Two Views of The Bell Curve
The following book reviews will appear in the May 1995 (Volume 40, Number
5) issue of Contemporary Psychology, APA's journal of book reviews.
Richard J. Herrnstein and Charles Murray
The Bell Curve: Intelligence and Class Structure in American Life
New York: Free Press, 1994. 845 pp. ISBN 0-02-914673-9. $30.00
Reviews by Thomas J. Bouchard, Jr., and Donald D. Dorfman
Breaking the Last Taboo
Review by Thomas J. Bouchard, Jr.
"We hold these truths to be self-evident, that all men are created equal,
that they are endowed by their Creator with certain inalienable Rights,
that among these are Life, Liberty, and the pursuit of Happiness." With
these words Jefferson introduced one of America's most treasured
documents, the Declaration of Independence. Successive generations of
Americans have not only embraced Jefferson's noble sentiments, they have
embellished them. Equality of political rights and legal standing has
been expanded into a belief in literal equality; today, differences in
outcome are taken as prima facie evidence of unequal opportunity. In an
egalitarian society such as ours, the existence of significant and
enduring individual or group differences in intelligence is seen as a
challenge to our highest ideals. This challenge is taken up by Richard J.
Herrnstein and Charles Murray in The Bell Curve.
The Bell Curve has a simple but powerful thesis: There are substantial
individual and group differences in intelligence; these differences
profoundly influence the social structure and organization of work in
modern industrial societies, and they defy easy remediation. In the
current political milieu, this book's message is not merely
controversial, it is incendiary. As scholars such as Daniel Moynihan,
Arthur Jensen, and E. O. Wilson have learned, the mainstream media and
much of the scientific community have little tolerance for those who
would question our most cherished beliefs. Herrnstein and Murray have
received similar treatment. They have been cast as racists and elitists,
and The Bell Curve has been dismissed as pseudoscience, ironically by
some commentators who broadly proclaim that their critique has not
benefited from a reading of the book. The book's message cannot be
dismissed so easily. Herrnstein and Murray have written one of the most
provocative social science books published in many years. The issues
raised are likely to be debated by academics and policymakers for years
The emergence of a cognitive elite
Commentators from across the political spectrum have documented the
profound social changes that all industrialized societies are undergoing
at the end of the 20th century--erosion of the middle class, loss of
well-paying manufacturing jobs, and an emerging information age in which
individual success will depend on brains not brawn. The Bell Curve tells
a similar story regarding the United States. It differs from other works
by focusing on intelligence, rather than education or social class as a
causal variable. The authors tell us that true educational opportunity as
a function of ability (measured by IQ tests) did not arrive in the United
States until about 1950. Until that date only about 55 percent of high
school graduates in the top IQ quartile went directly to college. From
1950 to 1960, this number jumped to 72 percent, and in 1980 over 80
percent of graduates in the highest ability quartile went to college. In
addition, sorting by cognitive ability continues as students move through
college. It also occurs across colleges, with the elite schools selecting
the more intellectually talented students. Finally, it continues across
careers in the world of work. The authors argue that intellectual
stratification through occupations is driven by powerful economic
pressures. This argument is based on a number of different and compelling
lines of evidence. If Herrnstein and Murray are correct, current social
inequalities reflect, in large part, the achievement of a meritocracy
based on cognitive ability.
The notion of a meritocracy is not, in itself, an affront to American
sensibilities. Social scientists have carefully documented that social
mobility does occur from one generation to the next and that cognitive
ability is a major factor in determining whether an individual will
achieve greater or lesser social status than did his or her parents
(Waller, 1971). When each generation resorts this way, the elements of
fairness and opportunity are preserved. If, however, as The Bell Curve
asserts, the heritability of IQ is quite high and there is a strong
tendency for those similar in ability to marry, there will be less
regression toward the mean in the cognitive ability of children of the
intellectually talented and, therefore, less intergenerational
reassortment. Under these circumstances a meritocracy begins to look like
an aristocracy, a perception that is strongly reinforced when the
intellectual elite segregate themselves from the rest of society by
living in separate neighborhoods, sending their children to private
schools, and supporting social institutions that cater to their own
The authors do argue that general cognitive ability (i.e., "g") is a
major determiner of social status and that variance in general mental
ability is largely attributable to genetic factors--propositions that are
certainly endorsed by many experts in the field. The book explicitly
disclaims, however, that general mental ability is the only determinant
of social status, or that g is the sum total of an individual's social
The role of social class of origin
The Bell Curve carefully documents in table after table, graph after
graph that cognitive ability has become a more important determinant of
social status than social class of origin. Although this may come as a
surprise to many, it is consistent with a large body of evidence.
