THE g FACTOR The Science of Mental Ability Arthur R. Jensen Human Evolution, Behavior, and Intelligence Seymour W. Itzkoff, Series Editor Westport, Connecticut PRAEGER London Library of Congress Cataloging-in-Publication Data Jensen, Arthur Robert. The g factor : the science of mental ability / Arthur R. Jensen, p. cm.—(Human evolution, behavior, and intelligence, ISSN 1063-2158) Includes bibliographical references and index. ISBN 0-275-96103-6 (alk. paper) I. General factor (Psychology) 2. Intellect. 3. Nature and nurture. I. Title. II. Series. BF433.G45J46 1998 153.9—dc21 97-22815 British Library Cataloguing in Publication Data is available. Copyright © 1998 by Arthur R. Jensen All rights reserved. No portion of this book may be reproduced, by any process or technique, without the express written consent of the publisher. Library of Congress Catalog Card Number: 97-22815 ISBN: 0-275-96103-6 ISSN: 1063-2158 First published in 1998 Praeger Publishers, 88 Post Road West, Westport, CT 06881 An imprint of Greenwood Publishing Group, Inc. Printed in the United States of America The paper used in this book complies with the Permanent Paper Standard issued by the National Information Standards Organization (Z39.48-1984). Every reasonable effort has been made to trace the owners of copyright materials in this book, but in some instances this has proven impossible. The author and publisher will be glad to receive information leading to more complete acknowledgments in subsequent printings of the book and in the meantime extend their apologies for any omissions. Quote from Charles Murray on the jacket reprinted with permission of National Review, 215 Lexington Avenue, New York, NY 10016. P In order to keep this title in print and available to the academic community, this edition was produced using digital reprint technology in a relatively short print run. This would not have been attainable using traditional methods. Although the cover has been changed from its original appearance, the text remains the same and all materials and methods used still conform to the highest book-making standards. Dedicated to the memory of CHARLES EDWARD SPEARMAN the discoverer of g Contents Preface ix Acknowledgments xiii Chapter 1. A Little History 1 Chapter 2. The Discovery of g 18 Chapter 3. The Trouble with “Intelligence” 45 Chapter 4. Models and Characteristics of g 73 Chapter S. Challenges to g 105 Chapter 6. Biological Correlates of g 137 Chapter 7. The Heritability of g 169 Chapter 8. Information Processing and g 203 Chapter 9. The Practical Validity of g 270 Chapter 10. Construct, Vehicles, and Measurements 306 Chapter 11. Population Differences in g 350 Chapter 12. Population Differences in g: Causal Hypotheses 418 Chapter 13. Sex Differences in g 531 Chapter 14. The g Nexus 544 Appendix A. Spearman’s “Law of Diminishing Returns” 585 Appendix B. Method of Correlated Vectors 589 viii Contents Appendix C. Multivariate Analyses of a Nexus 593 References 597 Name Index 635 Subject Index 643 Preface This book about the g factor has its origin in the aftermath of an almost book- length article (my 77th publication) that I wrote almost thirty years ago, titled “How Much Can We Boost IQ and Scholastic Achievement?” and published in the Harvard Educational Review in 1969. It had five main themes: (1) the malleability of IQ (or the latent trait it measures) by special psychological and educational interventions in the course of children’s mental development; (2) the heritability of IQ; (3) social class and race differences in IQ; (4) the question of cultural bias in mental tests; (5) the need for universal education to tap types of learning ability that are relatively unrelated to IQ in order to achieve the benefits of education for all children throughout the wide range of abilities in the population. It made four main empirically based claims: (1) individual dif­ ferences in IQ are largely a result of genetic differences but environment also plays a part; (2) the experimental attempts to raise the IQs of children at risk for low IQ and poor scholastic performance by various psychological and ed­ ucational manipulations had yielded little, if any, lasting gains in IQ or scholastic achievement; (3) since most of the exclusively cultural-environment explana­ tions for racial differences in these important variables were inconsistent and inadequate, genetic as well as environmental factors should be considered; (4) certain abilities, particularly rote-learning and memory, had little relation to IQ, which suggested that these non-IQ abilities could to some extent compensate for low IQ to improve the benefits of schooling for many children at risk for failure under traditional classroom instruction. According to the Institute for Scientific Information (ISI), which publishes the Science Citation Index (SCI) and the Social Science Citation Index (SSCI), this 1969 article soon became what the ISI terms a “citation classic”—an article (or book) with an unusually high frequency of citations in the scientific and professional journals. The onslaught of critiques and commentaries on the arti­ cle, in both the popular media and the professional literature, made it clear that X Preface there was sufficient misunderstanding and misinformation, as well as reasonable criticism and argument, concerning some of the article’s main topics to warrant a more thorough explication of the issues and the empirical evidence than was possible in the 124-page journal article. Moreover, certain