The Bell Curve: Intelligence and Class Structure in American Life
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“MOTIVATION TO TRY.” Suppose that the nature of cultural bias does not lie in predictive validity or in the content of the items but in what might be called “test willingness.” A typical black youngster, it is hypothesized, comes to such tests with a mindset different from the white subject’s. He is less attuned to testing situations (from one point of view), or less inclined to put up with such nonsense (from another). Perhaps he just doesn’t give a damn, since he has no hopes of going to college or otherwise benefiting from a good test score. Perhaps he figures that the test is biased against him anyway, so what’s the point. Perhaps he consciously refuses to put out his best effort because of the peer pressures against “acting white” in some inner-city schools.
The studies that have attempted to measure motivation in such situations have generally found that blacks are at least as motivated as whites.33 But these are not wholly convincing, for why shouldn’t the measures of motivation be just as inaccurate as the measures of cognitive ability are alleged to be? Analysis of internal characteristics of the tests once again offers the best leverage in examining this broad hypothesis. Two sets of data seem especially pertinent.
The first involves the digit span subtest, part of the widely used Wechsler intelligence tests. It has two forms: forward digit span, in which the subject tries to repeat a sequence of numbers in the order read to him, and backward digit span, in which the subject tries to repeat the sequence of numbers backward. The test is simple in concept, uses numbers that are familiar to everyone, and calls on no cultural information besides knowing numbers. The digit span is especially informative regarding test motivation not just because of the low cultural loading of the items but because the backward form is twice as g-loaded as the forward form—that is, the backward form is a much better measure of general intelligence. The reason is that reversing the numbers is mentally more demanding than repeating them in the heard order, as readers can determine for themselves by a little self-testing.
The two parts of the subtest have identical content. They occur at the same time during the test. Each subject does both. But in most studies the black-white difference is about twice as great on backward digits as on forward digits.34 The question arises: How can lack of motivation (or test willingness or any other explanation of that type) explain the difference in performance on the two parts of the same subtest?35
A similar question arises from work on reaction time. Several psychometricians, led by Arthur Jensen, have been exploring the underlying nature of g by hypothesizing that neurologic processing speed is implicated, akin to the speed of the microprocessor in a computer. Smarter people process faster than less smart people. The strategy for testing the hypothesis is to give people extremely simple cognitive tasks—so simple that no conscious thought is involved—and to use precise timing methods to determine how fast different people perform these simple tasks. One commonly used apparatus involves a console with a semicircle of eight lights, each with a button next to it. In the middle of the console is the “home” button. At the beginning of each trial, the subject is depressing the home button with his finger. One of the lights in the semicircle goes on. The subject moves his finger to the button closest to the light, which turns it off. There are more complicated versions of the task (three lights go on, and the subject moves to the one that is farthest from the other two, for example), but none requires much thought, and everybody gets almost every trial “right.” The subject’s response speed is broken into two measurements: reaction time (RT), the time it takes the subject to lift his finger from the home button after a target light goes on, and movement time (MT), the time it takes to move the finger from just above the home button to the target button.36
Francis Galton in the nineteenth century believed that reaction time is associated with intelligence but could not prove it. He was on the right track after all. In modern studies, reaction time is correlated with the results from full-scale IQ tests; even more specifically, it is correlated with the g factor in IQ tests—in some studies, onlywith the g factor.37 Movement time is much less correlated with IQ or with g.38 This makes sense: Most of the cognitive processing has been completed by the time the finger leaves the home button; the rest is mostly a function of small motor skills.
Research on reaction time is doing much to advance our understanding of the biological basis of g.For our purposes here, however, it also offers a test of the motivation hypothesis: The consistent result of many studies is that white reaction time is faster than black reaction time, but black movement time is faster than white movement time.39 One can imagine an unmotivated subject who thinks the reaction time test is a waste of time and does not try very hard. But the level of motivation, whatever it may be, seems likely to be the same for the measures of RT and MT. The question arises: How can one be unmotivated to do well during one split-second of a test but apparently motivated during the next split-second? Results of this sort argue against easy explanations that appeal to differences in motivation as explanatory of the B/W difference.
UNIFORM BACKGROUND BIAS. Other kinds of bias discussed in Appendix 5 include the possibility that blacks have less access to coaching than whites, less experience with tests (less “testwiseness”), poorer understanding of standard English, and that their performance is affected by white examiners. Each of these hypotheses has been investigated, for many tests, under many conditions. None has been sustained. In short, the testable hypotheses have led toward the conclusion that cognitive ability tests are not biased against blacks. This leaves one final hypothesis regarding cultural bias that does not lend itself to empirical evaluation, at least not directly.