Research methodology in the domain of individual differences has changed
dramatically in the past 20 years. Many investigators in this domain now
accept two major methodological principles: that single studies based on
small samples are inherently uninformative and that correlations
calculated from data gathered within biological families are seriously
confounded. Understanding both of these principles is important when
evaluating evidence often brought to bear against The Bell Curve.
Results from a single modest study carry little more weight than does a
single anecdote, no matter how compelling the finding. Most social
scientists, but certainly not all, have adopted the methodology of
meta-analysis, a statistical tool that systematically combines the
results from many studies to provide a single reliable conclusion. In a
similar fashion, behavioral geneticists combine the results from numerous
kinships weighted by their sample sizes to provide the best estimate of
the degree of environmental and genetic influence on any particular
trait. Any single study is viewed as providing only weak evidence on its
The confound generated by data drawn from within biological families
provides numerous pitfalls when assessing this book's claims and
reviewers' counterclaims. Within a biological family, correlations (e.g.,
parental socioeconomic status x child's IQ) are ambiguous because the
cause of the correlation could be the family environment or the parent's
genes. Within biological families, the correlation between parental
socioeconomic status (SES) and child's IQ, based on a meta-analysis of
the literature, is .333 (White, 1982). However, in studies where genetic
effects are held constant, through twin or adoption designs, the
correlation drops dramatically (Bouchard, Lykken, McGue, Segal, &
Tellegen, 1990; Scarr & Weinberg, 1978). Another striking exemplar of
this phenomenon is the IQ correlation between unrelated individuals
reared together who share a common family environment but lack a common
genetic background. When the cognitive ability of these "unrelated
siblings" is measured in adulthood the correlation is zero (McGue,
Bouchard, Iacono, & Lykken, 1993). Thus the correlation between parental
SES and offspring IQ in biological families is due, in some measure, to
genetic endowment. Consequently, when examining the relationship between
IQ and a dependent variable, to "hold constant" the SES of biological
parents (on the grounds that SES is a competing "environmental
explanation") results in an underestimate of the true influence of IQ. As
early as 1970, Paul Meehl warned that "the commonest error in handling
nuisance variables of the `status' sort (e.g., income, education, locale,
marriage) is the error of suppressing statistically components of
variance that, being genetic, ought not be thus arbitrarily relegated to
the `spurious influence' category" (pp. 393-394). In this book, intended
for lay readers as well as academicians, the authors have purposefully
provided simple and straightforward analyses of SES and cognitive
ability. They have, in many instances, understated the role of cognitive
ability by holding SES constant. We can expect to see numerous reanalyses
and the presentation of many more complex models derived to support both
sides of the debate. The careful reader will remember Meehl's caution
when examining the data and drawing conclusions.
Cognitive classes and social behavior
Part II of The Bell Curve reviews the role of cognitive ability in areas
of social dysfunction. In this section, the data are more complicated,
conclusions more equivocal. In spite of claims to the contrary by some
reviewers, the book makes it clear that with regard to the issues
discussed in this section of the book (e.g., poverty, schooling,
unemployment, idleness and injury, family matters, welfare dependency,
parenting, crime, civility, and citizenship), IQ "almost always explains
less than 20 percent of the variance, . . . usually less than 10 percent
and often less than 5 percent" (p. 117). These analyses deal only with
non-Latino Whites and make use of the National Longitudinal Survey of
Labor Market Experience of Youth (NLSY). This large nationally
representative survey, begun in 1979, incorporated the Armed Forces
Qualification Test (AFQT). The AFQT provides an excellent measure of g,
and the survey contains sufficiently detailed information that questions
regarding the influence of g on the outcomes listed above can now be
I discuss the results regarding poverty as an exemplar. First, it must be
noted that the decline in poverty from 1940 to 1970 is dramatic and
linear, dropping from over 50 percent to less than 15 percent. It has
remained nearly constant since 1970. This means that the rise in crime,
drug abuse, and many other discontents over the past 25 years cannot be
ascribed to poverty per se. It also means the analyses in The Bell Curve
are being carried out on a very different population than would have been
used had the analysis been carried out before 1970. Consequently,
comparisons with earlier research are problematical. The evidence
strongly supports the conclusion that high IQ is an important protective
factor, and low IQ is an important risk factor. Parental SES is not
nearly as protective or nearly as debilitating. IQ has an effect even
when education is held constant. When one looks at poverty among women
with children, the situation is quite different. For separated, divorced,
or never married White mothers with very low IQs, the probability of
being in poverty is almost 70 percent. For the same group of mothers with
very high IQs, the risk of poverty is about 10 percent. For married
mothers, however, the range is from under 20 percent to near zero. IQ is
influential, but marriage is clearly more important. Thus poverty among
children is strongly associated with the marital status of their mothers.
