 54, 66, 68, 72, 435437,439 Gualtieri 24, 31 Guariglia 122 Guiard 355 Guidetti 57, 68 Guilford-Zimmerman 323 Guillemin 65, 122 Gumayagay 540 Gunn 439 Gur 47, 54, 68, 151, 154, 163, 164, 327, 334, 335,339,419,422,436 Guroff 438 Gussow 44, 65 Gutezeit 179, 191 Guthrie 464, 483 Guze 468,484

H Habib 75, 98, 101, 124, 464,471, 476 Haefner 227, 249 Haith 126 Hallett 436 Halpern 53, 68, 75, 76, 78, 94, 324, 337, 464, 467, 471, 473, 474, 476, 482, 508, 522, 533, 535, 536, 542 Hamberger 55, 68 Hamilton 75, 94 Hamm 127, 303, 318 Hammond 54,68 Hampson 230, 250, 325, 335, 338, 351, 354, 359, 367, 399, 408 Hampton 354, 367 Handy 42, 72

Hanley 324, 329,334, 338 Hannay 349, 367, 384, 408, 409, 411 Hansen 107, 123 Hara 56, 74 Harburg 55, 56, 68, 244, 249, 252, 459, 474, 482, 527, 542 Hardwyck 76, 344 Hardyck 24, 31, 77, 80, 94, 95, 133, 151, 220, 222, 229, 249, 287, 293, 295, 304, 316, 322, 323, 337, 345, 349,367,373377,388, 397, 408, 444, 455, 477, 482, 485, 489, 490,503,522,523,543 Hare 428, 429, 436, 437, 486,487, 491, 503 Harkins 226, 231, 233, 249,250,494,505,515 Harlap 467, 482 Harley 322, 323, 338, 344,346,368 Harnad 94, 501 Hamadek 286 Harrington 42,54,68, 73 Hams 5, 12, 26, 31, 54, 68, 75, 76, 80,85, 95, 162, 164, 170, 174, 176-178,180,181,186, 191,192,195-197,199, 203, 210, 222, 223, 225, 227, 230, 232, 233, 237, 247, 250, 257, 262, 263, 270, 288, 319, 321, 324, 326, 329, 334, 335, 337-339,343,355,367, 369, 510, 512, 516, 517, 520, 543 Harrison 54,65, 80, 96, 505 Harshman 223, 230, 250, 252, 325, 326, 328, 334, 335, 338, 351, 354, 359, 367 Hartamn 367 Hartley 301, 316 Harvey 288 Hanvood 457, 482 Haslam 114, 124 Hassler 356, 367, 500, 503 Hatta 219, 250, 288 Haub 235, 249 Healey 155, 164, 349,367

Healy 260, 273, 288, 445, 446,448, 451, 455 Hebben 141, 151 Hkcaen 77, 80, 95, 153, 164, 349, 350, 367, 374-377, 379, 380, 385, 388, 391, 392, 402, 407, 408, 416, 427, 437, 498, 503, 519, 543 Hegedus 463,484 Heieck 479, 483 Heilman 490, 491, 503, 507 Heim 95 Heimbuch 226, 255 Helenius 69 Heller 359, 369 Hellige 349, 355, 368 Helm 80 Hemingway 474 Hendrix 527 Heninger 431, 440 Henry 6, 32, 46, 72, 80, 96, 133, 152, 153, 155, 165, 237, 256, 321, 340, 343, 371, 439, 444, 445, 456, 516, 544 Hermann 323, 324, 335, 338, 341, 398, 411 Hermelin 443, 455 Hem 286 Herrmann 347, 371, 489, 506 Herron 73, 94, 95, 192, 250, 288, 339, 366, 370, 372, 505 Herschkowitz 59, 68 Hertz 197, 203, 205, 250, 489,503, 508, 543 Hess 493, 506 Hetzler 447, 455 Hicks 24, 31, 52, 55, 68, 75, 77-81, 94, 95, 133, 145, 150, 151, 216, 244, 251, 295, 315, 316, 323, 327, 338, 344, 349, 367, 403, 407, 416, 426, 435, 437, 445, 453, 489, 493, 503, 504, 519, 521, 541, 543 Hier 110, 114, 124, 126 Higenbottom 381, 385, 408 Hildreth 75, 76, 78, 95, 216, 222, 227, 251,

Name Index 293, 295, 302, 304, 316, 343, 367 Hill 70, 493, 504 Hillman 135, 150, 151 Hindelang 494, 504 Hines 110, 124, 349, 367, 381, 384, 389, 408, 493, 494, 504 Hiorns 80, 96, 505 tiiramatsu 289 Hirota 439 Hirskowitz 490,504 Hiscock 131, 135, 150, 151, 241, 381, 408 Hitler 40 Hobson 442, 455 Hochberg 58, 68, 101, 124 Hoff 349-351, 369, 370, 384, 399, 406, 409, 410, 493, 504 Hoffman 303, 315, 423, 437, 488, 505 Hofmann 107, 123 Hogan 531 Holbien 416 Holden 525, 533, 543 Hollis 195, 251 Holloway 105, 125 Holt 55, 70, 466, 482, 526, 543 Holzinger 432, 438 Hommes 431, 437 Homzie 226, 227, 251 Hoogland 127 Hopkins 267, 287 Hornstein 301. 316 Honvitz 445, 455 Houang 324,336 Houston 122 Howell 336, 338 tlua 220, 248, 278, 287 Hubbard 79, 82, 83, 95 Hudson 77, 95, 240, 245 Hughes 322, 336 Humes 164,444,445,454 Humphrey 195, 223, 251, 494 Humphreys 114, 115, 117, 124 Hunter 436 Hutton 127 Huusko 486, 507 Hyde 200, 251 Hyrtl 205, 251

553

I

K

Iacono 47, 54, 69, 156, 163, 164, 419, 437 Ibbs 540 Igna 122 lizuka 253 Inch 150 Ingham 493, 502 Inglis 288, 325, 326, 338, 345, 367 lngraham 125 lngram 191 Ingvar 490,504 lnhelder 329, 340 lnnocenti 104, 124 Irwin 58, 68, 172, 244, 251, 459, 463, 482 hral 322, 337, 353, 366

Kaczenska 228, 256 Kaes 241, 251 Kagan 494, 504 Kalat 82, 96, 136, 137, 152, 240, 255 Kameyama 289,420,422, 437 Kandel 125 Kaplan 164,444, 454 Karacan 490, 504 Karp 304,318 Karpinos 53, 69,458,482 Kashihara 322, 323, 338, 345, 368 Kassens 212, 252 Katsanis 47, 54, 69, 156, 163, 164, 419, 437 Katzman 453 Kauffman 8, 31 Kaufman 353, 368 Kaufmann 115, 124, 125 Kayamura 60,73 Kee 342, 349, 353, 368 Keelor 54, 55, 70, 154, 165, 458, 483 Kellar 357 Keller 125, 368 Kellogg 198, 252 Kellor 438 Kelly 106, 125, 433, 506 Kemper 114, 123, 498, 503 Ken-ichi 437 Kennard 31 Kennedy 19, 30 Kerbeshian 445, 453 Kerr 220 Kershner 301, 318 Kertesz 336, 338 Kessler 445, 455 Khamis 55, 72 Kharabi 54, 67 Khayyam 539 Kibbee 55, 70, 466, 482, 526, 543 Kidd 226, 255, 513, 543 Kido 101, 125, 498, 504 l g a r 101, 127 Kilshaw 112, 122, 185, 190, 223, 245, 324, 336, 350, 357, 361, 364,368,500 Kilty 79, 95, 518, 543 Kim 165 Kimberling 126 lmura 180-182, 185,

J

Jacklin 324, 339, 466,483 Jackson 148, 151, 192, 198, 210, 251, 263, 288, 322, 336, 384, 390, 410, 474, 536 Jacobs 19, 31, 205, 251 Jacobson 55, 68 Jaffe 241, 247 James 54,58, 65, 68,204, 251, 294, 315 Jansson 467, 482 Jantz 57, 69 Jasper 252 Jayaram 165 Jaynes 94, 501 Jeffery 518, 544 Jeffrey 218, 256, 322, 492, 506 Jerussi 494, 503 Jobert 197, 205, 251 Johns 124 Johnson 123, 126, 322-324,338,344,346, 357,368,493,495,502 Johnstone 81, 82, 85, 93 Jones 41, 69, 78, 95, 124, 213, 251 Jordan 5, 31, 214, 251 Joseph 495, 504 JurAnkovd 123 Juraska 104, 106, 124 Just 324, 338

554

Name Index

191, 201, 223, 244, 252, 324, 326, 335, 336,338, 399,408 Kingstone 125 Kinsbourne 75, 77, 78, 95, 96, 133, 145, 150, 151, 172, 191, 295, 315, 322, 336, 337, 345, 365, 381, 394, 403, 407, 408, 410, 435, 445, 450, 453, 455, 489, 490, 493, 501,503,504,521,541 Kirk 365 Kirkby 59, 74 Kitterle 409 Klaiber 399, 409 Klee 527 Klein 54, 74 Klintenberg 346,361,368 Kloss 288 Knapp 228, 258 Knobloch 33, 34, 44,45, 69, 71 Knop 462,482,483 Knowlton 445, 453 Knox 82, 95 Kobayashi 399, 409 Kobyliansky 220, 252 Kocel 60,69, 349, 368 Koch 260,288 Koella 439 Koff 490,501 Kohn 490,504 Kolata 60, 69 Koles 428, 436 Komai 207,219, 223.239. 252 Konishi 119 Kopcik 104, 106,124 Korner 494, 504 Kosaka 127 Kourilsky 124 Kovelman 54, 72 Kraemer 443, 456, 466, 483 Kraft 353, 368 Kramer 57, 60, 69, 467, 482, 526, 543 Krauthamer 94, 501 Krieg 286 Krinicki 59, 69 Kristensen 417, 418, 437 Kristiansen 457, 482 Kroonenberg 218, 239, 242, 246, 260, 261, 280-282, 286 Krouse 8, 31

Kruesi 127 Krutsch 390, 391, 409 Krynicki 486, 504 Kubos 165 Kuh 60,64 Kuperman 154,165,438 Kupyers 26, 31 Kuse 329, 334, 341 Kutas 322, 338

L L'Abbat 204, 252 Lacoste 125 Lake 351, 368, 381, 384, 409 Landesman-her 110, 125 Lane 323, 336, 345, 365, 398, 407 Lansdell 237, 252, 320, 324, 339 Lansky 56, 60, 69, 260, 288,323,324,339-341, 356, 370, 493, 500, 504,505,509,543 Lapointe 127 Laponse 489, 504 Lappon 324,336 Larsen 519, 543 Lassek 47, 48, 69 lassen 490, 504 Laszlo 495, 504 Latimer-Sayer 242,257 Laver 531 Lavers 286 Lawlor 56, 67 Lawson 288, 325, 326, 338,345,367 LeMoal 56, 64,524, 543 Leboyer 451, 455 Lecours 241,258 Ledbetter 467, 481 Ledlow 344,368 Lee 126, 219, 257, 262, 290 Lee-Feldstein 244, 252 Leiber 82, 95, 226, 252, 277, 288 Lele 37, 69 Lelong 523, 543 LeMay 58, 68, 101, 124, 125,498,503,504 Lenneberg 241, 252, 300, 302, 303, 316 Lepore 252 Lerman 473,482 Lesinski 19, 31