questions raised in my article could not be answered adequately without doing further research based on adequate data—inquiries that had not been undertaken by anyone at that time. Hence some of the issues raised by my 1969 article in the Harvard Educa­ tional Review determined my research and publication agenda during the sub­ sequent years—empirical studies, methodological papers, and reviews, which, over a period of nearly thirty years, averaged over ten publications a year in journals and book chapters. The main themes in much of this work, I decided, should be consolidated into separate books, each dealing with one of the key topics of my 1969 article. The first book in this series was Educability and Group Differences (1973), which dealt almost entirely with social class and racial differences in IQ and other psychometric abilities and their important role in accounting for individual and group differences in scholastic achievement. Probably the book’s most con­ troversial conclusion was that all of the most popular and purely environmental theories of the causes of the well-established average black-white differences in IQ and scholastic achievement were either contradicted by the factual evidence or were inadequate as a scientific explanation, and that the total body of evidence at that time was better explained by the hypothesis that the racial differences involved both genetic and environmental factors and in about the same propor­ tions as they determined individual differences within either racial group. ISI, in its journal Current Contents (1987), announced that this book had also be­ come a “citation classic.” The second book in the series was Bias in Mental Testing (1980), in which I examined as comprehensively as was possible at that time the then controver­ sial question of whether the psychometric tests of mental ability that were widely used in schools, colleges, industry, and the armed services yielded biased scores for those racial and cultural minority groups in the United States that, on av­ erage, score below the mean of the rest of the population. My conclusion from this research was that the currently most widely used standardized tests of mental ability yield unbiased measures for all native-born English speaking segments of contemporary American society, regardless of their sex, race, or social class background, and that the observed mean differences between various groups are not an artifact of the tests themselves, but are attributable to factors that are causally independent of the tests. In brief, the tests do not create the observed group differences, they simply register them. This conclusion has since been accepted and affirmed by the majority of experts in the field of psychometrics. This book, too, was later written up as a “citation classic” in the ISI’s Current Contents (1987). The following year I wrote a smaller, popular book, Straight Talk about Men­ Preface xi tal Tests (1981), to explain the gist of the two previous books to readers without a background in psychometrics and behavioral genetics. (Those about to delve into the present volume may find this little book a helpful introduction.) Having addressed those points, I realized that the critical issue was the ex­ istence and nature of the g factor itself. Although it was mentioned in my 1969 article, g was largely taken for granted, as if there had long ceased to exist any serious controversy about the sovereignty of g in the study of human mental abilities. Yet some people, mostly from outside the field, viewed g not as a phenomenon of nature, but as merely an artifact created by subjecting a partic­ ular set of mental tests to the arcane mathematical machinations of factor anal­ ysis. And I discovered that more than a few psychologists had misconceived notions or prejudices about g. It became clear to me that the real basis of my 1969 article was g itself and that it deserved a book-length exposition in its own right, even more than the other topics that, at the time, I thought were most interesting and in need of investigation. So this— The g Factor—became the third volume in the series of books grow­ ing out of my 1969 article. Charles Spearman’s great work, The Abilities of Man (1927), in which he summarized the results of his pioneer studies of g, was then the best exposition of the subject, and it is still well worth reading. But Spear­ man’s book, of course, does not take account of the important research involving g that has accumulated during the seventy years since its publication. Also, not all of the issues related to g that are the focal point of psychometrics and dif­ ferential psychology today are the same as the problems faced by Spearman in his day. Therefore, since the publication of my last major book, in 1980, I have de­ voted my research to the empirical study of g. As this line of study was actually begun by Sir Francis Galton more than 100 years ago, I decided to take up where he left off in his attempt, which appeared unsuccessful at the time, to relate measurements of reaction time to other criteria of general mental ability. The initial success of this work in my chronometric laboratory encouraged me to institute a long-term research program using modern electronic techniques for precisely measuring an individual’s reaction time (RT) in performing extremely simple elementary cognitive tasks (ECTs, as they are now called), and deter­ mining how these RT measures are related to performance on complex tests of mental