Holding IQ constant washes out any influence of parental SES for both
types of mothers but leaves a large marital effect. Similar empirical
demonstrations, with numerous twists and turns, are made regarding the
other dependent variables enumerated above.
The national context
Part III of The Bell Curve contains the most controversial chapter in the
book, "Ethnic Differences in Cognitive Ability." The data reviewed here
are neither new nor surprising and find strong support in the current
psychological literature (Humphreys, 1988). East Asians, living in Asia
or America, score above White Americans in tests of cognitive ability;
the best estimate of that difference is about three points with findings
ranging from no difference to a 10-point spread in test scores. The
difference in measured IQ between African Americans and Whites has
remained at about 15 IQ points for decades, although there is some
indication of very modest convergence due to fewer low scores in the
African American population. Controlling for SES reduces but does not
eliminate this difference, and of course, controlling for SES in ethnic
group contrasts may eliminate a valid source of IQ variance. Moreover,
ethnic differences on cognitive tests cannot be attributed to test bias.
As described earlier, The Bell Curve asserts that differences in
cognitive ability between individuals are due in part to differences in
their genetic endowment. A great deal of research supports this
conclusion (Bouchard, 1993; Pedersen, Plomin, Nesselroade, & McClearn,
1992). The question is, What can we infer from these findings about the
origins of ethnic group differences? As any graduate student knows, the
source of individual differences in a trait cannot be taken as evidence
for the source of group differences in the same trait. A great deal of
indirect evidence points to both genetic and environmental contributions
to ethnic group differences in IQ. None of this evidence, however, is as
firm as the evidence for genetic influence on individual differences in
IQ. Many experts in the field (Snyderman & Rothman, 1988) agree with
Herrnstein and Murray when they state that "it seems highly likely to us
that both genes and the environment have something to do with racial
differences. What might the mix be? We are resolutely agnostic on the
issue; as far as we can determine, the evidence does not yet justify an
estimate" (p. 311).
Science, ethics, and social policy
The Bell Curve closes with a review of the policy implications of their
findings. What is the role of the social scientist in the formulation of
social policy? I agree with Kendler (1993) that it is clearly within the
scientific realm to comment on the likely consequences of competing
social policies. Judging the value, as opposed to the costs, of such
policies is, however, a matter of political rather than scientific
discourse. As Kendler documents, many social scientists confuse these two
functions. Herrnstein and Murray have been vigorously chastised for
discussing policy implications on the basis of the work reviewed and the
data analyzed in their book. Similar assertions are, however, regularly
made by many investigators in the social sciences. For example, the
implications of specific research projects are regularly found in grant
applications where they are used to justify the request for funds. Seldom
are the value judgments underlying these implications explicitly stated,
but they are easily inferred. Herrnstein and Murray have, in my opinion,
been much more "up front" about these matters than many social
scientists, and their discussions fall clearly within the boundaries
discussed by Kendler. They argue, for example, with regard to affirmative
action, "Our contribution (we hope) is to calibrate the policy choices
associated with affirmative action, to make costs and benefits clearer
than they usually are" (pp. 387-388).
In writing the Declaration of Independence, Jefferson was attempting to
give birth to a shared political goal--freedom, as expressed in the right
to life, liberty, and the pursuit of happiness. Herrnstein and Murray
also address this important theme. They make it clear that a meritocracy
need not be a Darwinian jungle and that a responsible society should make
a place for everyone. Their description of the ideal meritocracy will not
be to everyone's taste, but it is neither more foolish nor more naive
than many proposals that have been suggested in the past. Nevertheless,
predicting the future is an extremely hazardous enterprise. We have
recently seen the virtual collapse of a number of societies that were
based on a totally different conception of human nature than that
underlying The Bell Curve. Virtually no one predicted this dramatic
outcome for one of history's largest social experiments. Undoubtedly,
Herrnstein and Murray's arguments are wrong in some of the details, and
they may be wrong about the larger picture. Nevertheless, one of the
goals of the intellectual enterprise is to question received wisdom, to
ask difficult questions, and to seek novel and "better" solutions to both
new and old problems. They have succeeded admirably at this task.
