Race differences in IQ: Hans Eysenck's contribution to the debate in the light of subsequent research

Abstract

Hans Eysenck was one of the earliest protagonists in the controversy over race and intelligence. He believed that the observed variability in IQ scores is genetically determined to a high degree (80% heritability) and that, in consequence, the Black–White IQ gap in the US is due predominantly to genetic factors. Subsequent investigations have confirmed that IQ is indeed heritable, though at a level substantially below 80%, and a deeper understanding of population genetics has shown that race differences in IQ could be determined entirely by environmental factors even if its heritability were as high as Eysenck believed it to be. Several lines of research, notably racial admixture studies, racial crossing studies involving interracial parenting or adoption, and especially investigations using more recent techniques of molecular genetics, have provided evidence suggesting that the Black–White IQ gap is not determined significantly by genetic factors.

Full text

Race Differences in IQ 1
Colman, A. M. (2016). Race differences in IQ: Hans Eysenck’s contribution to the debate in the light
of subsequent research. Personality and Individual Differences, 103, 182–109. doi:
10.1016/j.paid.2016.04.050
Race differences in IQ: Hans Eysenck’s contribution to the debate in the light of
subsequent research
Andrew M. Colman
School of Psychology, University of Leicester, Leicester LE1 7RH, United Kingdom
ABSTRACT
Hans Eysenck was one of the earliest protagonists in the controversy over race and intelligence. He
believed that the observed variability in IQ scores is genetically determined to a high degree (80%
heritability) and that, in consequence, the Black–White IQ gap in the US is due predominantly to
genetic factors. Subsequent investigations have confirmed that IQ is indeed heritable, though at a
level substantially below 80%, and a deeper understanding of population genetics has shown that race
differences in IQ could be determined entirely by environmental factors even if its heritability were as
high as Eysenck believed it to be. Several lines of research, notably racial admixture studies, racial
crossing studies involving interracial parenting or adoption, and especially investigations using more
recent techniques of molecular genetics, have provided evidence suggesting that the Black–White IQ
gap is not determined significantly by genetic factors.
Keywords: Black–White IQ gap; Genetics; Genome-wide association study; Heritability; H. J.
Eysenck; Intelligence; Racial admixture; Racial crossing
1. Introduction
To many people, Hans Eysenck’s name is principally associated with certain claims
that he first published in 1971 about the heritability of intelligence and race differences in IQ
scores. This article will begin with personal reminiscences and impressions of Eysenck the
man, and it will then review and comment critically on his views on race and intelligence in
the light of what we now know. Although there are aspects of IQ, heritability, and race
differences that remain obscure, much has been learnt since the 1970s, and I believe that
Eysenck’s interpretation of these issues is hardly tenable today.
I met Eysenck for the first time in 1970, soon after arriving in the UK as an immigrant
from South Africa. I was 26 years old and had managed to secure a lectureship at the
University of Leicester in the English Midlands. The psychiatrist Griffith Edwards, a
distinguished authority on alcohol and drug addiction, introduced me to Eysenck. Edwards
had visited South Africa in 1966, where I met him at the home of my maternal uncle Harold
Cooper, a consultant psychiatrist in Cape Town. I was intrigued to learn that Edwards worked
at the Institute of Psychiatry in London, because I knew that Eysenck was based there. I had
read many of Eysenck’s journal articles and most of his early books, starting with Sense and
Nonsense in Psychology, a book that I stumbled on while I was at school and that played a
major part in my decision to study psychology at university. It turned out that Edwards knew
Eysenck quite well, and he offered to introduce me to him if I ever found myself in London.
As soon as I arrived in 1970, I got in touch with Edwards, and he made arrangements for me
to visit the Institute in south London, where I had lunch with Eysenck and Mike Berger,