Lessel 58, 69 Levander 217, 222, 252, 288, 346, 361, 368 Levat 55, 72 Leviton 20, 31, 79, 95, 518, 543 Levitsky 99, 101, 103, 123, 124, 366 Levy 24, 31, 77, 78, 95, 96, 140, 145, 151, 236, 237, 252, 253, 301, 316, 320, 321, 324, 326, 327, 334, 335, 339, 341, 343, 344, 350, 351, 359, 368, 369,511,543 Levy-Nagylaki 95 Lewandowski 57, 67 Lewine 204,254 lxwis 54, 69, 80, 230, 319, 335, 336, 339, 355, 369, 425, 437, 474 Lewy 449, 452, 453 Li 440 Liben 338 Liberman 157 Lichtman 104, 126 Liederman 77, 96, 155, 164, 260, 288, 543 Light 374, 410 Lilienfeld 44, 69, 71 Lindahl 69 Lindblom 174, 191 Lindesay 42, 52, 56, 57, 69, 113, 125, 288 Lindsay 226, 227, 251 Lindsley 117, 126 Lindzey 93, 315 Linn 324, 327-329, 334, 339 Linnarsson 55, 68 Liiinoila 464, 483 Lipper 49, 72, 126, 518, 544 Lipsitt 55, 72 Lishman 154, 164, 244, 253, 383, 409, 421, 427, 437, 439 Little 39, 44, 60, 64,69, 70 Littlejohn 221, 253 Lloyd 38, 70, 203, 253 Lobstein 154, 165 Loche 58, 70, 125 Lockshin 117, 125 Lockyer 445,456 Lombroso 3, 31, 40, 41,

Name Index 70, 75, 96, 428, 437, 486, 504 London 53, 55, 57, 70, 457,458,460462,464, 466-474,476,480,482, 483, 526, 543 Long 209,253 Longoni 219, 222, 256, 260,261,263,280,289 Lonner 246 Lonton 295, 300, 316 Loosen 464,483 Lott 312, 317 Lotter 441, 442, 445, 455 Louis 219, 228, 229, 253 Lucas 295, 300,306, 308, 317,445,447,455,507 Lucci 164, 44345, 454 Luchins 154, 164, 420, 422, 424, 437 Ludlow 322, 337, 348, 366 Luria 373, 375, 409, 491, 504 Lynch 479, 483

M Ma 440 Maccoby 324, 339, 466, 483 Macdonald 435 Macfarlane 44, 71 MacGregor 465, 483 Mackenzie 42, 70 MacNeilage 174, 191 Maehara 222, 240, 242, 253 Maer 490,506 Magilavy 57, 72 Magnus 218, 256 Maher 47, 54, 70, 154, 165,437 Mahon 252 Mahowald 127 Major 198, 199, 232, 253 Makatsuka 219 Makowska 57, 66 Malahy 467, 482 Malgaigne 253 Malone 349, -367, 384, 408 Mankin 358, 369 Manning 49, 59, 64, 70, 73, 172, 190 Manoach 47, 54, 70, 154, 165, 421, 422, 437

Manowitz 263, 289 Manschreck 47, 54, 70, 154, 165, 421, 437 Marceau 416 Marein 65 Margetts 60, 64 Marin 54, 153, 165 Marino 349, 351, 369, 384, 393, 394, 399, 409, 410, 493, 504 Markowitz 442, 454 Marrion 221, 253, 282,

288 Marsh 290,489, 493,507 Marshall 287, 509,542 Martin 250 Marx 416, 449, 489, 495, 504, 527 Mani 407 Mascarello 467, 481 Mascie-Taylor 244, 253 Massakowski 57,66 Master 54, 66, 154, 164, 422, 435 Mateer 336,339,355,369 Matson 317 Matsunaga 20, 31 Mattis 349, 367 Maudsley 422,427 Maxwell 332, 336 Mayeri 192 McAllister 192 McBride 470,483 McCall 332, 339 McCalley-Whitters 55, 70, 154,165,420,438, 458, 483 McCann 444,445, 455 McCarthy 314, 322, 338 McCartney 527 McCormick 113, 125, 442,454 McCreadie 154, 165,244, 253 McCulloch 113, 125 McEnroe 531 McEwen 493, 503 McGee 225, 240, 253, 322, 339, 448, 455 McGhee 347, 369 McGivern 493, 504 McGlone 105, 125, 322, 326, 339,448,455 McGough 55, 73, 322, 341 McGurk 542 McKeever 80, 230, 238, 253,335,339,344-346,

555

348-352, 361, 369, 370, 373, 381, 383-385, 390, 391, 393-397, 399401,406, 409412, 493, 504 McKie 286 McLaren 445, 455 McLean 500, 505 McManus 48, 70, 77, 79, 92, 96, 133, 151, 170, 172, 173, 178, 186, 191, 261, 288, 294, 295, 300, 307, 308, 310, 312, 315, 317, 403, 410, 425, 437, 511, 515,543 McMeekan 154,164,244, 253, 383, 409, 421, 427; 437 McMullin 214. 254 McPherson 491, 503 Meade 324, 338 Mebert 324, 339, 356, 370, 500, 505 Mednick 40, 54, 56, 67, 73,429,436,438,462, 482, 486, 491, 502, 503, 505 Mehler 126 Mellon 288 Meltzer 54, 66 Melzack 92 Menyuk 442, 455 Merat 519, 542 Merrin 153,154,165 Mesibov 454, 455 Metzler 329, 334, 341 Meudell 243, 254 Meyers 82, 93 Michel 110, 118, 125, 226, 250, 304, 315, 324, 339, 356, 370, 494,500, 505, 515 Michelangelo 356, 416 Michelsson 69 Micle 220, 252 Milberg 141, 151 Miller 57, 60, 69, 321, 340, 344, 370, 467, 482, 490, 505, 526, 543 Milner 145, 150, 152, 209, 220, 237, 255, 320, 321, 340, 373, 411,498, 505 Minkowski 258 Minor 365 Mintz 317

556

Name Index

Mirsky 152 Mittler 432, 437 Moberg 165 Mobius 41 Moffitt 438, 503, 505 Mohay 126,438,543 Molfese 31, 191, 192, 251, 338, 367, 543 Monroe 249 Montagu 19, 20, 31 Monteiro 459, 483 Monzon-Montes 350, 367, 375, 376, 408, 498, 503 Moore 79, 95, 223, 254, 490,505 Morel 40, 41 Morgan 10, 30, 31, 301, 305, 306,308-310, 315 Mori 439 Morley 226, 254 Morris 322, 337, 350, 366,502 Morse 494,505 Mortillet 254 Moscovitch 389,411,426, 427,434,437 Mosley 301, 316 Moss 464,483 Moutier 208, 210, 254 Mukherjee 435 Mulick 317 Mullen 282, 288 Murata 49, 73 Murphy 58, 66, 123, 255, 300,301,317,463,481 Murray 347, 370, 400, 410 Mussen 339 Myers 49, 60, 65 Myslobodsky 119, 125

N Nachshon 429, 438, 486, 487,492,500, 505 Naeser 373, 407 Naeye 44,52, 60, 70 Nagae 322, 323, .340 Nagylaki 77, 95, 96, 301, 316, 343,369,511, 543 Nahos 59, 69 Nakane 439 Nakatsuka 250, 288 Nasrallah 54, 55, 65, 70, 154, 165, 420, 422, 438,458, 483 Nass 58,70, 110, 125, 126

Natchshon 288 Naylor 293, 317 Neale 53, 58, 71 Nebbs 286 Nebes 82, 96, 136, 137, 152, 217, 240, 247, 255, 321, 322, 326, 329, 336, 340, 344, 345, 365, 370, 381, 384,407,489, 501 Needham 70, 75, 78, %, 203. 243, 247, 249, 250, 253, 254, 503, 507, 508, 543 Neft 204,254 Negishi 253 Neils 343, 370 Nelson 39,44,48,54,59, 68, 71, 155, 163-165 Neophytides 54, 74 Netley 57, 71, 113, 125 Nettleton 80, 81, 97, 105, 122, 319, 322, 336, 347, 365, 398, 407, 489, 501 Neumann 56, 67 Neuringer 58,67 Neveu 56,64,524,543 New 58, 70, 125 Newcombe 80, %, 133, 250, 288, 322, 338, 340, 345, 349, 370, 315, 379, 384, 388, 410, 489, 505 Newlin 164 Newman 432, 438 Nice 213, 254 Nichols 19, 30, 300, 316 Niwa 289, 437, 439 Noble 493, 504 Nolan 390, 410 Nosten 449, 455 Nottebohm 108, 125 Noumair 461,481 Ntumba 219, 220, 223, 257,290 Nuutila 486, 507

0 O h y l e 75, 342, 348, 349, 351, 355, 359, 365, 370 O’Callaghan 120, 126, 432,438,518,543 O’Connor 443, 455 O’Gorman 152, 439 O’Leary 103, 126

O’Neil 474 O’Reilly 324, 339, 341, 500,506 O’Rielly 289 Oakley 44,71 Obler 221, 256, 290, 366 Oblzut 351, 371 Ockwell 79, 81, 93, 310, 314, 355, 364 Oddy 154, 165 Ogle 208, 209, 211, 234, 254 Oh 56, 67 Ohta 439 Ojeman 355, 369 Ojemann 336, 339 Okrent 204, 254 Oldfield 24, 31, 82, 96, 113, 126, 175, 222, 254, 260, 288, 329, 340, 342, 370, 458, 483, 500,505 Olds 437 Oliver 301, 318 Olsen 156, 163, 421, 435, 498, 500 Olson 117, 126 Oppenheim 104,105,126 Oreland 439 Orme 244, 254, 323, 340, 426, 438 Orsini 6, 32, 46, 72, 80, 96, 133, 134, 148, 150-153, 155, 165, 171, 192, 237, 256, 306, 309, 317, 318, 321, 340, 343, 370, 371, 374, 383, 384, 410, 439, 444, 445, 456, 516, 521, 544, 545 Orton 43, 71, 114, 151 Osherson 449, 455 Osmond 60,64 Ostrosky 190 Ostrovsky 222, 245 Otemaa 532, 541 Otsuki 253 Otto 490, 491, 505 Ottoson 122, 123, 127, 338,367 Ovrut 494,505 Ozgoren 56, 68

P Paganini 4i6 Paget 209, 210, 238, 254

Name Index Pallie 127 Palmer 531 Panhuyssen 431,437 Pank 294, 295, 300-302, 317 Papagno 122 Papinicolaou 328, 337 Pappas 109, 126 Papsdorf 55, 68,459, 474, 482,527, 542 Parker 474 Parkinson 423, 424, 435 Parkison 54, 66 Parlow 394, 410 Parnas 54, 73 Parre 69 Parson 254 Parsons 215, 233, 234 Pasamanick 33,34,4345, 69, 71 Pasquino 287 Patterson 338 Patzke 435 Paumgartten 222, 247, 286 Payne 176, 191, 219, 220, 254,261-263,282,288, 289 Pearlson 156, 163, 165 Pedersen 141, 152, 223, 255 Peiper 207, 254 Pelecanos 289 Pellegrini 55, 68,79, 95, 244, 251, 425, 437, 489, 504, 519, 543 Pellegrino 328, 337 Pennington 116, 126, 442, 445,454 Pepper 248 Perelle 263, 289 Perez 417,418,438,439 Perlman 59, 60, 71 Perlo 124 Peron-Magnan 439 Perpere 117, 124 Perrone 498, 502 Peters 44, 70, 124, 141, 151, 152, 167, 174, 179,181-185, 191,219, 223, 234. 240, 242, 254, 255, 289 Petersen 122, 286, 324, 327-329,334,339,399, 410 Peterson 56, 60, 69, 260, 288, 289, 323, 324, 334,339-341,356,370,