This is a superbly written and exceedingly well-documented book. It
raises many troubling questions regarding the organization of our
society. It deserves the attention of every well-informed and thoughtful
Bouchard, T. J., Jr. (1993). The genetic architecture of human
intelligence. In P. A. Vernon (Ed.), Biological approaches to the study
of human intelligence (pp. 33-93). Norwood, NJ: Ablex.
Bouchard, T. J., Jr., Lykken, D. T., McGue, M., Segal, N. L., & Tellegen,
A. (1990). Sources of human psychological differences: The Minnesota
study of twins reared apart. Science, 250, 223-228.
Humphreys, L. G. (1988). Trends and levels of academic achievement of
Blacks and other minorities. Intelligence, 12, 231-260.
Kendler, H. H. (1993). Psychology and the ethics of social policy.
American Psychologist, 48, 1046-1053.
McGue, M., Bouchard, T. J., Jr., Iacono, W. G., & Lykken, D. T. (1993).
Behavior genetics of cognitive ability: A life-span perspective. In R.
Plomin & G. E. McClearn (Eds.), Nature, nurture and psychology (pp.
59-76). Washington, DC: American Psychological Association.
Meehl, P. E. (1970). Nuisance variables and the ex post facto design. In
M. Radner & S. Winokur (Eds.), Minnesota studies in the philosophy of
science IV (pp. 373-402). Minneapolis: University of Minnesota Press.
Pedersen, N. L., Plomin, R., Nesselroade, J. R., & McClearn, G. E.
(1992). A quantitative genetic analysis of cognitive abilities during the
second half of the life span. Psychological Science, 3, 346-353.
Scarr, S., & Weinberg, R. A. (1978). The influence of family background
on intellectual attainment. American Sociological Review, 43, 674-692.
Snyderman, M., & Rothman, S. (1988). The IQ controversy: The media and
public policy. New Brunswick, NJ: Transaction Books.
Waller, J. H. (1971). Achievement and social mobility: Relationship among
IQ score, education and occupation in two generations. Social Biology,
White, R. K. (1982). The relation between socioeconomic status and
academic achievement. Psychological Bulletin, 91, 461-481.
Soft Science With a Neoconservative Agenda
Review by Donald D. Dorfman
"Is there a danger that current welfare policies, unaided by eugenic
foresight, could lead to the genetic enslavement of a substantial segment
of our population? The possible consequences of our failure seriously to
study these questions may well be viewed by future generations as our
society's greatest injustice to Negro Americans" (Jensen, 1969, p. 95).
So said Arthur Jensen in 1969 in a Harvard Educational Review article on
race and general intelligence. General intelligence is often called IQ
for short. In the most controversial parts of The Bell Curve, a book
written for the general reader, Richard J. Herrnstein and Charles Murray
present much the same theories and general concerns as did Jensen with
regard to Black cognitive intelligence, while extending the analysis to
include Latinos. They greatly expand on the evidence, present possible
causal links between IQ and socially undesirable behaviors, and at the
end of the book suggest implications for public policy. They are
especially worried about a supposed downward pressure on the distribution
of IQ in the United States, which they call dysgenic pressure. Dysgenic
is a term borrowed from population biology. As does Jensen, the authors
believe that Blacks "are experiencing even more severe dysgenic pressures
than Whites" (p. 341). Part of the problem may be differences in
reproductive strategies among the races, according to J. Philippe
Rushton's theory discussed in the book (pp. 642-643). Herrnstein and
Murray mention Rushton's theory that Blacks have the largest genitals and
the highest frequency of sexual intercourse among the three major races
(p. 642). Consistent with customary academic standards of scholarly
objectivity and neutrality, Herrnstein and Murray reserve judgment on
whether Rushton is right or wrong: "We expect that time will tell whether
it [Rushton's theory] is right or wrong in fact" (p. 643).