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another young South African psychologist working there who went on to become a Professor
of Psychology at Royal Holloway, University of London.
Although Eysenck was more than twice my age and already the most prominent
psychologist in Europe—indeed, one of the most prominent in the world—and among the
most highly cited researchers across all the social and behavioural sciences (Garfield, 1978;
Rushton, 2001), he was polite and friendly, and he interacted with me without any trace of
condescension. I found him much more down-to-earth and easy to talk to than I had expected.
His surprisingly marked German accent seemed to add gravitas to everything he said. His
most striking personal characteristic was the sureness of his opinions on all issues. I found his
self-confidence inspiring and reassuring, but in retrospect, for reasons that will become clear,
I believe that he was overconfident, at least on some topics. He was obviously highly
intelligent and widely read, with many aesthetic preferences and tastes that I shared, and I
warmed to him immediately. At that initial meeting, I even felt relaxed enough to raise the
issue of the South African-born psychologist Arnold Lazarus, a relative of mine then working
at Yale University who had recently been ejected from the editorial board of Eysenck’s
journal, Behaviour Research and Therapy, in spite of being one of the founders of behaviour
therapy and the person who had introduced the term into the scientific literature (Lazarus,
1958). Without hesitation or the slightest hint of embarrassment, Eysenck replied that
Lazarus was in the process of developing an entirely different approach (later to be called
multimodal therapy), that he no longer subscribed to the mission statement of the journal, and
that he ought really to have resigned without having to be asked.
One topic that did not come up during our first meeting was the debate over race and
intelligence. The US psychologist Arthur Jensen (1969) had recently published an article
partly on the subject, propounding a hereditarian interpretation. It had stirred up a hornet’s
nest of controversy, and I was in the process of writing a critical response to it. What I did not
know was that Eysenck was simultaneously writing a book, Race, Intelligence and Education
(Eysenck, 1971), defending Jensen’s position. Jensen turned out to be a close friend of
Eysenck’s who had worked as a postdoctoral researcher at the Institute of Psychiatry in the
1950s. Eysenck’s book presented the hereditarian interpretation in a more accessible and
polemical style than Jensen’s rather technical article. When my own article eventually
appeared (Colman, 1972), it incorporated a critique of Eysenck’s recently published book. He
revisited the topic in later publications (Eysenck, 1991; Eysenck & Kamin 1981), and I
updated and expanded my counter-arguments (Colman 1987, 1990).
Eysenck’s characteristically well-written and persuasive book was published a few
months after our first meeting. As soon as it appeared, Peter Broadhurst, the Head of the
Psychology Department at the University of Birmingham, invited me to participate in a
public debate with Eysenck. The debate took place at the university on 28 January 1972,
preceded by a lunch at which Eysenck chatted openly and disarmingly about professional and
personal matters, addressing his comments not only to his old friend and colleague Peter
Broadhurst, but also to me and the two other academics with whom he was shortly to cross
swords in debate, the Jamaican-born sociologist Stuart Hall and the geneticist David Jones,
neither of whom he had met before. What was remarkable, in the circumstances, was his
candour and openness—further evidence, I believe, of his self-confidence. I recall him
mentioning that his son Michael, who had graduated with a degree in psychology several
years earlier and had begun lecturing at Birkbeck College, had still not completed his PhD.
“What can I do?” he asked us in despair. (Michael Eysenck completed his PhD soon after and
went on to become a very successful research psychologist in his own right.)
In the debate itself, Eysenck presented his hereditarian point of view quietly,
forcefully, and plausibly, while we three adversaries flailed about ineffectually, trying to
refute his arguments. That was when I discovered that Eysenck was not only the most

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engaging writer in psychology but also the most brilliant public speaker. It is fair to say that
he won the debate hands down. The rest of us had brought voluminous notes with us, but
Eysenck spoke from a handful of bullet points scribbled on a tiny scrap of paper—I could
see, because I was sitting right next to him. He brushed our criticisms aside politely but
firmly, quoting published and unpublished sources and research findings from memory, using
arguments that seemed scientific and irrefutable. Nevertheless, I believed at the time that he
was wrong, and having followed with interest subsequent scientific developments in the field,
I have even stronger grounds for believing that now. At the risk of appearing to serve out the
tennis match after the opponent has left the court, I shall try to explain how the hereditarian
interpretation gradually collapsed under the weight of accumulating evidence and a
deepening understanding of behaviour genetics, although a few researchers with entrenched
hereditarian views continued to believe it (e.g., Lynn, 2006; Rushton & Jensen, 2005, 2006).
2. Eysenck’s arguments and evidence
Black Americans score about 15 points (one standard deviation) lower, on average,
than White Americans on cognitive ability or IQ tests (Cottrell, Newman, & Roisman, 2015;
Neisser et al., 1996; Roth, Bevier, Bobko, Switzer, & Tyler, 2001), although this Black–
White IQ gap, as it has come to be called, may be narrowing slightly since Eysenck wrote his
book (Dickens & Flynn, 2006; Mackintosh, 2011; Nisbett, 2005; Nisbett et al. 2012). In
interpreting the Black–White IQ gap, Eysenck (1971) followed Jensen’s (1969) arguments
quite closely, and his later writings on the subject (Eysenck, 1973; Eysenck & Kamin 1981)
added no significant additions or modifications. In the paragraphs that follow, I discuss his
arguments, quoting from his original book; I comment critically on them, drawing freely
from my previous published evaluations (Colman, 1972, 1987, 1990); and I summarize some
important evidence that has come to light more recently.
2.1. Disclaimers
Eysenck (1971. p. 8) began with a plea for scientific evidence over dogma: “I can
only draw attention to what has been done, and warn against over-interpreting data that admit
of different ways of looking at their implications.” He added that this approach: “is not likely
to appeal to those who feel they already know the truth. The problems associated with race
are difficult enough when viewed calmly and from the scientific point of view; they become
completely impossible of solution when emotion is allowed to enter in.”
Realizing that he might be accused of racism, Eysenck (1971, pp. 9–10) declared his
own social and political attitudes and beliefs:
“My recognition of the importance of the racial problem, and my own attitudes of
opposition to any kind of racial segregation, and hatred for those who suppress any
sector of the community on grounds of race (or sex, or religion) were determined in
part by the fact that I grew up in Germany, at a time when Hitlerism was becoming
the very widely held doctrine which finally prevailed and led to the death of several
million Jews whose only crime was that they belonged to an imaginary ‘race.’ ”
However, he added (p. 11), we have to face up to the scientific facts:
“A benevolent attitude towards non-white races, coupled with admiration for their
many outstanding qualities, and deep sympathy for their suffering, should not blind
one towards such evidence as may exist to indicate that with respect to certain
qualities there may be genetic differences favouring one race (or ethnic subgroup) as
against another.”

