The observed ethnic differences in IQ could be explained solely by the environment if the mean environment of whites is 1.58 standard deviations better than the mean environment of blacks and .32 standard deviation worse than the mean environment for East Asians, when environments are measured along the continuum of their capacity to nurture intelligence.76 Let’s state these conclusions in percentile terms: The average environment of blacks would have to be at the 6th percentile of the distribution of environments among whites, and the average environment of East Asians would have to be at the 63rd percentile of environments among whites, for the racial differences to be entirely environmental.
Environmental differences of this magnitude and pattern are implausible. Recall further that the B/W difference (in standardized units) is smallest at the lowest socioeconomic levels. Why, if the B/W difference is entirely environmental, should the advantage of the “white” environment compared to the “black” be greater among the better-off and better-educated blacks and whites? We have not been able to think of a plausible reason. An appeal to the effects of racism to explain ethnic differences also requires explaining why environments poisoned by discrimination and racism for some other groups—against the Chinese or the Jews in some regions of America, for example—have left them with higher scores than the national average.
Environmental explanations may successfully circumvent these problems, but the explanations have to be formulated rather than simply assumed. Our initial objective is to warn readers who come to the discussion with firmly held opinions on either side. The heritability of individual differences in IQ does not necessarily mean that ethnic differences are also heritable. But those who think that ethnic differences are readily explained by environmental differences haven’t been tough-minded enough about their own argument. At this complex intersection of complex factors, the easy answers are unsatisfactory ones.
Reasons for Thinking that Genetic Differences Might Be Involved
Now we turn to some of the more technical arguments, beginning with those that argue for some genetic component in group differences.
PROFILE DIFFERENCES BETWEEN WHITES AND EAST ASIANS. Races differ not just in average scores but in the profile of intellectual capacities. A full-scale IQ score is the aggregate of many subtests. There are thirteen of them in the Wechsler Intelligence Scale for Children (WISC-R), for example. The most basic division of the subtests is into a verbal IQ and a performance IQ. In white samples, the verbal and performance IQ subscores tend to have about the same mean, because IQ tests have been standardized on predominantly white populations. But individuals can have imbalances between these two IQs. People with high verbal abilities are likely to do well with words and logic. In school they excel in history and literature; in choosing a career to draw on those talents, they tend to choose law or journalism or advertising or politics. In contrast, people with high performance IQs—or, using a more descriptive phrase, “visuospatial abilities”—are likely to do well in the physical and biological sciences, mathematics, engineering, or other subjects that demand mental manipulation in the three physical dimensions or the more numerous dimensions of mathematics.
East Asians living overseas score about the same or slightly lower than whites on verbal IQ and substantially higher on visuospatial IQ. Even in the rare studies that have found overall Japanese or Chinese IQs no higher than white IQs (e.g., the Stevenson study of Japanese, Taiwanese, and Minnesotans mentioned earlier),77 the discrepancy between verbal and visuospatial IQ persists. For Japanese living in Asia, a 1987 review of the literature demonstrated without much question that the verbal visuospatial difference persists even in examinations that have been thoroughly adapted to the Japanese language and, indeed, in tests developed by the Japanese themselves.78 A study of a small sample of Korean infants adopted into white families in Belgium found the familiar elevated visuospatial scores.79
This finding has an echo in the United States, where Asian-American students abound in engineering, in medical schools, and in graduate programs in the sciences, but are scarce in law schools and graduate programs in the humanities and social sciences. Most people reflexively assume that this can be explained by language differences. People who did not speak English as their first language or who grew up in households where English was not the language of choice choose professions that are not so dependent on fluent English, we often hear. But the explanation becomes less credible with every passing year. Philip Vernon, after reviewing the evidence on Asian-Americans, concluded that unfamiliarity with the English language and American culture is a plausible explanation only for the results of the early studies. Contemporary studies of Asian-Americans who are thoroughly acculturated also show the typical discrepancy in verbal and visuospatial abilities. American Indians and Inuit similarly score higher visuospatially than verbally; their ancestors migrated to the Americas from East Asia hundreds of centuries ago.80 The verbal-visuospatial discrepancy goes deeper than linguistic background.
Vernon’s overall appraisal was that the mean Asian-American IQ is about 97 on verbal tests and about 110 on visuospatial tests.81 Lynn’s 1987 review of the IQ literature on East Asians found a median verbal IQ of 98 and a median visuospatial IQ of 106.82 As of 1993, for Asian-American students who reported that English was the first language they learned (alone or with another language), the Asian-American SAT mean was .21 standard deviation above the national mean on the verbal test and .43 standard deviation above the national mean on the math test. Converted to an IQ metric, this amounts to a 3.3 point elevation of mathematical scores over verbal scores for the high IQ Asian-American population that takes the SAT83
Why do visuospatial abilities develop more than verbal abilities in people of East Asian ancestry in Japan, Hong Kong, Taiwan, mainland China, and other Asian countries and in the United States and elsewhere, despite the differences among the cultures and languages in all those countries? Any simple socioeconomic, cultural, or linguistic explanation is out of the question, given the diversity of living conditions, native languages, educational resources, and cultural practices experienced by Hong Kong Chinese, Japanese in Japan or the United States, Koreans in Korea or Belgium, and Inuit or American Indians. We are not so rash as to assert that the environment or the culture is wholly irrelevant to the development of verbal and visuospatial abilities, but the common genetic history of racial East Asians and their North American or European descendants on the one hand, and the racial Europeans and their North American descendants, on the other, cannot plausibly be dismissed as irrelevant.