In addition to supposed downward pressures on the distribution of
intelligence in this country produced by high fertility rates in Blacks,
Herrnstein and Murray believe that Latinos are also experiencing more
severe dysgenic pressures than Whites (p. 341) and that Latino
immigration is putting downward pressure on the distribution of American
national intelligence. So should we be worrying about dysgenic pressure
on our national cognitive intelligence? They conclude, "Putting the
pieces together--higher fertility and a faster generational cycle among
the less intelligent and an immigrant population that is probably
somewhat below the native-born average--the case is strong that something
worth worrying about is happening to the cognitive capital of the
country" (p. 364).
The authors present a large number of research analyses that they
performed themselves, in which they pit parental socioeconomic status
(SES) against IQ on a variety of economic and social behaviors. They
conclude that the major cause of economic and social behaviors is IQ, not
SES. The authors' research analyses are based on data collected in the
National Longitudinal Survey of Youth (NLSY). None of their research
analyses on the relation between IQ, SES, and social behaviors has ever
been published in peer-reviewed scientific journals. The Bell Curve is
written for the general reader and does not assume that the reader has
had a course in statistics. The authors have even included an appendix
for those readers who are sure they can not learn statistics, titled
"Statistics for People Who Are Sure They Can't Learn Statistics"
(Appendix 1, pp. 553-567). Scientists first publish their research in
peer-reviewed scientific journals, not in books written for the general
reader who may not have the technical background needed to detect flaws
in data and misinterpretations of data analyses. It is inappropriate for
a scientist to do otherwise.
Herrnstein and Murray's research analyses--never published in
peer-reviewed scientific journals--investigate the relation of IQ and SES
to marriage, to divorce, to illegitimacy, to welfare dependency, and to
parenting. They conclude that IQ is the primary problem, not SES: "People
with low cognitive ability tend to be worse parents" (p. 232). The
authors believe that low birth weight and high infant mortality are
indications of poor parenting and are probably caused by "prenatal
negligence" (p. 233) on the part of mothers with low cognitive ability
rather than inadequate prenatal medical care on the part of society. They
also present unpublished research analyses on the relation between crime
and low cognitive intelligence, and between civility and high cognitive
intelligence. "A smarter population is more likely to be, and more
capable of being made into, a civil citizenry" (p. 266), according to the
In the final part of The Bell Curve, titled "Living Together," Herrnstein
and Murray propose a solution to the supposed dysgenic downward pressures
on our national intelligence caused by the large number of children born
to "low-IQ women," and to the recent large-scale Latino immigrations to
the United States. They argue that America's current fertility policy
"subsidizes births among poor women, who are disproportionately at the
low end of the intelligence distribution" (p. 548). They seem to urge
eugenic foresight to counteract dysgenic pressure: "We urge generally
that these policies, represented by the extensive network of cash and
services for low-income women who have babies, be ended" (p. 548). With
regard to the supposed dysgenic effects of Latino immigration on national
intelligence, their central thought about immigration "is that present
policy assumes an indifference to the individual characteristics of
immigrants that no society can indefinitely maintain without danger" (p.
549). "But," they conclude, "we believe that the main purpose of
immigration law should be to serve America's interests" (p. 549). For
those members of groups who will not be excluded from the American dream
by eugenic foresight or new immigration laws, Herrnstein and Murray
propose "that group differences in cognitive ability, so desperately
denied for so long, can best be handled--can only be handled--by a return
to individualism" (p. 550).
Who are the authors of The Bell Curve? Are they right? The first author,
Richard Herrnstein, was a professor of psychology at Harvard University
for 36 years. He died a very short time ago. One would presume that The
Bell Curve represents Herrnstein's final summing up of a lifetime of
objective scholarly research published in peer-reviewed scientific
journals on the genetic basis of IQ. Regrettably, the media seem to be
totally unaware of the fact that the deceased Harvard professor never
published any scientific research on the genetic basis of IQ and its
relation to race, poverty, or social class in peer-reviewed scientific
journals in his entire 36-year academic career. Richard Herrnstein's
actual area of expertise is the experimental analysis of decision making
in pigeons and rats, and he never studied the genetic basis of any
behavior in those laboratory animals. The first presentation of his
theory on the genetic basis of IQ, social class, and poverty appeared in
a magazine article titled "I.Q." published in the September 1971 issue of
the Atlantic Monthly magazine. As we all know, scientists publish their
data and theories in peer-reviewed scientific journals or in
peer-reviewed technical books, not in popular magazines or in
nontechnical books written for the general reader.