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In any case, he pointed out (p. 11), acknowledging the existence of race differences in IQ
does not amount to racism:
“I am not a racist for believing it possible that negroes may have special innate gifts
for certain athletic events, such as sprints, or for certain musical forms of expression. .
. . Nor am I a racist for seriously considering the possibility that the demonstrated
inferiority of American negroes on tests of intelligence may, in part, be due to genetic
causes.”
This last comment betrays a patronizing stereotype of African Americans as fleet-
footed but dull-witted bongo drummers or spiritual singers. However, I believe that what
inspired Eysenck to write his book was not any prejudice against Black people but rather a
distinctive enjoyment of argument and controversy. Jensen’s (1969) article had whipped up a
blizzard of controversy in the US: members of the American Psychological Association had
petitioned (unsuccessfully) to expel him, students had disrupted his classes at the University
of California, Berkeley, and a bodyguard had been hired to protect him. As he described in
the preface to his book Genetics and Education (Jensen, 1972), he and his family received
death threats, and he was accused of being a racist and a fascist. When Eysenck’s book was
published, the reaction in the UK was almost as emotional though not as violent, as he
described in his autobiography (Eysenck, 1990, pp. 216–220). He was also accused of being a
racist and a fascist, but this could hardly have come as a surprise to him. In fact, he admitted
decades later in his autobiography that it was precisely the reaction to Jensen’s article that
had inspired him to write his book: “This book was written because of the considerable
uproar caused by the publication, in 1969, of an invited article by Arthur Jensen” (p. 215). I
am quite certain, having known Eysenck for many years, that his book, and more generally
his views on race and intelligence, were not motivated by racism. He was neither a racist nor
a fascist—his political views were liberal, and on some issues even left-wing. But there was
nothing that he enjoyed more than acrimonious and heated debate, and he was always in his
element when embroiled in controversy. It is worth noting that he came from a theatrical
background: his parents were both thespians, and his grandmother who brought him up was
an opera singer.
2.2. Scientific consensus
In a move that has become more familiar in recent debates on climate change,
Eysenck (1971, p. 15) claimed that the vast majority of experts shared his views: “I would be
prepared to assert that experts (real experts, that is) would agree with at least 90% of what I
am going to say—probably the true figure would be a good deal higher, but there is no point
in exaggerating.”
In fact, Eysenck was exaggerating wildly because, even in 1971, many real experts
disagreed, and he knew this. Literally scores of criticisms of the hereditarian interpretation,
many of them from leading researchers, had already been published by the time his book
appeared, even in the same journal as Jensen’s article; in fact, Eysenck (1971) mentioned in
his book “seven eminent authorities, critical of Jensen’s thesis” (p. 29) who had already
commented in that journal. What is more, a few years earlier, in a prominent publication that
Eysenck could hardly have overlooked, Pettigrew (1964) had reported that a survey of the
literature had uncovered only three psychologists who had stated in print that genetic factors
were significantly involved in the observed race differences in IQ. Pettigrew, undeniably a
“real expert” himself, believed that the existing evidence pointed to “a non-genetic
interpretation of the typically lower IQ test score averages of Negro groups” (pp. 104–105).
My claim that Eysenck was exaggerating about the consensus of expert opinion is really an
understatement.

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2.3. A priori assumption of a race difference in IQ
Eysenck (1971, p. 20) next advanced a kind of a priori argument:
“Th[e] myth of racial equality, while more acceptable in principle to any liberal and
well-meaning person than its opposite, is still a myth: there is no scientific evidence to
support it. Indeed, as Jensen has pointed out, the a priori probability of such a belief is
small: ‘The fact that different racial groups in this country have widely different
geographic origins and have had quite different histories which have subjected them
to different selective social and economic pressures makes it highly likely that their
gene pools differ for some genetically conditioned behavioural characteristics,
including intelligence or abstract reasoning ability. Nearly every anatomical,
physiological, and biochemical system investigated shows racial differences. Why
should the brain be an exception?’ ”
What is most seductive about this argument is that it appears to settle the issue
without the bother of examining the empirical evidence. If it is obvious from first principles
that there must be genetic race differences in IQ, then evidence seems hardly necessary. But,
in fact, most anatomical, physiological, and biochemical systems, including human brains, do
not show racial differences. When Eysenck wrote his book it was already known, as an
authoritative review of research findings concluded, that “there is no acceptable evidence for
. . . difference in the brains of these two racial groups [Black and White Americans], and
certainly nothing which provides a satisfactory anatomical basis for explaining any
differences in IQ or in other mental or performance tests, in temperament or in behaviour”
(Tobias, 1970, p. 22). Later neuroimaging research (reviewed by McDaniel, 2005) suggests a
small but consistent relationship between brain volume and intelligence, especially in frontal
and parietal brain areas (Jung & Haier, 2007). Nevertheless, research using modern methods
of molecular genetics, advanced brain imaging technology, and large sample sizes, has failed
to detect any relation between genes, brain anatomy, and IQ, or between genes, brain
anatomy, and race (Balaresque, Ballereau, & Jobling, 2007; Mekel-Bobrov et al., 2007;
Richardson, 2011; Timpson, Heron, Smith, & Enard, 2007).
Accumulating evidence since the sequencing of the human genome in 2003 suggests
that genetically determined race differences in IQ are a priori unlikely, rather than likely. We
now know that there are approximately three billion nucleotide base pairs in the haploid
human genome, and direct assessment of genetic variation has revealed that the average
proportion of these bases that differ between a human being and a chimpanzee is less than
2%; that the difference between a randomly chosen pair of human beings is approximately
0.1%; and that only 10% of that 0.1%, hence 0.01% of human DNA, differs between
European, African, and Asian populations (Barbujani & Colonna, 2010; Jorde & Wooding,
2004)—far less than had previously been assumed. Given that the human genome comprises
approximately 22,000 genes, this might be taken to imply that only about 22 genes differ
between populations, but that would be quite wrong. For one thing, 98% of human DNA
consists of noncoding regions (Elgar & Vavouri, 2008); and second, genes within the coding
regions typically contain thousands of DNA bases that show slight variations in allele
frequencies between populations. The parts of the human genome that differ systematically
between racial groups include coding regions containing genes that influence skin colour,
hair type, and facial features; but race, like beauty, is evidently only skin deep, and the total
genetic difference must be much smaller than had previously been assumed. Furthermore,
recent research has revealed that racial admixture has blurred whatever genetic differences
might have existed previously. Within the United States, White Americans are descended
predominantly from European populations and Black Americans from West African slaves;