Suppose our society is so steeped in the conditions that produce test bias that people in disadvantaged groups underscore their cognitive abilities on allthe items on tests, thereby hiding the internal evidence of bias. At the same time and for the same reasons, they underperform in school and on the job in relation to their true abilities, thereby hiding the external evidence. In other words, the tests may be biased against disadvantaged groups, but the traces of bias are invisible because the bias permeates all areas of the group’s performance. Accordingly, it would be as useless to look for evidence of test bias as it would be for Einstein’s imaginary person traveling near the speed of light to try to determine whether time has slowed. Einstein’s traveler has no clock that exists independent of his space-time context. In assessing test bias, we would have no test or criterion measure that exists independent of this culture and its history. This form of bias would pervade everything.
To some readers, the hypothesis will seem so plausible that it is self-evidently correct. Before deciding that this must be the explanation for group differences in test scores, however, a few problems must be overcome. First, the comments about the digit span and reaction time results apply here as well. How can this uniform background bias suppress black reaction time but not the movement time? How can it suppress performance on backward digit span more than forward digit span? Second, the hypothesis implies that many of the performance yardsticks in the society at large are not only biased, they are all so similar in the degree to which they distort the truth—in every occupation, every type of educational institution, every achievement measure, every performance measure—that no differential distortion is picked up by the data. Is this plausible?
It is not good enough to accept without question that a general “background radiation” of bias, uniform and ubiquitous, explains away black and white differences in test scores and performance measures. The hypothesis might, in theory, be true. But given the degree to which everyday experience suggests that the environment confronting blacks in different sectors of American life is not uniformly hostile and given the consistency in results from a wide variety of cognitive measures, assuming that the hypothesis is true represents a considerably longer leap of faith than the much more limited assumption that race prejudice is still a factor in American life. In the matter of test bias, this brings us to th
e frontier of knowledge.
Are the Differences in Overall Black and White Test Scores Attributable to Differences in Socioeconomic Status?
This question has two different answers depending on how the question is understood, and confusion is rampant. We will take up the two answers and their associated rationales separately:
First version: If you extract the effects of socioeconomic class, what happens to the overall magnitude of the B/W difference? Blacks are disproportionately in the lower socioeconomic classes, and socioeconomic class is known to be associated with IQ. Therefore, many people suggest, part of what appears to be an ethnic difference in IQ scores is actually a socioeconomic difference.
The answer to this version of the question is that the size of the gap shrinks when socioeconomic status is statistically extracted. The NLSY gives a result typical of such analyses. The B/W difference in the NLSY is 1.21. In a regression equation in which both race and socioeconomic background are entered, the difference between whites and blacks shrinks to .76 standard deviation.40 Socioeconomic status explains 37 percent of the original B/W difference. This relationship is in line with the results from many other studies.41
The difficulty comes in interpreting what it means to “control” for socioeconomic status. Matching the status of the groups is usually justified on the grounds that the scores people earn are caused to some extent by their socioeconomic status, so if we want to see the “real” or “authentic” difference between them, the contribution of status must be excluded.42 The trouble is that socioeconomic status is also a result of cognitive ability, as people of high and low cognitive ability move to correspondingly high and low places in the socioeconomic continuum. The reason that parents have high or low socioeconomic status is in part a function of their intelligence, and their intelligence also affects the IQ of the children via both genes and environment.
Because of these relationships, “controlling” for socioeconomic status in racial comparisons is guaranteed to reduce IQ differences in the same way that choosing black and white samples from a school for the intellectually gifted is guaranteed to reduce IQ differences (assuming race-blind admissions standards). But the remaining difference is not necessarily more real or authentic than the one we start with. This seems to be a hard point to grasp, judging from the pervasiveness of controlling for socioeconomic status in the sociological literature on ethnic differences. But suppose we were asking whether blacks and whites differed in sprinting speed, and controlled for “varsity status” by examining only athletes on the track teams in Division I colleges. Blacks would probably still sprint faster than whites on the average, but it would be a smaller difference than in the population at large. Is there any sense in which this smaller difference would be a more accurate measure of the racial difference in sprinting ability than the larger difference in the general population? We pose that as an interesting theoretical issue. In terms of numbers, a reasonable rule of thumb is that controlling for socioeconomic status reduces the overall B/W difference by about a third.
Second version: As blacks move up the socioeconomic ladder, do the differences with whites of similar socioeconomic status diminish? The first version of the SES/IQ question referred to the overall score of a population of blacks and whites. The second version concentrates on the B/W difference within socioeconomic classes. The rationale goes like this: Blacks score lower on average because they are socioeconomically at a disadvantage in our society. This disadvantage should most seriously handicap the children of blacks in the lower socioeconomic classes, who suffer from greater barriers to education and occupational advancement than do the children of blacks in the middle and upper classes. As blacks advance up the socioeconomic ladder, their children, less exposed to these environmental deficits, will do better and, by extension, close the gap with white children of their class.