In 1973, Herrnstein published a nontechnical Atlantic Monthly Press book
titled I.Q. in the Meritocracy that expanded on his theory of the genetic
basis of IQ and poverty. Herrnstein had never collected data on IQ, so
the book drew on the work of others, especially the "data" of Sir Cyril
Burt. According to Leslie Hearnshaw (1979), Burt's biographer and
distinguished historian of British psychology, Burt had probably invented
much of his highly cited data on the genetic basis of IQ. While doing
research on Burt's data for an article that I later published in Science
(1978), I discovered that Herrnstein had in fact laundered Burt's own
descriptions of Burt's widely publicized and highly cited study
"Intelligence and Social Mobility" (Burt, 1961). Burt had described his
own study "merely as a pilot inquiry" (p. 9) and his data as "crude and
limited" (p. 9). Burt had not even reported the number of subjects he had
tested in his crude and limited study. In describing Burt's study,
however, Herrnstein (1973) failed to tell the reader about the
deficiencies that Burt, himself, had mentioned. In addition, Herrnstein
(1973) said Burt's sample size was "1,000" (p. 203), later revising that
figure to "40,000" in response to criticism. In reply to a critical
letter by Jerry Hirsch (1975), Herrnstein (1975) revised his 1973 figure:
"It is true that Burt's sample was 40,000, not 1,000 as I said" (p. 436),
while failing to acknowledge that Burt had never reported the number of
subjects he had tested. Leon Kamin (1974) appears to have been the only
psychologist to notice and publicly report that Burt had failed to give
the sample size of his celebrated 1961 study of IQ and social mobility.
Presumably, Herrnstein and other psychologists who had publicized the
results of that study had never noticed that Burt had not reported the
sample size of his famous study.
The second author of The Bell Curve, Charles Murray, has a doctorate in
political science from the Massachusetts Institute of Technology and is
currently a Bradley Fellow with the American Enterprise Institute, a
conservative research group in Washington, DC. Murray often publishes his
research and theories in The Public Interest (e.g., Murray, 1994), a
neoconservative magazine edited by Irving Kristol, also a fellow of the
American Enterprise Institute and sometimes considered the founding
father of neoconservatism (Atlas, 1995). In an article recently published
in The Public Interest, Murray listed the first priority of his political
agenda: "And so I want to end welfare" (1994, p. 18). Inasmuch as the
media sometimes refer to The Bell Curve as Murray's book, perhaps the
book represents Murray's summing up of a body of objective scholarly
research that he had published in scientific journals on the genetic
basis of IQ and poverty. But like his coauthor Richard Herrnstein, Murray
has never conducted or published any research in scientific journals on
the genetic basis of IQ and poverty in his entire career.
The Bell Curve is not a scientific work. It was not written by experts,
and it has a specific political agenda. Still, it is possible that the
major scientific premises of the book may be correct. If two monkeys were
put before a typewriter, it is theoretically possible for those two
monkeys to produce a Shakespearean sonnet. Perhaps Herrnstein and Murray
produced a valid scientific work. I will now evaluate the major premises
of The Bell Curve.
The rewriting of history: The Burt affair
In 1972, Leon Kamin exposed the empirical unsoundness of the most
important evidence in support of the IQ hereditarian position, Sir Cyril
Burt's data (Hearnshaw, 1979). He later published his results in a book
attacking Burt's data as well as the secondary sources who publicized
those data (Kamin, 1974). In 1979, Leslie Hearnshaw (1979) published a
biography of Burt in which he concluded on the basis of personal diaries
and other material that it was highly likely that Burt had fabricated
some of his most celebrated data. Hearnshaw, distinguished historian of
British psychology, delivered the memorial address at Burt's Memorial
Service and was later asked by Marion Burt, Burt's sister, to write a
full-length biography of Burt. The result was the well-known Cyril Burt:
Psychologist (1979). In their discussion of the Burt affair, Herrnstein
and Murray suggest that some of Burt's "leading critics were aware that
their accusations were inaccurate" (p. 12), suggesting a possible
conspiracy against Burt. There is, however, no mention whatsoever of
Hearnshaw's book in their half-page synopsis of the Burt affair, and
Hearnshaw's book does not appear anywhere in their 57-page bibliography
of references. This misrepresentation of the Burt affair by omission of
important historical facts is not uniquely associated with The Bell
Curve. In 1982, Richard Herrnstein published an article in The Atlantic
Monthly in which he attacked the media for misrepresenting the evidence
in the IQ controversy (Herrnstein, 1982). In that magazine article, the
Harvard professor wrote "that most psychometricians had stopped trusting
Burt's data years before, partly because of inconsistencies first noted
in a 1974 article by Arthur Jensen" (p. 70), while omitting any mention
of Leo Kamin, the psychologist who in reality first noted inconsistencies
in Burt's data.