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but the latest and best evidence, using high-density genotype data, shows that the proportion
of European ancestry in the Black American population is as high as 24% (Bryc, Durand,
Macpherson, Reich, & Mountain, 2015). Taken together, these findings on genetic race
differences render it unlikely, a priori, that Black and White Americans have any genetically
based psychological characteristics that distinguish them sharply.
In addition to this, it is worth reminding ourselves that “race is a socially constructed
concept, not a biological one” (Sternberg, Grigorenko, & Kidd, 2005, p. 49). In the United
States, people of visibly mixed Black and White descent tend to be classified as Black or
African American (Ho, Sidanius, Levin, & Banaji, 2011)—a phenomenon called
hypodescent—whereas in most parts of Latin America, a person with even a small amount of
recognizable European ancestry tends to be classified as White (Skidmore & Smith, 2005, p.
152). This further reduces the a priori likelihood of significant genetically based race
differences, although it is going too far to infer that it makes them literally impossible, as
Gould (1986) and others have claimed. But even two relatively homogeneous populations
seem hardly likely to show very marked genetic differences, especially on complex traits like
intelligence that are now known to be influenced by very many genes scattered across the
entire genome, each contributing only a small effect (Davies et al., 2011; Kirkpatrick,
McGue, Iacono, Miller, & Basu, 2014). Given so many genes determining IQ differences,
and the fact that each of the implicated genes contributes so little, it seems unlikely that a
large race difference in IQ could arise from the tiny DNA difference of only a 0.01%. It
follows, fortiori, that two socially defined groups with substantial genetic admixture, such as
Black and White Americans, are even less likely to show marked differences. Darwin (1871,
p. 225), whose powers of naturalistic observation are legendary, suspected as much a century
and a half ago, when he wrote: “It may be doubted whether any character can be named
which is distinctive of a race and is constant.”
The a priori argument in favour of the hereditarian interpretation of the Black–White
IQ gap is evidently not as persuasive as it may appear. But no firm conclusions can be
reached by a priori reasoning alone. The question is empirical, and the evidence therefore
needs to be examined, as Eysenck himself always insisted.
2.4. Heritability of IQ scores and race differences
The central plank in Eysenck’s argument relates to the heritability of IQ scores. On
this pivotal issue, Eysenck (1971, p. 25) quoted from Jensen’s article:
“Individual differences in intelligence—that is, IQ—are predominantly attributable to
genetic differences, with environmental factors contributing a minor portion of the
variance among individuals. The heritability of the IQ—that is, the percentage of
individual differences variance attributable to genetic factors—comes out at about 80
per cent, the average value obtained from all relevant studies now reported.”
Eysenck (p. 117) linked this to race differences as follows:
“The argument is simply that this discovery of a strong genetic involvement in the
determination of individual differences in IQ between members of a given population
is an essential precondition for going on to argue in favour of the genetic
determination (in part at least) of racial differences in IQ. For clearly if all within-race
differences could be accounted for in environmental terms, we would have no
business to look further than that in our search for between-race differences.”
Eysenck (1970, p. 30) assumed that, if IQ variability in the general population is due
mainly to heredity, then it is “a not unreasonable hypothesis that genetic factors are strongly