ability, such as those used for measuring IQ. This Galtonian paradigm has proved a successful tool for probing the essential nature of psychometric g at the behavioral level, and it has pointed up fruitful hypotheses for further investigations of g at a physiological level, the new frontier of research on mental ability. All of my own empirical and methodological studies related to g, as well as virtually all of the research by the great many other investigators cited in the present book, have been published in the peer-reviewed scientific literature. I hope that my synthesis and theoretical interpretation of this massive body of research have done it justice. In any case, it had to be done. My research has led me to regard the g factor in a much broader perspective xii Preface than I had envisaged at the outset. I have come to view g as one of the most central phenomena in all of behavioral science, with broad explanatory powers at least as important for understanding human affairs as E. L. Thorndike’s Law of Effect (or Skinner’s reinforcement principle). Moreover, it became apparent that the g construct extends well beyond its psychometric origin and definition. The ^factor is actually a biologically based variable, which, like other biological functions in the human species, is necessarily a product of the evolutionary process. The human condition in all of its aspects cannot be adequately described or understood in a scientific sense without taking into account the powerful explanatory role of the g factor. Students in all branches of the behavioral and social sciences, as well as students of human biology and evolution, need to grasp the essential psychometric meaning of g, its basis in genetics and brain physiology, and its broad social significance. A MOTE TO THE READER Although much of the material in this book is admittedly, though unavoidably, at a fairly difficult conceptual level, I have tried to present it in such a way that it can be understood not only by specialized readers with a background in psy­ chology, psychometrics, statistics, or behavioral genetics, but by any interested persons of whatever educational background whose reading comprehension is up to the level of what I presume is typical of college graduates. I had thought of providing a glossary of the more specialized terms, but discovered that nearly all of the entries I would have included are given quite adequate definitions in the Random House Unabridged Dictionary (Second Edition, 1993). Each chapter is preceded by a brief summary of its content, as an “advance organizer” for the reader. Notes at the end of each chapter are keyed by nu­ merical superscripts in the text; they are of two kinds: (1) definitions or expla­ nations of technical terms or statistical concepts, or a more detailed explanation or analysis of some point in the text, that appear as end-notes to avoid inter­ rupting the main text (indicated in the text by superscript numbers); and (2) literature citations accompanied by little or no commentary (indicated in the text by bracketed superscript numbers). Germane but more specialized topics are explicated in the appendices. The references (all of them cited at some point in the text) provide a comprehensive bibliography of the scientific literature on human mental ability. Acknowledgments Mere thanks to all of those who have helped me in a variety of ways that eventually led to my writing this book, and indeed provided the conditions that made it possible, seems hardly enough. Many persons are owed my gratitude— my graduate research assistants and postdoctoral fellows at Berkeley who helped me in conducting many of the studies cited herein, my distinguished colleagues and friends who generously offered their expertise in specialized areas by read­ ing portions of the manuscript with a critical eye and providing advice for improving it, and those experts in fields relevant to certain topics in this book who were always willing to engage in helpful and encouraging discussions about my inquiries, often providing reprints and references. Especially deserving of credit for supporting much of the empirical research I have done on the g factor and its educational, social, and biological correlates, at a time when few foun­ dations or granting agencies would consider supporting research aimed at ex­ ploring the nature and implications of g in areas considered politically sensitive, are The Pioneer Fund and its admirably intrepid president, Harry F. Weyher, whose mission has been to lend support to pioneering efforts in scientific re­ search areas that in academe are often considered unpopular or even taboo, at least initially. Similarly, I am grateful to the publishers of this book, particularly Dr. James Sabin, Director, Academic Research and Development, and Professor Seymour Itzkoff, the series editor, for supporting this book on a topic that other firms may have thought unwise or unprofitable to consider publishing. Finally, and above all, I must acknowledge how very indebted I am to my remarkable wife, Barbara, who has not only been of direct assistance in my work, but whose superior capability, ingenuity, and efficiency in managing all of the practical and financial responsibilities of daily life have completely freed me from every chore and care that is not directly germane to my research work. For granting permission to reprint the figures or graphs in this book (indicated in parentheses), I am grateful to the following publishers: Ablex Publishing