Does the distribution of IQs follow a bell curve?
The distribution of IQ test scores cannot be expected to follow a bell
curve unless it is constructed by the tester to do so (Dorfman, 1978).
The shape of the distribution of IQ test scores will depend on the
average difficulty of the test items as well as their intercorrelations.
The high item intercorrelations in IQ tests imply that the IQ
distribution can take a variety of shapes. The central limit theorem does
not apply to random variables with positive intercorrelations (Lamperti,
1966). Frederic Lord (1952), one of the fathers of modern test theory and
former president of the Psychometric Society, gave results on this
question: "The results given are sufficient to show that the distribution
of test scores cannot in general be expected to be normal, or even
approximately normal. The question naturally arises as to what possible
shapes the frequency distribution fs, as given in (76)
[Lord's Equation (76)], may assume. The answer is that this function may
assume any shape whatsoever, provided the item intercorrelations are
sufficiently high" (Lord, 1952, pp. 32-33). The symbol fs refers to
the distribution of test scores.
Does cognitive ability consist of a single general factor?
The book uses factor analysis to infer the existence of a single
hypothetical general factor of cognitive intelligence that is presumed to
account for most of cognitive performance. One of the problems with
factor analysis as a tool for determining the underlying structure of a
system is that neither the factors nor the loadings are uniquely defined
if you have more than one factor (Lawley & Maxwell, 1963), and it is
difficult to determine if you have only one factor. In experimental
cognitive psychology, factor analysis is virtually never used as a tool
to determine the underlying cognitive structure. It is a tool for
correlational cognitive psychology, not experimental cognitive
psychology. I inspected the subject index of some well-known texts in
experimental cognitive psychology and found that the term factor analysis
never appears in the subject index (e.g., see Anderson, 1985; Matlin,
1994; Reed, 1982). Why not? Kendall and Stuart (1966) may provide the
answer: "Application of the same technique [factor analysis] to physical
systems very often results in weighted sums of variables to which no
clear interpretation can be given" (p. 310). In short, "The main
difficulty, as a rule, is to know what the results mean" (p. 310),
Kendall and Stuart point out.
Can you measure the heritability of IQ?
The most direct way of estimating heritability is from data on
monozygotic twins reared apart (MZA) and separated in early infancy
(Bouchard, Lykken, McGue, Segal, & Tellegen, 1990). This MZA design
allows for the estimation of heritability if the following major
assumptions are met: (a) environments are a random sample from the
population of environments, (b) genotypes are a random sample from the
population of genotypes, (c) there is no genotype-environment
correlation, and (d) there is no genotype-environment interaction. If the
pairs of MZAs differ in age, then these assumptions will not be met. If
these assumptions are met, then the intraclass correlation between IQ
scores of MZA twin pairs directly measures heritability. Sir Cyril Burt's
(1966) study of 53 MZAs appears to have met the first three assumptions.
Unfortunately, Burt's data appear to have been invented (Hearnshaw,
1979). Bouchard et al.'s (Minnesota) survey of MZAs provides the next
best data set. Unfortunately, to the best of my knowledge, the detailed
case-study records of the Minnesota MZAs have never been released and
have therefore not been subjected to public scrutiny to determine the
degree to which assumptions have been met and the degree to which the
MZAs told the truth to the Minnesota group. Finally, if there is
genotype-environment interaction--then the fourth assumption is not
met--and heritability is undefined. But this is the most controversial
assumption underlying the MZA design. Herrnstein and Murray present no
convincing evidence to justify the fourth assumption.
Does high within-group heritability of IQ imply between-group
heritability of IQ?