493, 500, 504, 505, 509,543 Petrinovich 24, 31, 76, n,80, 94, 95, 133, 151, 220, 222, 229, 249, 287, 293, 295, 304, 316, 322, 323, 337, 344, 345, 349, 367, 397, 408, 444, 455, 477, 482, 485, 489, 490, 503, 522, 523, 543 Petti 8, 30 Phillips 447, 452, 453 Piaget 329, 340 Piazza 351, 370, 383-385, 392, 410 Picasso 416, 527, 531 Pickenhain 493, SO6 Pickersgill 294, 295, 300-302, 317 Pieper 56, 74 Pipe 45, 52, 57, 59, 71, 293, 294, 300-303, 308, 309, 311-314, 317, 403, 410, 415, 438, 444, 445, 455, 521, 544 Piran 54, 59, 71, 420, 422, 438 Pizzamiglio 122 Plant 38, 41, 71 Plato 37, 38, 71, 260, 289 Plesset 42, 72 Ploen 355 Plornin 494, 506 Plutarch 198 Pogady 421, 438 Polder 82, 93 Polednak 20, 32 Polen 336, 339, 369 Polk 336, 338 Pollin 424, 433, 437, 438 Ponton 174, 175, 178-181, 188, 189, 192 Porac 6, 15, 18, 20, 21, 24, 26, 30, 32, 45, 48, 58, 61, 66, 71, 73, 75-82, 84, 85, 94, 96, 97, 105, 106, 145, 152, 170, 175, 192, 195, 222-224, 235, 240-242, 248, 255, 259, 262, 263, 266, 268, 276, 277, 280, 281, 286, 287, 289, 293, 294, 303, 304, 310, 317, 318, 342, 370, 395,

557

411, 427, 429, 438, 451, 453, 492, 493, 501, 505, 509, 510, 512-514,516-519,522, 523, 528, 531, 533, 534, 537, 541, 544 Porjesz 463, 472, 477, 481 Porter 30,527 Post 438 Poston 351, 371 Poulos 226, 257 Pratt 350, 372, 375, 376, 379, 412, 498, 507 Preuss 37, 71 Price 325, 337 Prior 415, 423, 438 Provins 78, 96, 220 Ptito 252 Purdue 181, 182, 184, 185 Purves 103, 126 Putnam 200, 255, 428, 438

Q

Qi 440 Quadfasel 320, 337, 373, 408 Quatember 235, 249 Quill 442, 455 Quinan 43, 71 Quintilian 199, 255 Quitkin 54, 6.5

Rabinovitch 148, 150 Rankowski 82, 96, 136, 137, 152, 240, 255 Rademaker 124 Raggi 209,255 Rakic 102. 124 Ralphe 122 Ransil 289. 472.. 483.. 490.. 493,566 Rapheal 416 Rapoport 127 Rasmussen 54, 65, 152, 209, 222, 237, 249, 255, 320, 340, 373, 411, 498, 505 Ratcliff 80, 96, 133, 288, 322, 328, 340, 375, 379, 384, 388, 410, 505 Ratcliffe 345, 349, 370

558

Name Index

Rattan 220, 248, 278, 287 Rauscher 54,65 Raveli 416 Raven 325,340 Reade 212, 255 Reagan 521 Records 226, 255 Reddy 58, 66 Redford 527 Redlich 3, 32, 41, 72, 519, 544 Reed 33, 34, 47, 64, 79, 81, 93, 309,310, 314, 343, 364, 492, 501, 518,519, 540 Rees 16, 259 Reich 470, 484 Reichler 529, 533, 544 Reid 82, 140, 145, 350, 369 Reilly 55, 72 Resek 46, 54,73 Reuter 111, 128 Reveley 425, 435, 437, 438 Rhawn 493,506 Rhoades 289 Rice 494, 506 Rich 344, 352, 361, 369, 310,383,399,401,411 Richards 218, 256, 290, 353, 372, 492, 506, 518,544 Richardson 222, 247,540 Riemann 43, 72 Rife 77, 96, 301, 317 Riley 41, 55, 74, 110, 128 Rimland 443, 455 Rimmer 470, 484 Rintala 58, 72 Ripoll 356 Ritter 204,238,255 Rizzolatti 407 Roberts 133, 344,370 Rodenhauser 55, 72 Roeper 56, 68 Roland 204, 255 Rosanoff 42, 72 Rose 51, 72, 191 Rosen 102, 104, 106, 108, 115, 123, 126, 223, 249, 359, 366,367 Rosenberger 114, 124, 126, 300, 316 Rosenblatt 54,74 Rosenblood 221, 253 Rosenfield 226, 255 Rosenstein 295, 317,445,

455 Rosenthal 282, 288, 472, 481 Ross 48, 72, 107, 120, 126, 200, 222, 227, 229, 233, 248, 255, 518,544 Rosselli 190, 222, 245 Roszkowski 261, 289, 493,506 Rothman 192 Roubertoux 449, 455 Rovegno 344,366 Rover 57, 71 Rovet 113, 125 Roy 192 Rubens 101, 105, 127 Rucker 204, 238, 255 Rude 213, 255 Rude1 152 Rumsey 114, 127 Ruth 204, 238, 474, 531, 536 Rutter 442,443,445,450, 453,454,456 Rymar 289

S Sabatino 489, 506 Sackeim 54, 72,426, 427, 438 Sacks 261, 289, 493, 506 Saitoh 289, 437 Sakano 493, 506 Salcedo 57, 72 Salk 55, 51, 72 Salmaso 219, 222, 256, 260,261,263,280,289 Samson 36 Sanders 289, 324-326, 340, 346, 361, 370 Sandini 343, 366 Santiemma 122 Saporta 57, 65, 122 Sartorius 157, 166 Saslow 6, 32, 46, 72, 80, 96, 133, 152, 153, 165, 237, 256, 321, 340, 343, 371, 516, 544 Satel 472, 483 Satz 6, 9, 32, 4648, 54, 63, 67, 68,72, 73, 80, 82, 92, 96, 115, 119, 127, 133, 134, 148, 150-153,155,156,162, 164, 165, 171, 192, 237, 256, 294, 302,

305-310, 312, 314, 317, 318, 321, 322, 337, 340, 343, 345, 349, 350, 366, 367, 371, 374, 381, 384, 389, 403, 408, 410, 411, 416, 423-425, 439, 444, 445, 447, 450, 452, 456, 516, 521, 525, 544,545 Sauget 349, 367 Saugstad 44, 60, 72 Sauguet 17, 95, 374, 375, 408,427,437 Saunders 289, 493, 506 Saver 57, 65 Scarpelli 66 Schachter 57, 72, 289, 464, 472, 473, 477, 483,490,493, 506 Schaefer 207, 256 Schaffer 426, 435 Schalling 217, 222, 252, 288, 346, 361, 368 Scheibel 54, 72 Scheidemann 225, 233, 256 Scheinfeld 494, 503 Scheller 169, 192 Schenk 123 Schiff 57, 65 Schmiedel 123 Schmitt 340, 341 Schmuller 385,387, 392, 394, 411 Schonfeld 494, 503 Schopler 454, 455 Schroeder 54, 70 Schuckit 459,470,483 Schulman 155, 165, 444, 445, 456 Schulsinger 54, 73, 462, 482,483 Schwartz 43, 48, 73, 75, 79,81-8.5, 87, 96, 97, 119, 125, 127, 289, 349, 371, 490, 506 Searle 347 Searleman 18, 24, 26, 32, 48, 56-58, 61, 66, 73, 79-81, &I, 85, 94, 97, 170, 192, 224, 244, 248, 255, 263, 289, 294, 301, 302, 309, 310, 312, 313, 318, 335, 341, 349-352, 371, 383, 394, 398, 403, 411, 415, 439,

Name Index 489, 492, 501, 506, 513,516-519,521,523, 525,528, 541, 544 Seashore 230, 246 Sedgewick 38 Sedvall 55, 68 Segal 58, 73 Segalowitz 6, 31, 164, 191, 192, 196, 223, 245, 250, 251, 258, 320, 338, 341, 367, 373, 409, 411, 543 Seggie 104,127 Seidman 153, 165 Seitz 349, 369, 384, 385, 390, 391, 393, 399, 410, 411, 493, 504, 532, 541 Seltzer 47, 73, 417, 439, 520, 544 Selvin 20, 32 Selzer 201, 205, 206, 208, 210, 215, 223, 256 Semmes 152, 350, 371 Senf 122 Senter 323, 341 Serban 455 Sergent 328, 337 Servos 179, 182, 183, 185, 191 Sexton 79, 83, 84, 97 Shanonfelt 163 Shapiro 58, 73, 107, 124, 454 Shaw 46, 54, 73. 198, 256, 263, 289 Sheehan 322, 323, 341, 345, 354, 371 Shepard 329, 334, 341 Sherman 107, 116, 123, 126, 127, 223, 249, 322, 341, 353, 359, 366, 371 Sherwin 47, 73, 417, 418, 439, 520, 544 Shettel-Neuber 289, 324, 339, 341, 500, 506 Shields 424, 425, 436 Shimizu 224, 235, 256, 290 Shiota 60, 73 Shipley 110, 124 Shoitsuki 56, 74 Shryne 124 Shucard 223, 256 Siege1 443, 456, 534, 544 Sielicka 228, 256 Sik 288

Silbennan 438 Silva 6, 32, 46, 73, 133, 152, 294, 306, 307, 318, 448,456 Silverberg 221, 256, 290 Silverman 54,55,73, 322, 341 Simmonds 60, 64 Simonet 356 Simpson 323, 341 Sindrup 417, 418, 437 Sing 60,69 Sizelove 540 Skerry 126 Skinhoj 490, 504 Smail 199, 256,493,506 Small 8, 30 Smart 218, 256, 338, 492, 506, 518, 544 Smiley 177, 192 Smith 41, 54-56, 66, 68, 73, 116, 122, 126, 127, 133, 152, 155, 164, 197, 212, 244, 247, 256, 312, 322, 323, 341, 345, 352, 354, 371, 389, 441, 443, 446, 451, 453, 456, 458, 459, 470, 481, 483,500,507,525,544 Smuts 213 Snelbecker 261,289, 493, 506 Snow 355, 356, 371 Snowling 443,454 Snyder 227, 256,443,454 Soares 324, 340 socol 49, 73 Somit 504 Sommers 301, 318 Soper 152, 155, 156, 159, 162, 165, 171, 192, 307, 308, 312, 317, 318, 374, 410, 439, 444, 445, 447, 450, 452, 456, 521, 525, 544,545 Southam 124 Southard 42, 73 Speiser 58, 70, 125 Spennemann 195, 222, 256 Spennnian 75, 97 Speny 321, 341 Spiegler 57, 72,76, 92, 97, 176, 192, 217, 256, 493,507 Springer 58, 73, 75, 97,

559

342, 350, 356, 371, 381, 394, 411, 510, 519, 545 St. James-Roberts 133, 152 St. Vincent Millay 474 Stabenau 424, 433, 438 Stack 503, 505 Stafford 399, 412 Standage 55, 74, 78, 94, 154, 164, 218, 248, 287, 436, 486, 507, 509,542 Stanfield 126 Stanley 365 Star 531 Stark 540 Starkey 301, 318 Steenhuis 190, 222, 239, 245,256 Stein 167, 356 Steinbeck 474 Steiner 349, 367 Steingrubber 82, 97 Stellingwerf 78 Stewart 54,s Stiasny 471, 482 Stiehm 56, 67 Stockler 123 Stone 322, 337, 353, 366 Straus 57, 72 Strauss 101, 127, 437 Streissguth 41, 122, 125 Stringer 495, 507 Strom 52, 67 Strumpf 494, 503 Studdert-Kennedy 174, 191 Sturner 55, 72 Suberi 238,253, 384,410 Subirama 79, 97 Subirana 373, 375, 412 Suchenwinh 493, 507 Suchy 365 Sugiyama 441, 456 Sullivan 384,412 Sumi 122 Sumiyoshi 253 Sunseri 345, 371 Supramaniam 507 Sutton 290 suzuki 253 Swaab 107, 127 Swanson 344,368 Swash 31 Sweet 58, 73 Szelozynska 228, 256