Race Differences in IQ 7
implicated in the average negro–white intelligence difference” (again quoting Jensen). This
argument is not only pivotal but also slightly technical, so some explanation is in order. The
heritability of a trait in a specified population is the proportion of the variance in the trait that
is attributable to genetic differences between individuals. The heritability of IQ is therefore
the proportion of the variance in IQ scores that is attributable to genetic variance in the
population, variance being a measure of the degree of variability or scatter in a set of scores.
This has nothing whatever to do with how much of an individual’s IQ is determined by
genes: that is a meaningless question, because without the requisite genes there could be no
brain, and without a brain, no IQ. The broad heritability (h2) of IQ is the proportion of the
phenotypic or measurable variance in IQ scores (VP) that is attributable to genetic variance
(VG) in a particular population at a particular time. Hence, h2 (broad) = VG/VP, and because it
is a proportion, it ranges from 0 (observed variance entirely attributable to non-genetic
factors) to 1 (observed variance entirely attributable to genetic factors), although it is often
expressed as a percentage.
Eysenck had a high opinion of his own mathematical prowess and was not shy about
saying so, even in print (see, e.g., Eysenck, 1990, pp. 26, 53, 55, 56–57, 95, 117), but I
believe that his talents lay in writing, public speaking, and perhaps managing organizations,
rather than in mathematics. He certainly did not understand the concept of heritability. In
particular, he failed to grasp the fundamental point that heritability is a parameter of a
population, with no meaningful application to an individual. He (Eysenck, 1971, p. 71) wrote:
“Another qualification, not often mentioned, is that the figure of 80% heritability is an
average; it does not apply equally to every person in the country. For some people
environment may play a much bigger part than is suggested by this figure; for others
it may be even less.”
The reason why this is “not often mentioned” is that it is not true: a heritability value
is certainly not an average. What is more, the variance of an individual score is invariably
zero. As I pointed out immediately (Colman, 1972, p. 145), it follows that Eysenck was being
far too modest in claiming that 80% of an individual person’s IQ is attributable to genetic
factors. He could have pointed out that 200% is, because (200/100) × 0 = 0; but then the
nonsense would have been obvious.
One problem with the argument from heritability is that the estimate of h2 (broad) =
.80, on which both Jensen (1969) and Eysenck (1971) based their argument, is now known to
be too high. Over many decades, three classic methods were used to provide estimates of the
heritability of IQ: studies of separated identical twins, family studies, and adoption studies.
All three provide indirect estimates, and all have limitations that I will not address here (see,
e.g., Colman, 1987, 1990; Mackenzie, 1984; Neisser et al., 1996). More recently, direct
methods based on genome-wide association studies (GWAS) have become feasible, and they
have yielded direct estimates of heritability between .40 and .50 in one study (Davies et al.,
2011) and .35 in the other (Kirkpatrick et al., 2014). Population geneticists now believe that
the heritability of IQ usually lies somewhere between .35 and .70, depending on various
features of the sample being investigated; for example, it tends to be larger in older
populations and smaller among children, and it is close to zero at the lowest socioeconomic
status levels (Mackintosh, 2011, chap. 11; Nisbett et al., 2012).
The most serious problem with the argument from heritability is that it uses data about
the heritability of IQ within populations to draw conclusions about the genetic basis of
differences between populations. The argument is now known to be invalid, as Lewontin
(1970) demonstrated with the following famous Gedankenexperiment (thought experiment).
Take two handfuls of seed from the same sack and plant them in separate plots, the first rich

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and the second deficient in nutrients. The plants in the first plot will grow tall and those in the
second will be stunted. The average difference between the two groups of plants will be due
entirely to environmental factors (nutrients), because the seeds come from the same sack and
are therefore genetically the same. But the variability within each plot will be due entirely to
genetic factors, because the environment is identical for all seeds within the same plot. In
other words, heritability is 100% within each group, but the average difference between the
two groups is due entirely to environmental factors. Lewontin’s Gedankenexperiment shows
that, even if the heritability of IQ were as high as Eysenck and Jensen believed it to be, the
differences between racial groups could be caused entirely by non-genetic factors.
Eysenck’s (and Jensen’s) argument from heritability is fallacious, because heritability
within populations does not imply that differences between populations are caused by genetic
factors. As explained in a leading text on population genetics:
“Sometimes the argument is made that because a trait is heritable within two different
populations that differ in their mean trait value, then the average trait differences
between the populations are also influenced by genetic factors (e.g., Herrnstein and
Murray 1994). Because heritability is a within-population concept that refers to
variances and not to means, such an argument is without validity. Indeed, heritability
is irrelevant to the biological causes of mean phenotypic differences between
populations.” (Templeton, 2006, p. 285)
2.5. Racial admixture and crossing studies
Eysenck (1971) was adamant that “the evidence is circumstantial” and that “we must
look at many lines of research rather than at any single, decisive experiment” (p. 30).
However, among the classic studies there is a line of evidence, mentioned by Eysenck (pp.
98–99) only in passing, in relation to skin colour and without citing any US data, that yields
direct evidence. Racial admixture studies capitalize on the fact, mentioned earlier, that Black
Americans have varying amounts of European ancestry. This makes possible “the most direct
test of genetic versus environmental hypotheses” (Mackenzie, 1984, p. 1224). If the Black–
White IQ gap is mainly due to genetic factors, then those Black Americans with the most
European ancestry, and therefore the most European genes, should have higher average IQ
scores than those with no European ancestry.
The first study to use this method (Witty & Jenkins, 1935) focused on 63 children
with the highest IQ scores among 8,000 Black American children in the Chicago public
school system. When the researchers classified these high-IQ children according to their
ancestry as reported by their parents, they found no evidence that they had any more
European ancestry than a comparison group of ordinary Black Americans. For example, the
results showed that 14.3% of the high-IQ children had predominantly White ancestry,
compared to 14.8% of the comparison group. If genetic factors determine the Black–White
IQ gap to any significant extent, then there should be substantially more children with
predominantly White ancestry in the high-IQ group. In fact, the distribution of White
ancestry was remarkably similar in both groups of children, and the brightest child in the
entire sample, a girl with an exceptionally high IQ of 200, was one of those whose parents
reported no knowledge of any White ancestry at all.
This admixture evidence is devastating to the hereditarian interpretation of the Black–
White IQ gap. The Witty and Jenkins (1935) study has been criticized on the grounds that
parental reports do not provide entirely accurate measures of European ancestry and that the
comparison group was not ideally matched with the 63 high-IQ children. Nevertheless, if the
large Black–White IQ gap were significantly caused by genetic factors, then there would
have to have been far more children with predominantly White ancestry in the high-IQ group