PROFILE DIFFERENCES BETWEEN WHITES AND BLACKS. Turning now to blacks and whites (using these terms to refer exclusively to Americans), ability profiles have also been important in understanding the nature, and possible genetic component, of group differences. The argument has been developing around what is known as Spearman’s hypothesis.84 This hypothesis says that if the B/W difference on test scores reflects a real underlying difference in the general mental ability, g, then the size of the B/W difference will be related to the degree to which the test is saturated with g.85 In other words, the better a test measures g, the larger the black-white difference will be. Arthur Jensen began to explore this possibility when he looked at the pattern of subtest scores on the WISC-R, taking advantage of the fact that the WISC-R has thirteen subtests, each measuring a somewhat different skill. Converting their statistical procedures into a more easily understood form, here is the logic of what Arthur Jensen and his coauthor, Cyril Reynolds, did.86
On average, low-SES whites get lower test scores than high-SES whites. But suppose you were to go through a large set of white test scores from a low-SES and a high-SES group and pull out everyone with an overall IQ score of, say, 105. Now you have identical scores but very different SES groups. The question becomes, What does the pattern of subtest scores look like? The answer is, The same. Once you equalize the overall IQ scores, low-SES and high-SES whites also had close-to-identical mean scores on the indivi
dual subtests.
Now do the same exercise with blacks and whites. Again, let us say that you pull all the tests with a full-scale IQ score of exactly 105. Again, you examine the scores on the subtests. But this time the pattern of subtest scores is not the same for blacks and whites, even though the subtests add up to the identical overall score.87 Despite identical overall scores, whites are characteristically stronger than blacks on the subtests involving spatial-perceptual ability, and blacks are characteristically stronger than whites in subtests such as arithmetic and immediate memory, both of which involve retention and retrieval of information.88 As Jensen and Reynolds note, the pattern of subtest differences between whites and blacks differs sharply from the “no differences” result associated with SES. This directly contradicts the hypothesis that the B/W difference reflects primarily SES differences.89 What accounts for the different subtest profiles? Jensen and Reynolds proceeded to demonstrate that the results are consistent with Spearman’s hypothesis. Whites and blacks differ more on the subtests most highly correlated with g, less on those least correlated with g.
Since that initial study using the WISC-R, Jensen has been assembling studies that permit further tests of Spearman’s hypothesis. He concluded from over a dozen large and representative samples of blacks and whites90 that “Spearman’s hypothesis has been borne out significantly by every study (i.e., 13 out of 13) and no appropriate data set has yet been found that contradicts Spearman’s hypothesis.”91 There appears to be no dispute with his summary of the facts. It should be noted that not all group differences behave similarly. For example, deaf children often get lower test scores than hearing children, but the size of the difference is not positively correlated with the test’s loading on g.92 The phenomenon seems peculiarly concentrated in comparisons of ethnic groups.
Jensen’s most recent work on Spearman’s hypothesis uses reaction time tests instead of traditional mental tests, bypassing many of the usual objections to intelligence test questions. Once again, the more g-loaded the activity is, the larger the B/W difference is, on average.93 Critics can argue that the entire enterprise is meaningless because g is meaningless, but the hypothesis of a correlation between the magnitude of the g-loading of a test and the magnitude of the black-white difference on that test has been confirmed.94
How does the confirmation of Spearman’s hypothesis bear on the genetic explanation of ethnic differences? In plain though somewhat imprecise language: The broadest conception of intelligence is embodied in g. Anything other than g is either a narrower cognitive capacity or measurement error. Spearman’s hypothesis says in effect that as mental measurement focuses most specifically and reliably on g, the observed black-white mean difference in cognitive ability gets larger.95 At the same time, g or other broad measures of intelligence typically have relatively high levels of heritability.96 This does not in itself demand a genetic explanation of the ethnic difference, but by asserting that “the better the test, the greater the ethnic difference,” Spearman’s hypothesis undercuts many of the environmental explanations of the difference that rely on the proposition (again, simplifying) that the apparent black-white difference is the result of bad tests, not good ones.
Arguments Against a Genetic Explanation
The ubiquitous Arthur Jensen has also published the clearest evidence that the disadvantaged environment of some blacks has depressed their test scores. He found that in black families in rural Georgia, the elder sibling typically has a lower IQ than the younger.97 The larger the age difference is between the siblings, the larger is the difference in IQ. The implication is that something in the rural Georgia environment was depressing the scores of black children as they grew older.98 In neither the white families of Georgia, nor white or black families in Berkeley, California, are there comparable signs of a depressive effect of the environment.