This expectation is not borne out by the data. A good way to illustrate this is by using our parental SES index and matching it against the mean IQ score, as shown in the figure below. IQ scores increase with economic status for both races. But as the figure shows, the magnitude of the B/W difference in standard deviations does not decrease. Indeed, it gets larger as people move up from the very bottom of the socioeconomic ladder. The pattern shown in the figure is consistent with many other major studies, except that the gap flattens out. In other studies, the gap has continued to increase throughout the range of socioeconomic status.43
Black IQ scores go up with socioeconomic status, but the black-white difference does not shrink
How Do African-Americans Compare with Blacks in Africa on Cognitive Tests?
This question often arises in the context of black-white comparisons in America, the thought being that the African black population has not been subjected to the historical legacy of American black slavery and discrimination and might therefore have higher scores. Many studies of African students in primary and secondary schools, in both urban and rural areas, have included cognitive ability tests. As in the United States, it has been demonstrated in Africa that the same test items that discriminate best among blacks discriminate best among whites and that the same factors that depress white scores (for example, coming from a rural area) depress black scores. The predictive validity of tests for academic and job performance seems to be about the same. In general, the psychometric properties of the standardized tests are the same for blacks living in Africa as for American blacks.44
It has been more difficult to assemble data on the score of the average African black than one would expect, given the extensiveness of the test experience in Africa. In the same review of the literature that permitted the above generalizations, for example—a thirty-page article followed by a bibliography of more than 200 titles—not a single average is reported.45 One reason for this reluctance to discuss averages is that blacks in Africa, including urbanized blacks with secondary educations, have obtained extremely low scores. Richard Lynn was able to assemble eleven studies in his 1991 review of the literature. He estimated the median black African IQ to be 75, approximately 1.7 standard deviations below the U.S. overall population average, about ten points lower than the current figure for American blacks.46 Where other data are available, the estimates of the black African IQ fall at least that low and, in some instances, even lower.47 The IQ of “coloured” students in South Africa—of mixed racial background—has been found to be similar to that of American blacks.48
In summary: African blacks are, on average, substantially below African-Americans in intelligence test scores. Psychometrically, there is little reason to think that these results mean anything different about cognitive functioning than they mean in non-African populations. For our purposes, the main point is that the hypothesis about the special circumstances of American blacks depressing their test scores is not substantiated by the African data.
Is the Difference in Black and White Test Scores Diminishing?
The answer is yes with (as usual) some qualifications.
IQ TEST DATA. The most straightforward way to answer the question would be to examine the repeated administrations of the same IQ tests to comparable populations, but large, nationally representative IQ data are not produced every year (or even every decade). The NLSY data are among the most recent for a young adult population, and they have a B/W difference toward the high end of the range. The only post-1980 study reporting black and white adult averages that we have found is the renorming of the Wechsler Adult Intelligence Scale (WAIS-R) in 1981 in which the difference between blacks and a sample of whites (that apparently did not try to discriminate between Latino and Anglo whites) was 1.0 standard deviation.49
Recent data on children tell opposite stories. In a review of IQ tests of children conducted since 1980, Ken Vincent of the University of Houston reports results for four normative studies that showed a B/W difference of only seven IQ points for the Ravens Standard Progressive Matrices (SPM) and the Kaufman Assessment Battery for Children (K-ABC).50 Two other stud
ies involving the Stanford-Binet IV found B/W differences often points for children ages 7 to 11 and twelve points for children ages 2 to 6.51 Qualifications must be attached to these findings. The B/W difference on the K-ABC normative sample has in particular been subjected to reexamination suggesting that the diminished gap largely reflected psychometric and statistical artifacts.52 Nonetheless, the data on children that Vincent reviews may be read as encouraging. The most impressive of the findings is the comparatively small B/W difference of only seven IQ points on the Ravens SPM administered to 12-year-olds. This finding corresponds to Jensen’s 1992 study of black and white children in an upper-middle-class setting in which the difference on the Ravens SPM was similarly below the norm (a deficit corresponding to ten IQ points).53
In contrast to Vincent’s optimistic conclusions, the NLSY shows a growing rather than a shrinking gap in the next generation of blacks and whites. As discussed in Chapter 15, the B/W difference between NLSY children is currently wider than the B/W difference separating their mothers.
ACADEMIC APTITUDE AND ACHIEVEMENT TESTS. The most extensive evidence of a narrowing black-white gap can be found in longitudinal data from the National Assessment of Educational Progress (NAEP), the American College Testing (ACT) examination, the SAT, a comparison of the 1972 and 1980 national high school surveys, and some state-level achievement test data. We review the NAEP and the SAT here, and others (which tell the same story) in Appendix 5.