560

Name Index

T Tager-Flusberg 442, 456 Takahashi 253, 439 Talairach 439 Tallal 490, 506 Tambs 218,238,242,256 Tan 79, 81, 97, 217, 219, 239, 257, 346,371 Tanner 244, 257 Tannock 301, 318 Tapley 178, 192,222,257 Tarter 462,463,470,476, 477, 483, 484 Taylor 38, 74, 154, 166, 309, 315, 322, 336, 347, 365, 385, 398, 407,415,417420,422, 426, 427, 439, 489, 490, 501 Teague 469,472,482 Teasdale 462, 481, 482 Teng 219, 220, 229, 233, 238, 239, 257, 262, 263,278,290,390,412 Terman 214, 257 Terrell 214, 257 Terzian 70 Teuber 152, 321, 341 Theilgaard 462, 481 Thelliaz 523, 543 Thiessen 93, 315 Thomas 322, 337, 345, 366

Thompson 180, 192, 233, 257,290,489,493,507 Thornby 490,504 Thurstone 322 Ticknor 481 Tinkcom 351, 371 Tison 209, 257 Todor 177, 192 Toklas 167 Toone 417,418,439 Toran-Allerand 107, 127, 495, 507 Torres 539,545 Torrey 54, 74 Tough 19, 31 Touwen 81,97 Towbin 59, 74, 135, 152 Tramo 126 Traub 383, 384 Trehub 164, 196, 250, 258, 543 Trembly 59, 74 Trimble 417, 418, 438, 439

Tsai 54, 74, 156, 166, 253, 445, 456 Tsao 81, 97,492, 506 Tucker 153, 165, 490, 491,507 Tudehope 126,438, 543 Tune 165 Tupper 240, 242, 257 Turk 494, 507 Turkewitz 304,318 Turner 42, 74, 222, 240, 245, 294, 301, 314 Tweedy 350, 371, 394, 411

U Uchino 434,439 Uhrbrock 204,257 Ulleland 41 Umilta 407 Uribe 230, 245 Urion 116, 127 Uylings 252, 336, 338, 365

V Vaclav 153 Vaillant 469, 484 Valdin 203, 257 Valentich 495, 507 Vallar 122 Valverius 55, 68 Van der Kleij 150 Van der Vlugt 96, 306, 317 VanDenAbell 322, 337, 345, 366, 491, 503 VanDeventer 238, 253, 345,369,383-385,395, 396, 400,410 VanDyke 323, 324, 338 VanGorp 155, 165, 318, 521,545 VanHof-van Duin 60, 68 VanSchroeder 154, 165 VanStrien 48, 74, 120, 127,352,371,518,545 Vandenberg 289, 325, 329, 334, 340, 341, 346, 361, 370 VanEys 391, 394, 410, 412 Vargha-Khadem 152, 416, 439 Varney 350, 371 Vaughn 227, 257

Verdon 286 Verhaegen 219, 220, 223, 257,290 Vermess 127 Versiloff 209, 257 Veyn 227 Victoria 527 Vignolo 498, 502 Vincken 97 Virdis 58, 70, 125 Virkkunen 486,507 Vitiello 56, 64,524, 543 Viviani 304, 318 Vles 127 Vlugt 80 Voglmaier 54, 65 Volpe 54, 59, 60, 71, 74 von Hentig 489, 507 Von Knorring 427, 439

W Wada 102, 105, 127, 303, 318 Waddington 113, 125 Waddy 59, 74 Wadsworth 60,64 Wagner 157. 166, 236. 253 Wahl 154.166 Waldenstrom 222, 249 Waldrop 8, 30 Walker 154, 166, 423, 424,440 Wallin 41, 74 Wang 232, 257 Ward 51, 74, 113, 127, 191, 250, 504 Wardell 429, 440 Ware 490,504 Warrenburg 447, 454 Wamngton 350, 372, 375, 376, 379, 412, 498, 507 Waterhouse 164,44245, 454 Watkins 246 Watson 60, 68,211, 255, 257, 490, 507, 512, 545 Watters 152, 439 Watts 80,95 Webster 75, 226,227, 257 Wechsler 329, 341, 500 Weinberger 54, 74, 125, 154, 157,. 164, 166, 437 Weinstein 56, 74, 152

-

Name Index Weinstock 51, 67 Weisz 51, 74, 113, 127 Weller 242, 257 Wellman 80, 97, 353, 372 Wells 223, 258 Wernicke 210, 258 Werry 448, 456 Wesley 286 Wesman 230, 246 West 486, 488, 507 Wexler 431, 440 Wheatley 123 Wheeler 52, 67 Whipple 74, 213, 258 Whitaker 367, 408 White 261, 290, 314, 490, 501 Whitters 54, 70 Whittington 290,353,372 Wieschhoff 489,507 Wiesner 473, 481 Wile 3, 6, 32, 43, 74, 203, 208,258,293-295,304, 318, 508, 545 Williams 286, 474, 519, 531, 543 Williamson 491, 507 Wilson 199, 205, 208, 258, 289, 294, 295, 318, 325, 340, 346, 361, 370,483 Winblad 439 Winer 152 Wing 157, 166, 441, 451, 456 Winokur 470, 484 Winslow 154, 165, 244, 2.53 Winter 60,64

Witelson 57, 59,74, 101, 105, 125, 127, 168, 175, 188, 192, 336, 341, 351, 357, 372, 381,412,475,498,507 Witkin 329, 341 Wittenborn 322, 341 Witteenstein 173 WittYg 122 Wolf 282., 290.. 441., 456 Wolfe 474 Wong 288 Wood 204,258,535,536, 545

Woodruff 3, 32, 41, 74, 468,484 Woods 152 Worden 340,341 Wurtman 472,484 Wyatt 54, 74, 154, 157, 164, 166,437 Wynn 527

Y Yaghmai 122 Yakovlov 241, 258 Yan 421,422,428, 440 Yang 219, 257, 262, 290 Yen 325,341,347,372 Yeni-Komshian 76, 92, 97, 176, 192, 217, 256, 493, 507 Yeo 347, 372, 398, 400, 412, 490, 491, 505 Yeudall 427, 429, 436, 440, 486, 492,507 Yokoyama 56, 74 Yonekura 439

561

Young 164,196,228,250, 258,543 Yusuf 519, 542 Yutzey 123, 126

Z Zachal 457, 482 Zahler 57 Zaidel 371, 504 Zalma 353, 368 Zangwill 148, 151, 373, 412 Zawisza 57,66

Zekulin-Hartley 301, 318 Zener 82,94,239, 248 Zernicke 147,150 Zimmerberg 41, 55, 74, 110, 111, 128, 494, 503 Zoccolotti 122 Zonderman 36.5 Zuluaga 230, 245 Zurif 350, 372, 381, 383-385, 387-389,412 zvi 155, 165, 439, 444, 445,456 Zylberlast 249

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563

Subject Index

A abstracting ability 463 accident 53, 132, 473, 532 accident proneness 43, 52, 56,531, 532, 539 accidental factors 77,515 ACI'H 51 adrenal hyperplasia 58, 70,110,125 adrenalin 51 affective disorders 54, 415, 418, 421, 481 age 19-26, 28, 29, 31, 32, 34, 44, 49, 65, 66, 74-78, 83-91, 93-96, 109, 112,118-122,135, 136, 140, 143, 145, 151, 152, 154, 155, 157, 171, 187, 196, 198, 206, 207, 212, 213, 216, 218, 219, 221,224-227,230,232, 235, 239, 240-243, 2A7-249,254, 255, 258, 262, 264, 265, 267, 273, 282, 286, 287, 303, 315, 317, 318, 320, 353, 355, 368, 379,418420,423,426, 429, 432, 433, 439, 449, 467, 474, 481, 482, 487, 501, 502, 505,508-510,515,518, 520, 526-528, 530, 533-544 aggression 7, 8, 434, 505 agnosia 376 agraphia 235 agricultural societies 221 AIDS 57, 113 air pollution 63 alcoholism 7, 41, 55, 58, 59, 64,68-71, 73, 110, 128, 159, 238, 249, 457479,480484,499, 507, 517, 526, 527, 530,539,540,542,543 alexia 235 alexithymia 55, 72 allergies 7, 56, 57, 64, 66, 73, 75, 111, 112, 114, 117, 127, 318, 359,

371, 464, 467, 471, 476, 517, 522, 524526, 530, 543, 544 alpha activity 463 Alzheimer's disease 47, 52 ambidextrous (see also, ambihanded) 33, 36, 37, 42, 83, 99, 100, 101, 118, 212, 235, 346, 347, 389, 421, 428 ambihanded 221, 261, 270, 271, 274, 276, 278,280-282,284,285 amniogenesis 424 amniotic fluid 107, 117 Amytal test 101 anatomical anomalies 40, 57,59 androgen 108, 110, 122, 124, 369, 409, 495, 497 animal 7, 75, 99, 104, 105, 107-108, 110, 111, 116, 117, 493, 499, 519, 524, 525 anomalous cerebral dominance 114, 116, 450, 461, 462, 465, 474, 475, 477479 anoxia 44,120, 519, 520, 523, 530 anti-Ro antibody 117 anti-social behaviour, antisocial beh. 55, 438, 461 anxiety 55, 68, 228, 244, 251, 422, 426, 437, 504 apgar scores 48, 87, 92 aphasia 152, 209, 237, 246, 247, 239, 251, 253, 254, 258, 317, 337, 350, 373, 377-379,385,407409, 412, 431, 439, 477, 543 aplysia 169 apraxia 192, 376, 377 architects 75, 104, 115, 323, 340, 356, 370, 475, 505, 522 architecture 356, 364,

500 art 195, 200, 247, 252, 255, 257, 324, 356, 500,510 a r t e r i o v e n o u s malformations 54 articulatory disorder 377 artists 289, 340, 356, 370, 475, 505, 531 Asher Test 137, 141 asphyxia 39, 42, 65, 70, 71, 520 asthma 111, 116, 524, 525,530 asymmetric brain damage 131, 133, 134, 142, 149, 150 asymmetry 10, 14, 37,55, 57, 66, 73, 93-95, 99, 101-109, 111,113-115, 117, 118, 122-127, 136-138, 142-151, 160-162, 164, 165, 183, 185, 188, 192, 245, 247, 249, 252, 255, 286, 287, 290, 308, 312, 314, 317, 318, 328, 336, 339, 355, 364, 366, 368-370, 372, 381, 383, 385, 389-394, 403, 408410, 412, 435, 438, 455, 481, 482, 500-504, 507 ataxic 135 athetoid 135 athletes 111, 257. 355. 356, 364, 474, 500, 531, 536 atopic diseases 116 attention deficits 8, 35, 50, 472,475, 476, 478 autism 7, 8, 43, 54, 56, 65, 66, 74, 111, 116, 155, 156, 162, 164-166, 294, 300, 307, 308, 311, 315, 415, 423, 424, 432, 435,437439,441456, 467,481,517 autoimmune system 7,73, 127, 111, 116, 289, 311, 318, 371, 504, 517,525,544