The authors have made a fundamental error well-known by professional
geneticists. It is sometimes called "Jensen's error." Jensen made that
error in his famous 1969 Harvard Educational Review article. The critical
importance of that error was first clearly illuminated by Roger Milkman,
a professor of biology at the University of Iowa and a world authority on
population genetics and evolutionary biology. The article, "A Simple
Exposition of Jensen's Error," was published in the Journal of
Educational Statistics in 1978 (Milkman, 1978). Melvin Novick was editor
of that journal when Milkman's article was published. Novick, professor
of statistics and education at the University of Iowa at the time, later
became president of the Psychometric Society. What is Jensen's error? It
is that within-race heritability has no implications for between-race
heritability. The Bell Curve is therefore flawed with regard to inferring
between-race heritability in IQ from within-race heritability in IQ.
Does IQ or SES cause socially undesirable behaviors?
Herrnstein and Murray use logistic regression to determine which is more
important--IQ or SES--in determining socially undesirable behaviors.
Logistic regression is a form of regression in which the dependent
variable is binary. In all of their analyses, they assume a simple
additive model in which the logit (a transform of the sample proportion)
is assumed to equal B0 + B1IQ + B2SES + B3 age + random residual [numbers
after Bs should read as subscripts]. They assume no IQ-SES interaction.
They use the standardized beta weights to determine the relative importance
of IQ and SES in determining the probability of various undesirable or
desirable behaviors. Unfortunately, IQ and SES are highly intercorrelated
There are two major problems with Herrnstein and Murray's attempts to
determine whether IQ or SES is more important. First, there is the
collinearity problem. Weisberg (1985) describes the collinearity problem
in linear regression: "When the predictors are related to each other,
regression modeling can be very confusing. Estimated effects can change
magnitude or even sign depending on the other predictors in the model"
(p. 196). Next, there is the problem of deciding that the predictor with
the largest standardized beta weight is the most important. Weisberg
describes why this approach is faulty: "Unfortunately, this logic is
faulty because the scaling depends on the range of values for the
variables in the data" (p. 186). Perhaps these are the reasons why
Herrnstein and Murray never published their logistic analyses in
Were Herrnstein and Murray as lucky as the proverbial monkeys at a
typewriter? That depends on your point of view.
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Richard J. Herrnstein (deceased) was Edgar Pierce Professor of Psychology
at Harvard University (Cambridge, Massachusetts) and is author of IQ in
the Meritocracy and coauthor, with J. Q. Wilson, of Crime and Human
Nature and, with E. G. Boring, of A Sourcebook in the History of
Psychology. Herrnstein is coeditor, with R. C. Atkinson, G. Lindzey, and
R. D. Luce, of Stevens' Handbook of Experimental Psychology, Vol. 1:
Perception and Motivation, and Vol. 2: Learning and Cognition (2nd ed.);
and, with M. L. Commons, S. M. Kosslyn, and D. B. Mumford, of
Quantitative Analyses of Behavior, Vol. 8: Behavioral Approaches to
Pattern Recognition and Concept Formation, and Vol. 9: Computational and
Clinical Approaches to Pattern Recognition and Concept Formation.
Charles Murray, currently a Bradley Fellow at the American Enterprise
Institute, is author of Losing Ground: American Social Policy 1950-1980.
Thomas J. Bouchard, Jr., professor of psychology at the University of
Minnesota (Minneapolis) and director of the Minnesota Center for Twin and
Adoption Research, is immediate past president of the Behavior Genetics
Association and American Psychological Association Distinguished
Scientist Lecturer for 1995. Bouchard is author of the chapter "The
Genetic Architecture of Human Intelligence" in P. E. Vernon (Ed.)
Biological Approaches in the Study of Human Intelligence and of the
chapter "IQ Similarity in Twins Reared Apart: Findings and Responses to
Critics" in the forthcoming R. J. Sternberg and E. L. Grigorenko (Eds.)
Intelligence: Heredity and Environment. Bouchard is coeditor, with P.
Propping, of Twins as a Tool of Behavior Genetics.
Donald D. Dorfman, professor of psychology and radiology and faculty
member in the graduate program in applied mathematical and computational
sciences at the University of Iowa (Iowa City), is author of the chapter
"Group Testing" in S. Kotz and N. L. Johnson (Eds.) Encyclopedia of
Statistical Sciences, Vol. 3 and coauthor, with J. T. Cacioppo, of the
chapter "Waveform Moment Analysis: Topographical Analysis of Nonrhythmic
Waveforms" in J. T. Cacioppo and L. G. Tassinary (Eds.) Principles of
Psychophysiology: Physical, Social, and Inferential Elements.