Race Differences in IQ 9
than in the comparison group. This follows from the recently discovered fourth law of
behaviour genetics (Chabris, Lee, Cesarini, Benjamin, & Laibson, 2015), according to which
human behavioural traits are generally associated with very many genes, each contributing
only a small part of the effect. Direct evidence for this law in relation to IQ (Davies et al.,
2011; Kirkpatrick et al., 2014) has already been mentioned in this article. It follows that if the
Black–White IQ gap were due predominantly to genetic factors, then this would have to have
shown up, with large differences between groups, even in the rather crude admixture study
reported by Witty and Jenkins.
Later studies, using more objective methods for estimating racial admixture and in
some cases much larger samples, yielded the same result. Scarr, Pakstis, Katz and Barker
(1977) used 43 blood group markers to estimate European ancestry in a sample of 362 Black
American schoolchildren in Minnesota and found no significant correlation between
European admixture and scores on any of the five separate intelligence tests that they
administered. Furthermore, the children with the most European ancestry did not differ
significantly from those with the least. In fact, there was a marginally significant tendency (p
< .10) for the children with the most European ancestry to score lower on one of the tests
(paired associates). These findings corroborate those of Witty and Jenkins (1935) in failing to
find any relationship between degree of European ancestry and IQ scores in the Black
American population. They provide strong evidence suggesting that the Black–White IQ gap
is not predominantly attributable to genetic differences.
Loehlin, Vandenberg and Osborne (1973) used blood group markers to estimate the
degree of European ancestry in two groups of Black American adolescents (N = 40 and N =
44). There were no significant correlations between European ancestry and IQ scores in either
group. In one of the groups, the correlation was almost exactly zero, and in the other it was
non-zero but the non-significant difference was in the opposite direction—suggesting
marginally lower IQ scores for adolescents with more European ancestry.
Moore (1986) measured the IQ scores of 46 Black and mixed-race US children who
had been adopted by either Black or White middle-class parents. The half-White children
turned out to have virtually the same average IQ as the Black children, suggesting that having
50% European genes provided no advantage to the mixed-race children. However, both Black
and mixed-race children had IQ scores 13 points higher, on average, when they were adopted
by White parents than by Black parents, demonstrating that non-genetic environmental
factors had an effect on IQ large enough to account for almost the entire Black–White IQ
gap. Racial admixture studies have clearly failed to support the hereditarian interpretation
of the Black–White IQ gap. Instead, they have provided evidence that seems to contradict it.
Even Rushton and Jensen (2005) acknowledged that “blood groups distinguishing African
from European ancestry did not predict IQ scores in Black samples” (p. 262), but on the same
page they claimed inexplicably that “studies of racial hybrids are generally consistent with
the genetic hypothesis, [although] to date they are not conclusive”. Nisbett (2005, p. 309)
responded to Rushton and Jensen with what seems like a more reasonable conclusion: “The
most directly relevant research concerns degree of European ancestry in the Black
population. There is not a shred of evidence in this literature, which draws on studies having
a total of five very different designs, that the gap has a genetic basis.”
Racial crossing studies provide evidence of a different kind, and they have also failed
to support the hereditarian interpretation. These studies avoid the complications and
ambiguities involved in estimating White ancestry by focusing instead on Black, White, and
mixed-race children who, for one reason or another, happen to be raised in the same or at
least similar environments. If genetic factors are mainly responsible for the Black–White IQ
gap, then in these circumstances Black children should have lower average IQ scores than