564

Subject Index

ayurvedic medical tradition 37

B baseball 53, 204, 251, 254, 474, 505, 529, 531, 533, 535, 536, 544,545 behaviour problems 59 bias 50, 51, 70, 95, %, 107, 108,111, 120, 125, 162, 171, 174, 175, 179, 184, 195, 209, 212, 218, 220, 232, 241, 260, 262, 303, 374, 445, 494, 512,515,524,530-533, 536,539 Bible 6, 36, 37, 71, 203, 355 Bilateral Object Naming Latency Task 390, 411 bilingual 428 biochemical 39, 46, 102 biological 30, 31, 65-67, 70, 72-74, 77, 78, 93, 94, 98. 119, 121-125. 164, 195, 223, 249; 251. 252. 257. 282. 283; 285; 290; 304; 308, 315, 316, 336, 365,366,380,435438, 440, 442, 443, 452, 454, 458, 461, 462, 466, 467, 476, 478, 480483,488,490,502, 503, 505, 509, 522, 523,533,539,540,542 bipolar 54, 72, 415, 418, 421, 426, 427, 431, 434,438,467 birds 108, 125 birth asphyxia 520 birth order 17, 18, 30, 32, 33, 49, 64,83, 84, 86, 87, 93-95, 97, 119, 120, 122, 192, 314,318,364, 407,458,465,466,469, 473,478,479,481,483, 500, 501, 540, 543 birth patterns 457, 464, 465,467, 468, 471 birth position 17, 118, 125 birth process 5, 17, 39, 71 birth stress 3, 7, 17, 18-

20, 21, 22, 24, 28, 29, 32, 33, 38,43, 60, 61, 63, 64, 69, 70, 73, 74, 80-84, 87, 93, 94, 96, 97, 99, 118-120, 127, 170,192,288,309-314, 317, 318, 343, 364, 371, 404, 405, 429, 433, 443, 450, 467, 501,505,506,517-520, 523, 526, 540, 541, 544,545 birth weight 35, 44, 49, 58, 69, 85, 87, 88, 90, 120, 432, 433, 518, 530,543 blond hair 111, 472, 473, 476, 511 body stature 530 brain damage 6, 8, 32-34, 39, 42, 43, 46, 47, 52, 55, 64, 68, 71, 72, 80, 81, 93, 96, 115, 117, 120,127,131,133-135, 142, 144, 148, 149, 150, 152, 153, 156, 157, 164, 191, 227, 235, 237, 255, 294, 306-308,314,317,321, 328, 340, 343, 371, 378, 404, 411, 415, 416, 420, 428, 429, 432434,445,447,449, 450, 452, 454, 457, 462, 477, 505, 507, 517, 526 brain development 59, 68, 127, 166, 256,342, 343,507 brain organization 81, 176, 191, 250, 252, 336, 338, 339, 341, 349, 367, 411, 421, 430, 432, 433, 519 brain size 54 brain wave 459 breech birth 49,432,518, 530

C callosal agenesis 54 callosal morphology 104, 106 cancer 54,57,60,69,467, 468,474,478480,482, 483,526,527,530,543 candida infections 471

cardiovascular effects 41, 54, 60, 64, 117,434 carotid angiogram 101 carotid barbiturization 431 cats 104 Cattell Culture Fair Intelligence Test 344 celiac disease 56 central nervous system 41, 44, 47, 65, 195, 418, 520, 523 cerebral dominance 65-67, 72, 74, 95, 98, 107, 114, 116, 122-125, 164, 190, 208, 235, 247, 315, 337, 338, 365-367, 408412,435437,447, 450452, 454, 461, 462, 465, 474, 475, 477479,483,502-504, 542 cerebral dysfunction 43, 54, 79, 92, 309, 373, 433, 441, 4 4 5 4 7 , 452, 504 cerebral lateralization 67, 94, 110, 124, 164, 189, 190, 235, 241, 249, 303, 316, 319, 320, 326, 327, 335, 337, 341, 349, 355, 366, 369, 371, 408, 442, 446, 448, 452, 454, 482, 503, 542 cerebral maturation 52 cerebral organization 71, 188, 237, 252, 319, 321, 336-338, 343, 367, 371, 373-375, 380, 392, 396, 401407, 410, 411, 415, 428, 438, 443, 448, 450 cerebral palsy 33, 35, 39, 43, 64, 66, 132, 134, 135, 148, 433 cerebral situs inversus 100 cerebrovascular accidents 132 character disordered 430 child 4, 5, 30, 31, 34, 38, 43, 45, 48, 49, 64-72, 77-79, 81, 85, 86, 94, 97, 110,114, 117-120, 126, 131, 132,

Subject Index 134-138, 140-149, 151, 152, 153, 166, 168, 170,179,188,190-192, 196-199, 205-208, 212-234,239,240,243, 246-252, 254,255-258, 264, 265, 271, 277, 284,288-290,294,295, 300-305, 309,315-318, 322,323,337-342,345, 353, 355, 358, 359, 364-368,370,372,403, 405, 415, 416, 420, 423, 424, 426, 421, 429,430,433,435458, 462, 463, 465, 466, 473, 475, 476, 479, 481483,499-501,504, 506,511-513,516,518, 531,537,540, 541-544 childhood hyperactivity 463,473,475,476,499 childhood schizophrenia 54, 65, 66. 315, 423, 424, 453, 455 chlorpromazine 157 chromosomal factors 5, 7, 10, 13, 19, 20, 57, 467, 517 chronic diseases 38 chronic stress 34 cigarettes 51, 63, 459, 474. 527 circadian rhythms 107 circling direction 107 cleft palate 58, 72, 467 Clockface Reading Latency Task 393 clockwise bias 532 clumsiness 58, 65, 190. 246, 531 cocaine 472, 483 cognition 4, 32, 43, 50, 53, 59, 62, 65, 66, 69, 72, 74, 80, 95, 96, 112, 122, 124, 126, 132, 133, 151, 154, 165, 172, 176, 190, 191, 229. 236. 231, 245, 246, 248-250, 256, 287, 293,315,316,319-321, 323-328, 334-339, 341, 342, 345-348, 350, 353-355, 358, 360-362, 364-372, 407, 409412, 416, 435, 443, 446, 448456,475,482,492, 497, 498, 501, 503,

506, 520-522, 527, 530,541,542,544 cognitive abilities 65,69, 80, 124,133, 176,190, 229, 246, 250, 320, 321, 327, 334-339, 341, 345, 350, 353-355, 358, 360, 362, 364, 365, 367, 368, 369, 371, 372, 407, 409, 411, 443, 506, 520, 522, 542 cognitive crowding hypothesis 321, 325, 328 cognitive deficits 43, 50, 53, 59, 133, 191, 293, 316, 336, 338, 345, 347, 365, 367, 407, 446,448450,501,503 communication 39, 151, 191, 201, 224, 300, 316, 317, 410, 427, 432, 441, 455, 456, 490 compulsive behaviours 442 congenital abnormalities 19, 35, 37, 40, 62, 481 contralateral inhibitory regulation 434 convulsions 39, 375, 495 corpus callosum 27, 54, 57, 65, 74, 105, 122, 125-127, 192, 253, 341, 475, 477, 507 cortical 99, 102-104, 109, 114-117, 123, 126, 127, 213, 237, 241, 244, 307, 321, 327, 339, 343, 351, 369, 373, 447, 449, 450, 452, 453, 495, 502 cortical systems 447 corticospinal m o t o r system 147 country of origin 282, 283 covert left-handers 218 creativity 464, 474, 475, 477. 500 cretinism 41 criminality 3, 4, 7, 4042, 58, 63, 66, 75, 415, 428431, 440, 461, 485489, 491, 492, 495499,502,507 Crohn’s disease 57, 13,

565

289, 318, 371, 525, 530,544 cross-cultural 196, 226, 232, 234, 236, 246, 262, 263, 278, 282, 283, 285,288, 317 crystallized intelligence 230, 323, 327 CT scan 57, 101, 113, 114, 424 cultural 26, 29, 46, 76, 78, 96, 98, 107, 168, 170, 171, 173-176, 188-191, 195, 196, 200, 210, 2 1 2 , 218-220, 222, 223, 226, 228, 229, 231, 232, 234, 236, 239, 240, 243, 245, 246, 250, 254, 255, 262, 263, 270, 278, 282, 283, 285, 288, 289, 304, 317, 323, 344, 346, 402, 421, 494, 509, 510, 516, 531 customs 195, 198, 200, 221, 239, 241, 243 cVBR 158-162 cyclothymic 419

D DAT Space Relations Test 322, 350, 353 daylight 112 de la Tourette syndrome 58 defining handedness 208, 379, 380, 425 deformity 39, 42, 70 degeneracy 4, 4042, 75, 102, 243 degradation 41 delayed reading 445 delayed speech 56, 254 delinquency -t -ts 8, 40, 52, 56, 67, 429, 430, 436, 486488, 497, 500, 502, 507 delivery 5, 20, 39, 44, 49, 59, 85, 86, 121, 246, 518, 530 dementia 52, 73, 417, 439, 520, 530, 544 depravity 41 depression -s 54, 55, 65, 111, 134, 375, 421, 426428, 431, 434,

566

Subject Index

436,438,471,484,505

DES 543 development (see also age) 9, 10, 12, 13, 19, 27-31, 34, 3744, 5052, 54, 56, 58, 59, 62, 63, 65-69, 71, 72, 76, 79, 83-85, 95, 97-100, 102-105, 111-118, 120-128,132,144,146, 148,150-152,155,165, 166, 172, 179, 187, 190, 1%, 198, 205, 213, 221, 223, 225, 226, 229. 233. 236, 238, 240-247, 249-251, 253, 254, 256, 258, 288, 293, 302, 303, 305, 308, 310-313, 315-318, 320, 336-339, 341,342-344,350,351, 358,359, 361,365-368, 370, 372, 409, 410, 415, 418, 424, 430, 432, 433, 435, 439, 441-444, 446450,452, 453456,458,462,476, 483,495,497,499-504, 506, 507, 511, 516, 521, 523, 524, 528, 529, 540-544 developmental delay 58, 172, 241, 302, 303, 432,528, 529 developmental disorder -s, 52, 56, 165, 166, 441, 442, 447, 448, 450,453456,521, 529 developmentally disabled 135, 441, 446, 447, 452, 454 dextral bias 531 diabetes 34, 57, 73, 111, 289, 318, 371, 464, 467,525, 530, 544 dichotic listening 105, 110,134, 241,301-303, 306, 317, 322, 324, 335, 348, 351, 354, 357,381-383,388,407, 411,415, 437,503 Dichotic Object Naming Latency Task 390 diethylstilbestrol (DES) 31, 67, 72, 110, 124, 248, 251, 254 difficult labour 39, 70 dizygotic twins 104,

423425,432,434,511, 512 dopamine 51, 67 double personality 54 Down's syndrome 57, 300-302, 307, 310-313, 316-318,415,467,482, 521 drug abuse 7, 474, 499, 517 dN@ 34, 35,44,58, 60, 63, 68, 107, 191, 244, 251, 459, 463, 482 dyscalculia 235 dysgenesis 111, 115, 126 dysgraphia 376 dyslexia -ics 43, SO, 56, 75, 111, 114-117, 123, 124, 126, 127, 317, 343, 366, 371, 446, 448,454,475478,481, 482, 542, 544 dysphasia -ic 115, 122, 375, 376,453 dysphoric mood states 415, 427 dysplasia 114

E early onset dementia 52, 520, 530 early post-natal 103, 126, 416,434 eating 159, 176, 197, 198, 203, 207, 216, 219, 224, 229, 238, 239, 269,272,275,282,513 echolalia 442 ECT 372,412,426,507 eczema 56, 111, 116,524, 525,530 Edinburgh Handedness Inventory 31, 113, 254, 288, 340, 342, 358, 360, 370, 395, 396, 398, 458, 483 education 30, 31, 38, 63, 152, 154, 157, 190, 197, 198, 207-211, 214-216,220-222,225, 230, 235, 240, 242, 245,248-251,253,255, 256-258,287,288,290, 304, 314, 334, 337, 341, 357, 367, 368, 372, 502, 506, 515 EEG 46, 51, 57, 58, 66,

