The g factor, the science of mental ability - emilkirkegaard.dk

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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