Race Differences in IQ 10
mixed-race children, and mixed-race children should have lower scores than White children.
On the other hand, if non-genetic factors are all-important, then the Black, mixed-race, and
White children should have similar average IQ scores.
It is difficult or impossible to find children from different racial groups raised in
identical environments, and that may appear to expose this line of research to methodological
criticism. The most obvious objection is that it is literally impossible to raise Black, White,
and mixed-race children in identical environments if racism itself is a significant
environmental factor. If Black children in such a study score lower than White children, then
critics can always argue that the supposedly similar environments actually favoured the
White children, because they were not exposed to the negative environmental effects of anti-
Black racism. But this objection turns out to be entirely irrelevant, because Black and mixed-
race children do not, in fact, score lower than White children raised in similar environments.
In the racial crossing studies that have been published, children from the different racial
groups have, in almost every case, been found to achieve remarkably similar average IQ
scores. Eyferth (1961) studied Besatzungskinder (occupation children) who were the
illegitimate offspring of sexual liaisons between American (and a few French) occupation
troops and German women in Germany after the Second World War. Some of the children’s
American fathers were Black and some were White. All the children were raised by White
mothers or foster parents in post-war Germany, where anti-Black prejudice was largely
absent, because there had been no significant Black presence in Germany before the war. The
two groups were matched for location, school, and mothers’ circumstances. The average IQ
of the mixed-race children (96.5), who inherited half their genes from their Black fathers,
differed by less than one point from the average IQ of the White children (97.2), whose
genetic heritage was entirely European. In other words, children whose genomes were 50%
African American did not score significantly lower on IQ tests than those with exclusively
European genomes raised in similar environmental circumstances. Even Jensen (1998, p.
483) had to concede that this result is: “consistent with a purely environmental hypothesis of
the racial difference in test scores.”
The Minnesota Transracial Adoption Study (Scarr & Weinberg, 1976) focused on 145
Black, mixed-race, and White children between 4 and 12 years old raised in middle-class
White adoptive homes. The three groups of children shared essentially similar environments
including, importantly, any effects of having been adopted. The average IQ scores of the
three groups were 96.8, 109.0, and 111.5, respectively. There was no significant difference
between the mixed-race and White means, apparently contradicting the hereditarian
prediction, but the Black children scored significantly lower, on average, than the others. The
study was flawed, as the researchers acknowledged in their original article, because the Black
children had been adopted later in life and had therefore spent less time in their adoptive
homes when they were tested, and both their natural and adoptive parents were less well-
educated than those of the mixed-race and white children. Thus only the mixed-race and
White children were raised in reasonably comparable environmental circumstances, and the
researchers concluded that the similar average IQ scores of these two groups “support the
view that the social environment plays a dominant role in determining the average IQ level of
black children” (p. 739). If the Black–White IQ gap were determined predominantly by
genetic differences, then there would have to have been a large and significant difference
between the mixed-race and White children, but there was not.
It is remarkable that Jensen (1981) managed to cite this study in support of his
hereditarian interpretation. On page 224 of this book, he listed the mean IQ scores of the
children as follows: “Black/black adoptees: 96.8”, “White/black adoptees: 109.0”, and
“Natural children of adoptive parents: 116.7”, suggesting that the mixed-race children

Race Differences in IQ 11
(“White/black adoptees”) did indeed score significantly below the White children (“Natural
children of adoptive parents”). But the appropriate White group was the White adopted
children, whose mean score of 111.5 was hardly different from the mean of the mixed-race
group, not the irrelevant “natural children of the adoptive parents”—the biological offspring
of the parents in the study who happened to have natural as well as adopted children in their
homes. These natural children had not been adopted and had not experienced any of the
environmental deprivation of the adopted children, and their mean IQ was far above the
others. By omitting the appropriate adopted White group and substituting an irrelevant non-
adopted White group to compare with the adopted mixed-race and adopted Black children,
Jensen was able to convey the misleading impression that the children’s IQ scores increased
in an orderly fashion in line with the proportion of White genes that they had inherited. This
enabled him (p. 224) to conclude: “The relative differences among all these averages appear
to be consistent with a genetic hypothesis.” It seems ironic that this devious manoeuvre
appeared in a book entitled Straight Talk About Mental Tests!
The children were re-studied 10 years later when most were teenagers (Weinberg,
Scarr, & Waldman, 1992), and the pattern of results was generally similar. As usually
happens in longitudinal follow-up studies, the researchers did not manage to retest all
children from the earlier study, and unfortunately the White adopted group suffered attrition
of some of its lowest-scoring children, causing the mean IQ score for that group to be
significantly higher in the follow-up (117.6) than in the original study (111.5), whereas the
mean IQ scores of the mixed-race (109.5) and Black (95.4) children were not significantly
different from their original means—actually slightly lower, because of test renormalization.
This introduced a further methodological flaw into the follow-up, making interpretation
difficult. Hierarchical regression analyses showed that the IQ scores of the three groups of
adopted children were not significantly different after adjusting for pre-adoption measures.
Nevertheless, it is striking that the Black children had shown no increase in IQ after spending
many years being raised in middle-class White homes, and this seems more consistent with a
hereditarian than an environmental interpretation of the Black–White IQ gap, as Lynn (1994)
and Rushton and Jensen (2005), among others, were keen to point out. Taking everything into
consideration, especially the non-comparability of the Black children to the others in the
study, Weinberg, Scarr and Waldman (p. 133) concluded: “The results of the longitudinal
follow-up continue to support the view that the social environment maintains a dominant role
in determining the average IQ level of black and interracial [mixed-race] children.”
Racial admixture and crossing studies are uniquely important in the debate over the
interpretation of the Black–White IQ gap, because they offer the most relevant evidence on
the putative genetic origin of the gap, and none of them has provided persuasive support for
the hereditarian interpretation proffered by Eysenck. After a comprehensive review of these
studies, Nisbett (2009) went as far as to conclude that they provide no evidence whatsoever in
favour of this interpretation. On the contrary, racial admixture and crossing studies, and
especially studies using advanced techniques of molecular genetics, all provide rather
compelling evidence against the hereditarian interpretation.
Discussion and conclusion
Eysenck (1971, p. 7) began his discussion of race and intelligence with disarming
humility: “Most people who write on this topic seem to know all the answers, and are firmly
convinced that their point of view is correct; I know perfectly well that we do not know all
the answers, and feel little confidence that such views as I have formed are necessarily
correct.” He quoted Jensen’s cautious-seeming suggestion of a “not unreasonable hypothesis
that genetic factors are strongly implicated in the average negro-white intelligence
difference” (p. 30). It is revealing to trace the gradual evaporation of doubt and caution as