73, 251, 306, 307, 436, 447, 463, 504 electro-convulsivetherapy 375, 426, 438 embryogenesis 416, 418 embryonic 42, 43, 423, 424 emotionality 7, 8, 27, 40, 55.121.211. 229. 238. 2&4, &O, ,421; 426, 427, 431, 434, 436, 438, 442, 446, 490492,494,497499, 505 enteritis 464 environment 32, 37, 38, 45, 49, 58, 59, 70, 71, 76-78,92,98-100, 104, 106, 109, 118, 121, 122, 124, 145, 195, 196, 200, 216, 228, 232, 242-244,255-257, 261, 282, 283, 285, 289, 304, 305, 312, 326, 338, 365, 438, 458, 461, 464467, 473, 478, 479, 481, 488, 490, 492, 495, 499, 502, 509, 512, 514, 515, 523, 524, 528, 530-532, 538, 539, 544 environmental explanations 98, 282, 488, 512 environmental objects 514 environmental pressure -S 241, 282, 285, 509, 512, 514 environmental risk factors 530 epigenetic 98, 102, 104, 106,121 epilepsy 3, 6, 33, 35, 38, 40, 41, 43, 45, 46, 52, 69. 75, 295, 306, 307, 320, 343, 365, 415, 417, 418, 437439, 444, 451, 452, 495, 499, 517, 520, 530, 541 ERP 463 estradiol 107 estrogen 109, 110, 123 ethnic effects 76, 116, 135, 220, 221, 346, 469

A,

Subject Index ETS Card Rotation test 346 evoked cortical responses 463, 447 extraversion 426 eye colour 473, 478 eye dominance 14, 59, 62, 66, 132, 137, 141-143, 146, 147, 149, 165, 241, 248, 287, 289, 364, 395, 401, 419421,429,437,458, 488, 528, 541

F familial aggregation 45, 454 familial history 312 familial learning disabilities 449 familial sinistrality 69, 134, 135, 150, 232, 253, 256, 290, 301, 302, 309, 312, 315, 318,335,341,345-353, 355, 357, 360-364, 367-375, 377-379,381388,391406,409412, 415, 425, 426, 433, 435, 439, 445, 446, 448, 450, 453, 479, 501,506,521,541,544 family history 100, 309, 407, 425, 434, 458, 461, 463465,476478, 482, 483, 522, 543 family trait 522 February 112, 249, 253, 257,468 fencing 203, 204, 250, 252,255, 257,356,416 fetal 34, 37-39, 4042, 44, 49-51, 55, 59, 60,62, 64-66, 68-71, 73, 74, 77, 79, 85, 86, 87, 99, 100, 102, 103, 111, 113, 117-119, 127, 223, 257, 432, 448, 450, 467, 481, 497, 499, 523,524, 526 fetal alcohol syndrome 41 fetal brain 49, 100, 102, 117 fetal brain injury 117 fetal breathing movements 49,51

fetal disease 42 fetal organ injury 117 fetal position 117 fetal testosterone 223, 467, 526 fever 51, 60, 116, 525, 530 field dependence 55 fine manual skill 182, 185, 189 finger identification 137, 143, 148, 149 Finger Oscillation Test 400 finger tapping 134, 137, 138, 140, 143, 144, 178, 181, 182, 185, 368,395, 400 fishing societies 221 Flags Test 322, 346 fluid intelligence 230, 323, 324, 327 foetal development 450 foetal see fetal folklore beliefs 202 foot length 137, 141, 146, 149 foot preference 14, 49, 59, 62, 135, 137, 138, 140, 144, 145, 149, 151, 191, 241, 252, 395,419,429,488,528 forebrain 102 Fragile-X 443 fraternal twins 58 frontal lobe 57, 113 fusion malformations 58, 65

G gastrointestinal diseases 56 gender 22,24-26,50, 109, 122, 157, 273, 317, 346, 364, 370, 434, 436, 448, 455, 469, 525,530,537,539,542 gene -s 4546, 48, 51, 52, 77. 93. 96. 169. 186, 192, 403, 404, 478; 479.511 general 'intelligence 445, 446 generation 4, 41, 76, 78, 108, 176, 218, 219, 224, 228, 234, 238, 239, 251, 253, 257,

567

455,500 genetic effects 31, 93, 94, 98, 167-169, 172, 186, 315, 364, 404, 412, 435, 443, 450, 478, 479,484,501,511 genetic factors 5-7, 10, 13, 19, 26, 28, 29, 31, 23, 37, 4248, 51, 60, 65, 66, 68, 71, 73, 7681, 92-96, 98, 100, 102, 104, 110, 115, 118, 121, 124, 133, 145, 151, 169, 172, 185, 186, 191, 244246, 248, 251, 253, 255, 256, 282, 285, 287-289, 301, 305, 308, 311, 315-317, 338, 341, 343, 348, 364, 367, 369, 372, 402405, 423, 432, 436, 442, 443, 446448, 451, 454, 455, 461, 466, 478, 479, 481484, 493, 497, 501-503,505,510-512, 515, 516, 521, 530, 539-541, 543-545 geographic regions 98, 222, 510 Geschwind-Galaburda hypothesis 448451 gifted 75, 112, 176, 177, 348, 358-361, 416, 437,526 graphesthesia 137, 143, 147, 148, 421 grasping 198, 207 grip strength 137, 138, 140, 143, 144, 395

H Halstead Reitan Neuropsychologica I Test Battery 400 hand injury 272, 277 hand length 137, 146 hand posture 236, 394 hand preference 48, 72, 79, 93, 96, 97, 110, 118, 121, 126, 135-138, 140, 143, 144, 147, 155, 159, 162-164, 168, 171, 173-175, 177,179-181, 183, 185-188, 192,

568

Subject Index

196, 199, 216, 218, 220,222-225,231-233, 239-241,244,245,246, 247, 250, 252, 253, 256, 257, 260, 261, 263,267,270-272,280, 286-288,301,304-307, 309-312,314,316,317, 329,337,339-343,346, 349, 351, 353, 357, 360, 362, 363, 366, 368, 369, 372, 379, 381,383,395-399,401, 402,415,419421,425, 429,437,439,44547, 453, 505-507, 509, 527,532,533,542-544

Hand-preference Demonstration Test 159

hand strength 182, 185 hand training 198, 208, 209, 212, 213, 215, 219,221,224-228,231, 234-239, 243 handwriting 197, 201, 202. 207. 215. 218. 221; 225; 229; 234; 238,244,252,253 handwriting posture 76, 135, 137, 140, 149, 150, 252, 350, 253, 370, 381, 389, 394, 401, 406, 409,410 hare lip 58, 111

Hashimoto’s

thyroiditis

123, 125, 288, 502, 517 hormonal effects 50, 67, 70, 76, 79, 99, 100, 103-114, 116, 117, 120, 121, 124-127, 190, 310, 311, 408, 448, 454, 458, 464466, 483, 485, 492495, 497499, 501, 504, 506, 507, 522524,529,542 hunting 221, 222 HVc 108 hydrocephalus 300,316 hyperactivity 8, 41, 56, 449, 456, 463, 473, 475, 476, 499 hyperlexia -ic -ics 445, 446, 448, 453, 455, 456 hyperplasia 58, 70, 110, 125 hyperreflexia 135

hypersensitivity 365 hyperstriatum ventralis 108

hypogonadotropic hypogonadism 110 hypopigmented 473 hypoplasia 134, 148, 157 hypothyroidism 464, 465 hypoxia 34, 35, 44,47-52, 54, 55, 58-60, 63, 68, 70, 73, 79, 87, 343 hysteria 41

525

hay-fever 116 head trauma 157 headaches 157, 517 hemihypoplasia 155 hemiparesis 132 hemiplegia -ic 52, 59, 131, 132, 134-138, 140-149, 151, 209, 251 hemisphere density 425 hemispherectomies 168 high altitude 51, 60 high blood pressure 120 hippocampus 109, 502 historical 17, 35, 76, 94, 127, 163, 164, 191, 195, 197, 200, 222, 225, 245, 248, 250, 251,254,438,510,541 Hole Test 137, 141 homosexuality 4, 7, 42, 52,56, 57, 64, 69, 113,

I ID

352, 353, 355, 359-361, 363 identical twins 104, 425 IHP 389, 394, 395 ileitis 56, 525 immune disorders 50,59, 56, 75, 116, 117, 123, 126, 311, 312, 352, 359, 362, 363, 370, 399, 400, 448, 449, 452, 464, 465, 467, 471, 476, 522, 524-526,529, 539 immune effects 50, 51, 56, 57, 59, 64, 65,67, 74, 75, 114-117, 122124, 126, 190, 249, 311, 312, 316, 352, 359, 362, 363, 366,

370, 399, 400, 448, 449, 452, 464468, 471, 481, 483, 522-526, 529, 532, 533, 539, 543

411, 454, 476, 503, 530, 542,

immunological pathology 51

impulsivity 8, 426, 440, 491, 492

in utero 51, 64, 65, 107, 108, 110, 117, 120, 121,497 inattention 449, 503

incomplete sexual dominance 42 induction of labour 120 infant mortality 49, 53 infantile autism 54, 164, 308, 415, 454, 455

infection 34, 42, 44, 51, 55, 60,474 insomnia 517 instrument deliveries 39 intellectual 245, 293, 314, 324. 328, 335-337. 342; 343; 345, 353; 359-361, 365, 367, 368, 370, 371, 415, 416,482, 501,540 intellectual-cognitive 416 intellectual deficits 293 intellectual precocity 122, 336, 359, 360, 365, 370, 501, 540 intelligence 64, 95, 120, 122, 126, 133, 152, 190, 229, 230, 254, 288, 315, 320-324, 326, 327, 337, 338, 341, 344-346, 353, 365-368, 370, 371, 398, 407, 411, 426, 433, 441, 444-446, 449, 451, 452, 479, 489,506

intrafamilial handedness 233

intrauterine factors

37, 39, 42, 44, 48, 60, 98, 113, 523, 524, 528 inverted writing 140, 141, 151, 254, 289, 322 irritability 41

ischemic encephalopathy 54

Subject Index

J

June 69, 112, 247, 253. 287,468,469

K kidney disease 60 Klinefelter’s syndrome 57, 113

L labelling theory 489 language 6, 27, 32, 43, 50, 71, 73, 14, 99-101, 112, 115, 116, 120, 126, 127, 132, 14, 145, 152, 189, 191, 200,202,203,208-210, 226,221,235-237, 245, 247,250-254,256,258, 300-303, 306,310-322, 324, 327, 336, 337, 339-341, 344, 348, 350-352,354,355,357, 359, 365-377, 379-381, 383-392,395,398,401, 407412,415,421,428, 432,442,443,445447, 449456,462,491,492, 498, 500, 501, 503, 504, 507, 432, 453, 511,520, 543 language deficits 116, 300, 313, 421, 428, 445, 520 language effects 115,132, 200, 203, 236, 344, 350, 359, 372, 412, 451, 452, 490, 497 language lateralization 99. 145, 210, 253, 324, 337,340,348,350-352, 354, 357, 369, 372, 374,375,381,383-385, 387-390, 395, 408412, 504,507 late onset dementia 73, 439,544 lateral coherence 388, 389, 392 lateral neurodevelopment 50

lateral ventricle 158, 419 latitude 459, 465, 469, 470, 472, 482,483 law 491, 492, 500, 539

learning 8, 27, 34, 35,43, 50, 52, 56, 57, 59, 67, 73, 75, 78, 108, 111, 114, 116, 120, 121, 123, 125, 179, 215, 249,252,287-289,293, 302-305,311,343,366, 370, 442, 445, 446, 44840,452454,463, 476, 483, 489, 491, 493,499-501,503,5@4, 506,512,514,515,542 learning disorder -s 8, 34, 35, 43, 50, 52, 56, 57, 67, 75, 111, 114, 120, 123, 179, 249, 289, 311, 343, 366, 442, 445, 446, 448, 450,452454,463,476, 483, 489, 491, 499, 500,503,506,542 left occiput anterior (LOA) 117, 118 left shift 263, 266-271, 273-280, 282-284 leprosy 480 lesions 27. 29, 39, 48, 56, 73, 104,115, 134,152, 162, 235, 236, 249, 294,305-308,316,328, 350,373,375-380,385, 388,407,416418,428, 439, 447, 450, 454, 517, 541 lethargy 471 life span (see also longevity) 53, 54, 70, 96, 196, 241, 242, 264, 265, 338, 464, 467, 468, 473, 478, 509, 530,532,533,537-539, 544 - .. light pigment 464, 472, 473, 476 lights 51 limbic system 495, 499 limbs 10, 26, 132, 135, 143, 145, 155, 186, 187, 192, 199, 237, 266,519, 524 liver disease 474, 527 lobectomy 415, 417 longevity (see also life span) 508, 509, 519, 524, 525, 526, 529, 530, 532-534, 536-539 lupus erythematosis 72, 117, 125,476