But demonstrating that environment can depress cognitive development does not prove that the entire B/W difference is environmental, and in this lies an asymmetry between the contending parties in the debate. Those who argue that genes might be implicated in group differences do not try to argue that genes explain everything. Those who argue against them—Leon Kamin and Richard Lewontin are the most prominent—typically deny that genes have anything to do with group differences, a much more ambitious proposition.
CONFRONTING SPEARMAN’S HYPOTHESIS. If one is to make this case against a genetic factor on psychometric grounds, the data supporting Spearman’s hypothesis must be confronted. There are two ways to do so: dispute the fact itself or grant the fact but argue that it does not mean what Jensen says it does.
The most searching debate about Spearman’s hypothesis was conducted in a journal that publishes both original scholarly works and commentaries on them, Behavioral and Brain Sciences, where, in two separate issues in the latter 1980s, thirty-six experts in the relevant fields commented on Jensen’s evidence.99 A number of comments were favorable and provided further support for Jensen’s conclusion. Others were critical, for reasons that varied from the philosophical (research into such hurtful issues is not useful) to the highly technical (were Jensen’s results the result of varying reliabilities among the tests?). We summarize them in the notes, but the striking feature was that no commentator was able to dispute the empirical claim that the racial gap in cognitive performance scores tends to be larger on tests or activities that draw most on g.100
Several years after the exchange on Spearman’s hypothesis in Behavioral and Brain Sciences, Jan-Eric Gustafsson presented some data finding a considerably smaller correlation than Jensen and others do between g loading and B/W differences on a group of subtests.101 It is not clear why Gustafsson obtained these atypical results, but, as of this writing, they are still atypical. We have found no others for representative groups of blacks and whites. Our own appraisal of the situation is that Jensen’s main contentions regarding Spearman’s hypothesis are intact and constitute a major challenge to purely environmental explanations of the B/W difference.
CULTURAL EXPLANATIONS. Another approach has been taken by Jane Mercer, a sociologist and the developer of the System of Multicultural Pluralistic Assessment (SOMPA). Tests are artifacts of a culture, she argues, and a culture may not diffuse equally into every household and community. In a heterogeneous society, subcultures vary in ways that inevitably affect scores on IQ tests. Fewer books in the home means less exposure to the material that a vocabulary subtest measures; the varying ways of socializing children may influence whether a child acquires the skills, or a desire for the skills, that tests test; the “common knowledge” that tests supposedly draw on may not be common in certain households and neighborhoods.
So far, this sounds like a standard argument about cultural bias, and yet Mercer accepts the generalizations that we discussed earlier about internal evidence of bias.102 She is not claiming that less exposure to books means that blacks score lower on vocabulary questions but do as well as whites on culture-free items. Rather, she argues, the effects of culture are more diffuse. Her argument may be seen as a variant of the “uniform background radiation” hypothesis that we discussed earlier.
Furthermore, she points out, strong correlations between home or community life and IQ scores are readily found. In a study of 180 Latino and 180 non-Latino white elementary school children in Riverside, California, Mercer examined eight sociocultural variables: (1) mother’s participation in formal organizations, (2) living in a segregated neighborhood, (3) home language level, (4) socioeconomic status based on occupation and education of head of household, (5) urbanization, (6) mother’s achievement values, (7) home ownership, and (8) intact biological family. She then showed that once these sociocultural variables were taken into account, the remaining correlation between ethnic group and IQ among the children fell to near zero.103
The problem with this procedure lies in determining what, in fact, these eight variables control for: cultural diffusion, or genetic sources of variation in intelligence as ordinaril
y understood? Recall that we pointed out earlier that controlling for socioeconomic status typically reduces the B/W difference by about a third. To the extent that parental socioeconomic status is produced by parental IQ, controlling for socioeconomic status controls for parental IQ. One obvious criticism of SOMPA is that it broadens the scope of the control variables to such an extent that the procedure becomes meaningless. After the correlations between the eight sociocultural variables and IQ are, in effect, set to zero, little difference in IQ remains among her ethnic samples. But what does this mean? The obvious possibility is that Mercer has demonstrated only that parents matched on IQ will produce children with similar IQs—not a startling finding.
Mercer points out that the samples differ on the sociocultural variables even after controlling for IQ. The substantial remaining correlations indicate that “important amounts of the variance in sociocultural characteristics [are] unexplained by IQ,”104 evidence, she says, that they may be treated as substantially independent of IQ.105 But they are, in fact, not independent of IQ. They remain correlated. Her basic conclusion that “there is no justification for ignoring sociocultural factors when interpreting between-group differences in IQ” seems to us unchallengeable. 106 In the next chapter, we will present other examples of ethnic differences in social behavior that persist after controlling for IQ. But to conclude that genetic differences are ruled out by her analysis is unwarranted, because she cannot demonstrate that a family’s sociocultural characteristics are independent of their IQ.107
The Bell Curve: Intelligence and Class Structure in American Life Page 35