Race Differences in IQ 12
Eysenck’s book progresses: “there may be genetic differences favouring one race (or ethnic
subgroup) as against another” (p. 11); “individual differences in intelligence—that is, 1Q—
are predominantly attributable to genetic differences, with environmental factors contributing
a minor portion of the variance among individuals” (p. 25); “All the evidence to date suggests
the strong and indeed overwhelming importance of genetic factors in producing the great
variety of intellectual differences which we observe in our culture, and much of the
difference observed between certain racial groups” (p. 130); “the fact [is] that negroes show
some degree of genetic inferiority” (p. 142). What starts out as a “not unreasonable
hypothesis”, cautiously advanced by a writer who does not claim to know all the answers, is
progressively transmogrified into a simple “fact” of “genetic inferiority” backed up by “all
the evidence to date”.
Even when his book was written all those years ago, the proposition that the Black–
White IQ gap is predominantly attributable to genetic factors was debatable, and in fact I and
many others debated it at the time. We now have much stronger evidence and a deeper
understanding of population genetics, and it seems clear that the hereditarian interpretation is
not supported by the data. The one study that has been claimed to provide a scrap of
supportive evidence, namely the Minnesota Transracial Adoption Study (Scarr & Weinberg,
1976; Weinberg, Scarr, & Waldman, 1992), is interpreted by its own researchers and many
commentators as providing no such evidence. The other most relevant studies all appear to
contradict the hereditarian interpretation.
I believe that Eysenck’s (1971) ideas on this topic were seriously mistaken. The
Black–White IQ gap appears to be caused, not predominantly by genetic factors, but by
differences in socio-economic status, together with educational, demographic, cultural, and
other non-genetic factors, some of which have been discovered and at least partly understood.
An exhaustive discussion of these factors is clearly beyond the scope of this article (see
Dickens & Flynn, 2001, for a review and theoretical interpretation). It is nevertheless worth
commenting on one phenomenon, namely the Flynn effect, that illustrates how powerfully
non-genetic factors can influence IQ scores and that may also provide a crucial key to the
Black–White IQ gap. Flynn (1987) discovered large increases in IQ scores, typically three IQ
points per decade, that have been occurring in both White and Black American populations
since the introduction of IQ tests in the US in 1916 and in many other populations around the
world. These rapid IQ gains must be due to non-genetic factors, because natural selection—
the mechanism that underlies genetic changes in populations—is so gradual that differences
are imperceptible from one generation to the next. Flynn (2007, 2010, 2012, pp. 132–141)
suggested that the effect could be explained by cultural diffusion during the 20th century of
scientific “habits of mind”, including especially abstract and hypothetical reasoning, and that
this may also explain the Black–White IQ gap, in part at least. Scientific modes of reasoning
can be acquired only by sustained effort, and if there are aspects of the Black cultural ethos
that devalue or discourage such cognitive challenges, or that fail to engender as much
motivation in Black as in White families to encourage the intellectual effort required to
acquire them, then that may account for the gap.
The issue of race differences in IQ scores is interesting from a purely scientific point
of view, and it also carries a great deal of political and moral baggage. Eysenck (p. 12)
claimed: “I found it very difficult to look at the evidence detailed in this book with a detached
mind, in view of the fact that it contradicted certain egalitarian beliefs I had considered
almost axiomatic.” It is commendably rational to accept repugnant facts when empirical
evidence demands their acceptance. But maintaining unpleasant doctrines when the evidence
does not support them is no form of rationality; it is mere intellectual masochism—a form of
self-abuse, in the proper sense of that word. In Eysenck’s case, because there were dangerous
and reactionary interest groups and prejudiced individuals all too keen to capitalize on his

Race Differences in IQ 13
authoritative endorsement of the hereditarian doctrine for their own sinister ends, I believe
that it was also ill-advised.
Acknowledgements
I am grateful to Philip Corr, James Flynn, Ken Hughes, Mark Jobling, Lynn Jorde, Richard
Nisbett, Briony Pulford, and Caroline Salinger for helpful comments on the first draft of this article,
and to Nick Mackintosh (1935–2015) who helped me, over many years, to understand the trickier
twists and turns of the race and intelligence debate.
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