569

lymphocyte reactivity 524

IM madness 41, 67 magnetic resonance imaging 101, 114, 122, 126, 127, 156 malformations 19, 3537, 43, 54, 58, 65, 117, 122 malnutrition 35, 42, 51, 60,67,542 manic-depressive 54,420, 421, 426-428, 431, 432,436, 438,439 manual performance 144 135-138, manual skill 144, 145, 182, 185, 189, 3%, 428 MA0 58, 66, 427, 434, 435, 463, 477 March 112,212,255,468, 475, 501 masturbation 41 maternal age 19-25, 28, 29, 34, 44, 66, 83, 84, 86, 88, 90, 93, 120, 540 maternal anatomy 48 maternal disease 55 maternal effect 226, 250 maternal endocrine disorders 34, 44 maternal-fetal blood type incompatibilities 34 maternal fever 51 maternal health 60, 63 maternal height 63 maternal transmission 233 mathematical ability 112, 122, 176, 336, 360, 364,479, 500 mathematical precocity 358, 361 mathematics 112, 176, 329, 347, 357, 364, 398, 443, 500 maturation 7, 10, 13, 29, 30, 34,52,58,66, %, 102, 103, 116, 153, 223, 241, 242, 258, 302, 308, 312, 315, 503,509,528-530,541 medial preoptic nucleus 107 melancholia 41, 428

570

Subject Index

melatonin 112 mental and motor disability 39 mental deficiency 33, 43, 71, 165, 318, 544 mental retardation 7, 8, 35, 41, 45, 46,50, 52, 59, 62, 68, 95, 150, 151, 155, 156, 162, 165, 170, 171, 176, 213, 293, 294, 300, 302, 305, 306, 308, 309,312,313,315-318, 343, 403, 407, 411, 415,435,439,443445, 450, 452, 453, 455, 456, 506, 517, 521, 541, 544 mesial sclerosis 418 mice 73, 94, 116, 117, 127, 287, 304, 315, 494,524, 543 microangiomata 114 microdyigenesis 111,115, 117, 125 micropolygyria 114, 123 migraine 7, 56, 57, 67, 68, 75, 111, 123, 249, 365,454,503,517,542 military 53, 66, 247, 270, 481 Mill Hill Vocabulary Scale 344,346 Minnesota Paper Form Board 399 miscarriages 19 mixed handedness 120, 156, 175, 344, 356, 360, 361, 382, 419, 423, 425, 426, 458, 485, 487, 489, 491, 493,495,497,499, 521 monoamine oxidase activity 426, 439 monoamine oxidase levels 58 monozygotic twins 73, 104,126,423425,432, 433, 437, 438, 456, 511, 512 mood instability 422, 431 mood regulation 431 mortality 42, 49, 53, 64, 76, 83, 509, 529, 530, 532-536, 538, 539 motor area 49 motor asymmetry 142, 143, 147, 148, 151

motor development 146 motor impairments 300, 302 motor laterality 144, 156, 401,421,424,428,431 motor performance 31, 132,135,144-147,149, 187, 189, 191, 223, 442, 451, 474, 504 motor skills 64, 68, 69, 74, 94-97, 125, 150, 166,192,247-250,253, 255-258, 286, 288-290, 317, 336-341, 366, 368-372,408,412,452, 480, 482, 502, 505, 506,540,542 motor stereotypes 442 multiple birth 49, 94, 518,530 multiple personality 428, 438 music 75, 121, 197, 315, 356, 357, 365, 366, 368, 416, 443, 475, 500, 505, 527, 531 myasthenia gravis 56, 287,525 myelination 104, 127, 241

N negative symptomatology schizophrenia 421 neocortex 64, 111, 114, 490,492,494,497499 neonatal 43, 69, 71, 84, 86, 106-108, 117, 119, 123, 125, 126, 128, 504, 519, 520 neonatal head turning 119 neonatal lupus syndrome 117, 126 neoplasms 114 nervous sptem 4,41,43, 44,47,65,67,94, 117, 120, 125, 132, 133, 195, 418, 501, 503, 520, 523 neural networks 321,390, 406 neurasthenia 41 neumhemical 124, 454, 492,497 neurodevelopmental disorders 54, 65 neurological 5-8, 13, 26,

27, 30, 47, 48, 53, 54, 87, 120,125, 165, 168, 173. 177. 213. 252. 261; 282; 285; 316; 320, 338, 339, 352, 375, 379, 392, 400, 403, 404, 409, 417, 421, 430, 436, 448, 450-452, 464, 467, 468, 475, 476, 484, 486, 490, 492, 495, 497, 505, 517, 519, 522, 528, 529, 531, 490492,498 neurological damage 6, 13, 27 neurological deficits 47, 48, 87, 173, 177, 375, 400, 403, 404, 421, 430, 451, 464, 517, 519, 522,529, 531 neurological development 476, 495, 528 neurological marker 352 neurological phenomena 467, 475, 476 neuronal growth 523 neuropathology 115, 122, 124, 166, 307, 417, 435, 436, 441, 443, 450, 517, 519, 520, 524, 527, 529, 530, 532, 539 neurophysiological 431, 447, 481 neuroplasticity 132, 152 neuropsychological 30, 48, 54, 66, 67, 69, 7274, 94, 95, 97, 127, 134, 148, 151, 152, 164, 170, 189, 191, 192, 235, 237, 238, 243, 245, 250, 253, 257, 287-289,317-319, 326-328, 334, 339, 340, 342, 366-368, 370-372, 380, 400, 409-411, 419, 423, 431, 436439, 447, 454, 463, 475, 477, 502-507, 509, 541, 544, 545 neurosis 41 neuroticism 6, 8, 228, 244, 517 NHP 389, 394, 395 nicotine 34, 244 noise 51, 401

Subject Index noninverted writing posture 132, 145, 148 noradrenalin 51 November 112, 215, 468, 469 nutrition 35, 44, 63, 65

0 Object Naming Latency Task 350, 385, 390, 411 obsessional 442 occipital lobes 114 oculomotor system 147 offenders 64, 429, 430, 487 ontogenesis 108, 424

P Panglossian weakness 121 paralysis 46, 48, 55, 67 paranoid 165, 419421, 430,438 parietal 101, 106 parietal operculum 101 paternal chromosome 15 467 p a t h o l o g i c a l left-handedness 6,16, 17, 23-25, 27, 29, 31, 32, 36, 46, 52, 71-73, 80, 83, 92, 96, 119, 131, 133, 134, 142, 144,146-152, 155,162, 165, 170-173, 176-178, 191, 227, 237, 250, 294,306-311,313,314, 317, 318, 321, 338, 343, 367, 370, 371, 402, 404, 405, 410, 411, 415, 416, 425, 433, 434, 438, 439, 447, 455, 456, 506, 516, 521, 543, 544 pathological reflexes 135 p a t h o l o g i c a l right-handedness 16, 17, 23, 46, 58, 59, 61, 62, 133, 306,447 pathological shift 13-17, 22-25, 27, 28 pathology 3-7, 9, 10, 12-17, 19, 22-29, 31, 32, 35-38, 40, 41, 45-54, 57-64, 66, 67,

70-73, 76, 79-81, 83, 87, 92, 94, 95, 96, 98, 110, 114, 115, 119, 120, 124, 131, 133-135, 142, 144-152, 153, 155, 162, 164, 165, 169-173, 176-179, 181, 190, 191, 227, 237, 246, 249, 250, 256, 257, 285, 290, 294, 302, 304-318, 321, 338, 341, 343, 367, 370, 371, 402405, 407, 410, 411, 415, 416, 418, 425, 433435, 438, 439, 441, 445451, 453456, 482, 503, 506, 516, 517, 521, 522, 527-530, 538, 541-543, 544 paw preferences 78, 93, 96, 111, 304,315,494, 524, 543 pellagra 35 perceptual functions 132 perceptual problems 293 perceptual speed 354, 398 perceptual tasks 147, 187, 334 perinatal 17, 33-35, 39, 4145, 4749, 52, 53, 55, 58, 60, 62, 63, 65-69, 71, 72, 74, 80, 81, 83, 84, 120, 132, 135, 149, 152, 312, 313, 317, 343, 429, 433435, 492, 517, 519, 523, 529, 530, 539, 542 personality 4, 5, 43, 54-56, 59, 67, 70, 74, 186, 228, 245, 247, 253, 258, 419, 420, 428, 431, 433, 438, 439, 502, 507 personality disorders 43, 55, 56, 59, 74, 419, 420, 433, 438, 507 phenylketonuria 443 phototherapy 472, 481 placental blood 107 placental disorders 55 placental transfusion syndrome 432 planum temporale 79, 99,101,102,107,109,

571

114, 115, 123, 249, 366

Pointing Test 137, 141 polluted air 51, 60 positive symptomatology schizophrenia 421 post-natal 102-104, 111, 126, 416, 432, 434 Prader-Willi Syndrome 467, 481 pregnancy 5, 31, 33, 38, 42-44, 45, 49, 52, 55, 58, 63, 64, 67, 71, 73, 79, 81-83, 85, 86, 88, 90, 92, 96, 97, 100, 113, 118, 120, 127, 289, 343, 404, 405, 432 pregnancy complications 44, 52, 58, 73, 82, 83, 97, 127,289,404,405, 432 premature birth 19, 39, 43, 44, 48, 49, 55, 70, 72, 99, 111, 120, 121, 126, 312, 432, 445, 474, 518, 530, 544 premature graying 111 prenatal 30, 31, 33, 35, 3742, 44, 45, 47-56, 58, 60, 62, 63, 67, 69, 71, 73, 74, 75-77, 79-81, 92, 97, 104, 110-114, 119, 120, 122-124, 126-128,132, 135, 149, 289, 358, 416, 458, 464467, 473, 476479, 485, 492, 495, 497, 504, 517, 519, 522-524, 530,539 prenatal coitus 60 prenatal sex horniones 50,458, 522, 523 prenatal stress 51, 67, 73, 79,97, 127, 289 prenatal testosterone 50, 56,465,466,478,523, 524 Present State Exam 156 progesterone 74, 466 prolonged labour 120, 518, 530 prosody 121,446 psychiatric 43, 58, 66, 67, 72, 74, 93, 150, 157, 164, 166, 244, 253, 256, 419421, 425,

572

Subject Index

430,431,434439,456, 481, 482, 484 psychopathology 4, 7, 8, 27, 30, 43, 53, 55, 56, 59,66,69,70,72,154, 157, 162, 164, 165, 415, 416-422,424431, 434440,456,481,487, 502,503, 505,517 puberty 418, 493, 497, 528 85, Purdue Pegboard 137, 138, 140, 143, 144,181,182,184,185 pyramidal lesions 47, 48

race 76, 135, 157, 249, 262-264, 282-285, 287, 290,539, 543 radiation 31, 34, 44 rape 113,429 rare trait marker model 3, 9, 11, 13, 14, 19, 21, 24, 26, 28, 29, 517 rat -8 51, 55, 74, 78, 104, 106-109,111,115,116, 123-128, 493495,502, 504,520, 524 reaching 171, 174, 198, 247; 250, 251, 465, 505. 515 reading 6, 43, 71, 114, 123, 150, 151, 216, 230, 235, 236, 293, 316, 317, 328, 329, 345, 353, 393, 443, 445, 446, 448, 454, 462, 482, 490, 491, 502, 505, 517 reading disability 6, 43, 293, 317, 445, 517 reasoning 112, 190, 210, 230, 323, 325, 326, 328,334-337,341,346, 354, 355, 359, 360, 365, 366, 371, 491, 493,522,528,540,542 religious biases ,262 retardation 3, 5, 7, 8, 29, 35, 41, 45, 46, 50, 52, 56, 59, 62, 65, 67, 68, 71, 75, 95, 150, 151, 155, 156, 162, 165, 170, 171, 176, 213, 293-295, Mo-318, 343, 403, 407, 411, 415,

435, 438, 439, 441, 443-445,450-453,455, 456, 495, 506, 517, 521, 522, 541, 544 reticular formation 495, 499 RH incompatibility 518, 530 rheumatoid arthritis 56 rhinitis 525 right-handed world 179, 185, 188, 215, 242-244, 259, 262, 287, 304 right occiput anterior (ROA) 118 right shift 46, 51, 52, 64, 93,150,164, 186,191, 244, 245, 266-271, 273-280, 282, 283, 286, 288, 305, 361, 364, 403, 404, 492, 495, 497, 499, 500, 511,540 right-sided world hypothesis 512, 530 rodents 107, 493, 501 rubella 443

S sagittal resonance magnetic images 104 savant 443, 452, 455 schizophrenia -s -ic 42, 43, 47, 52-54, 64-75, 153-157, 159, 162-166, 253, 315, 415, 417427,429440,453, 455, 467, 481, 500 sclerotic lesions 417 scoliosis 111 scuny 35, 36 season of birth 44, 60, 66, 93, 457, 464471, 479482, 540 season of conception 458, 466, 473, 478, 479 seasonal affective disorder 462, 471, 472 seasonal sensitivity 462, 464,471 secular trend 53, 217, 218, 240, 241 seizure 59, 157, 209, 418, 439, 444,495, 499

septuagint 36 serotinergic deficit 461 serotonin 461463, 472, 481 sex 4, 18, 24, 31, 50, 60, 68, 69, 74, 83, 100, 105-113, 116,122-127, 140, 143, 150, 171, 190, 207, 208, 221-224, 229, 230, 232, 233, 245-253, 256, 267, 273, 276, 277, 286-288, 319, 324-330. 332-341, 346-355, 357, 360, 363,365-372,377-379, 381, 383-385, 392, 396, 398, 399, 407, 409, 410, 412, 417, 420, 426, 443, 448, 451, 455, 456, 458, 465, 466, 479, 483, 485, 486, 492-495, 497-504, 506, 507, 518, 522-525, 533, 536-538, 540-543 sex difference 24, 31, 50, 74, 105,106, 108, 109, 123-125, 127, 171, 190, 207, 208, 222-224, 232, 233, 246, 250-252, 276, 277, 324, 326, 327, 334, 337-341, 365, 366, 368, 370, 372, 377, 409, 412, 455, 493495,500-504,506, 507,524,525,540-542 sex hormones 50, 106-110, 112, 113, 116, 448, 458, 465, 466, 483, 485, 492494, 497499,522-524 shift attempts 142, 263, 268, 270, 271, 274, 276-278, 280-285 sickle cell anemia 172 sight dominance (see also eye dominance) 132, 135, 141, 143, 148, 151, 165, 395, 407 sleep difficulties (see also insomnia) 7, 41, 52, 56, 58, 66, 248, 528530,541 sleepwake cycles 107 smoking 35, 44, 49, 50, 55, 60,64, 68, 70, 73,

Subject Index 85, 87, 88, 90, 119, 249, 459, 474, 482, 526, 527, 530, 533, 539, 542 Sncllen chart 137, 141 social 29, 31, 44, 55, 56, 64, 71, 72, 112, 121, 171, 196, 200-202, 206, 208, 210, 211, 220, 224-226, 230-233, 235, 241, 246, 254, 257, 260, 290, 304, 328, 340, 379, 430, 438, 441, 442, 449, 450, 454456,482,483,488, 489, 494, 498, 504, 505,507,512-516, 531 social class 35, 44. 60, 135, 154, 157, 230, 231, 489 social pressure 121, 171, 206, 208, 210, 211, 220, 221, 224, 225, 230-232, 235, 241,246, 262, 326, 334, 379, 489, 498, 512-514 social problems 232, 504 somatosensory 132, 135, 137, 142, 147, 149, 151, 152 soper-satz model 447 Space Thinking (Flags) Test 344 spacing 458, 465, 466, 473,478,479 spacing of children 458, 465, 466, 479 spastic quadriplegia 135 spasticity 39, 48, 66, 135 spatial 7, 8, 74, 105, 110-112,122, 124, 155, 230,231,25238,245, 250, 253, 254, 289, 319-330, 333-341, 344-356,358, 361,365, 366, 368-372, 373-380, 392-394, 397-402, 407410,412,416,456, 490, 493, 504, 517, 522, 542 spatial abilities 7, 8, 110, 111, 124, 155, 238, 253, 319-321, 324-329, 334,335,337-3440,344, 345, 347-356, 361,365, 366, 368-370, 377-379, 392,398402,407,409, 410, 504, 542

spatial processing 237, 321, 351, 369, 373, 392-394,409 speech (see also language) 43, 46, 54, 56, 71, 93, 96, 121, 124,150-153,155,162, 165,199-201,208,209, 213-216,223,226,227, 235-238,244,246,247, 250,252,254-256,288, 300,302,305,313-320, 338-340,369-371, 373, 375, 376, 410, 411, 422, 447, 453, 446, 502, 504-507, 511 speech disorders 43, 214, 226, 235, 300 smech recovery 375 &rts 205, 22~(232, 233, 254. 258, 269. 275, 279; 356,416,474,532 Stafford Identical Blocks Test 399 stereognosis 137, 142, 143, 147-149 sterility 41 steroids 106-108, 112, 113, 399,400 stigmata 40 stillbirths 43, 86 strabismus 43, 58 stress 3, 7, 13, 17, 18-22, 24, 28, 29, 32, 34, 43, 47, 49, 51, 60, 61, 63, 64, 67, 70-74, 79-88, 90, 92-94, 96, 97, 113, 118-120,127,170,192, 288,289,309-313.314, 317, 318, 364, 371, 404, 405, 429, 501, 505,506,517-519,523, 524,529,539-541,544, 545 strokes 132 stuttering 56, 75, 111, 213, 214, 226, 227, 247,255,251,256-258, 293,475,476,478,505 subcortical 26, 27, 336, 447, 449, 450, 452, 453, 495, 497499 substance abuse 433,460, 471, 472, 474 suicide 7, 53-55, 66, 72, 247,426,458,481,537 suprachiasmatic nucleus 107

573

Sylvian fissure 101, 102 syphilis 480 systemic lupus erythematosis 72, 117. 125

T tactile perception 143, 147, 149, 371 tail bias 108, 111 talented 111, 112, 122, 190, 342, 355-358, 360-365, 475, 478, 479,500,540 talmud 36, 37 Taylor Manifest Anxiety Scale 426 technologicalenvironment 515,539 temporal 52, 68, 106, 115, 124, 164, 184, 218, 319, 359, 365, 366, 377, 415418, 432, 439, 455, 524, 541 testosterone 50-52, 56, 74, 79, 100, 106-113, 116, 117, 126, 127, 223, 310, 311, 358, 359, 369, 448, 458, 465467, 478, 479, 523-526, 542 thymus 50, 56, 116, 465, 524, 542 thyroid disorder 56, 57, 70, 116, 457,463465, 467,471,476478,483 thyrotropin 464, 483 tools 174, 195, 212, 216, 222, 232, 243, 510, 512, 514, 531, 532 Torque test 420 toxemia 42, 63, 82, 86 training 95, 198, 199, 206-216, 219, 221, 223-229, 231,234-239, 243, 246, 251, 261, 270, 282, 304, 316, 317, 326, 367, 494 trauma 33, 38,42, 43, 46, 60,97, 120, 157, 310, 343, 404, 409, 429, 433, 474, 492, 519, 528,529 tremor 58, 65 tryptophan 461 tuberculosis 480

574

Subject Index

tuberous sclerosis 443 Turner syndrome 57 twins (see monozygotic twins and dizygotic twins) 58, 65,68, 71, 73, 96, 98, 104, 126, 257, 317, 423425, 432437,438,343,454, 456,467,468,511,512

U ulcerative colitis 56, 57, 73,289, 318, 371, 464, 525,530,544 unipolar 415, 426428, 434,438 urticaria 56, 525

V Vandenburg Mental Rotation Test 344 vasopressin 51 VBR 157, 161-163 vegetarianism 7, 58, 66 ventricular brain ratio 158 ventricular dilatation 421 verbal (see also language) 7, 8, 105, 110, 112. 122, 126, 181, 235, 238, 249, 252, 253, 269,275,318-322,324, 326, 327, 339-341, 344-347, 350, 351, 353-355, 358-360, 365, 366,368-371, 374-377, 380, 383, 384, 388, 391,398,407410,422,

423,426,429431,446, 447, 462, 491, 499,

503,517,522 verbal ability (see also language) 7, 8, 110, 112, 181, 253, 320, 324, 327, 345, 346, 354,358-360,369,398, 446,491,517 verbally gifted 112 violinists 121 visual acuity 135, 141, 143, 146, 149 visual acuity 419 visual dysfunction 58, 60 visual evoked potentials 429 visuo-spatial functions 46, 105,328, 337,373, 377, 380, 392, 393, 397,400,407 vitamin deficiency 35 vulgate 36

W war 113, 238, 533 Wechsler Adult Intelligence Scale 320, 322, (WAIS) 323,325327,329,331, 341, 344, 344, 346, 347, 398, 419, , 423, 463 weight 32, 35, 44, 49, 58, 63, 64, 69, 73, 85,87, 88, 90, 120, 121, 432, 433, 471, 479, 518, 529,530,539

Wisconsin Card Sorting test 419 writing (see also handwriting) 31, 41, 43, 71, 74, 82, 94, 97, 114, 132, 137, 140, 141, 145, 148-150, 151, 152, 162, 163, 174, 175, 178-182, 184-186,188-190,192, 1%-198,201-211,213231, 233-240, 242, 244, 246, 249, 253-256, 261, 262, 269, 272, 274, 275, 278-280, 282, 286, 288, 289, 316, 322, 350, 366, 369, 376, 379, 393-396, 402, 411, 419422, 425, 428, 444, 451, 471, 473, 478, 489, 490, 499, 507, 513, 536-538.542

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