Explaining Race Differences in IQ The Logic , the Methodology , and the Evidence

The claim that there are genetic racial differences in IQ has been based mainly on two grounds: the evidence for high within-race heritability for IQ and the failure of various environmental explanations to account completely for mean racial differences. However, neither of these has direct relevance to the question of whether race differences in IQ have mainly a genetic or mainly an environmental origin. The assumption that these factors are relevant and that they support a genetic account is criticized as the "hereditarian fallacy. " The choice between a genetic and an environmental account of race differences is most properly based on "jointly genetic/environmental designs," which control for both genetic and environmental differences in a behavior genetic framework. The available evidence from such designs tends to support an environmental over a genetic account, although the evidence is not presently sufficient and gives little insight into what the relevant environmental differences might be. Identification of those environmental differences may require turning away from the traditional "heredity versus environment" question and focusing on the detailed causal influences on the development of intellectual level generally. In a number of articles in the past few years, I have criticized the logic, methodology, and assumptions involved in some of the attempts to demonstrate the likelihood of genetic racial differences in IQ (Mackenzie, 1979, 1980a, 1980b). Along with several other authors (e.g., Feldman & Lewontin, 1975; Flynn, 1980; Lewontin, 1970, 1975; Vetta, 1975, 1980), I have shown that in several cases the evidence and reasoning are not merely inadequate but irrelevant to the issues involved. Like Vetta (1975, 1980), but unlike the other authors cited, I have not tried to answer the substantive question of whether or not there are genetic factors involved in the measured race differences in mean IQ, particularly between the black and white populations in the United States. Instead, I have focused solely on the faults in some of the reasoning and evidence that have been presented on the question. This "agnostic" approach may seem restrictive and narrowly methodological, but it does have some advantages. The scientific issues involved in the debate over IQ and race are complex and technical, although some of the positions that have been advanced are simplistic and ideological. Even technical arguments and criticisms often seem to be developed and put forward on an ad hoc basis, because they can be used to justify an already favored position. To the extent that this impression is justified, it is very hard to assess the relevance and importance of particular arguments that are offered, even if we can accept their validity. An analysis of the logic and methodology in the area can throw more light on how the substantive questions may possibly be resolved, if it is not already tied to one particular resolution of them. In the present article I will extend these methodological analyses and discuss their significance for the problem of explaining black-white IQ differences. In particular, I will try to show that (a) much of the debate on the issue has suffered from confusion over what relevance most of the evidence has for genetic versus environmental explanatory hypotheses; (b) contrary to the claims of some writers, the inadequacy of the specific environmental hypotheses so far put forward does not provide support for a general genetic hypothesis; (c) the particular genetic hypotheses so far put forward have little or no scientific weight; (d) there is only a small body of existing evidence suitable for testing a general environmental hypothesis against a general genetic one; (e) although this body of evidence tends to support an environmental hypothesis, it is inadequate both in size and in quality to resolve the issue; (f) evidence potentially sufficient to resolve the issue can be gathered with currently available methods; (g) the general question of genetics versus environment is an extremely limited form of the question of how to explain race differences in IQ; and (h) progress in dealing with less limited forms of the question will be aided by closer relationships between theories of cognition, behavior genetic analysis, and research designs that focus on environmental manipulation and assessment. In criticizing the main arguments and evidence for genetic racial differences in IQ, I will focus 1214 November 1984 • American Psychologist Copyright 1984 by the American Psychological Association, Inc, Vol. 39. No. I I , 1214-1233 largely on the writings of Arthur Jensen. It is appropriate to focus on Jensen's statements because they contain the clearest, fullest, and most influential presentation of the hypothesis of genetic racial differences in IQ. For this same reason, a thorough criticism specifically of Jensen's position may have more general applicability. However, I will not be trying to show that Jensen in particular holds an untenable position on questions of race and IQ. Most of the arguments and evidence that Jensen puts forward have been offered by others as well, as I will show. Furthermore, Jensen's own position on the question is a more modest one than many critics have maintained. In a recent book he explains that one of his "chief aims" has been merely to show that the causes of race differences in IQ constitute "an unsettled question for which our present evidence cannot justify the pretense of a definitive answer"; he has proposed and argued for a hypothesis of genetic racial differences in IQ, he says, mainly in order to provide "goad and grist for scientific action" (Jensen, 198la, p. 214). Three Kinds of Evidence Throughout, I will assume the reliability of the general finding that blacks in the United States score, on average, about 15 points lower than whites on a variety of IQ tests. Equally, I will not dispute the conclusions emphasized by Jensen (1980) that the difference is greater on nonverbal than on verbal tests, that it is greater on culture-reduced than on culture-loaded ones, and that item analyses reveal few, if any, significant race-by-item interactions. The issue is whether this 15-point mean difference is due almost entirely to genetic differences between blacks and whiles, due almost entirely to environmental differences, or due to a significant amount of both. It may be noted that few, if any, serious researchers support the first of these positions, that the IQ difference is all or almost all genetic. Instead, those who maintain that there is a genetic difference in IQ usually support the third position, that both genetic and environmental factors are involved. Those who support an environmentalist position, however, typically take a stronger stand, insisting that there are no significant genetic differences in IQ between races. Three broad classes of evidence can be used to choose between the competing hypotheses. The first is genetic evidence: Genetic mechanisms or evolutionary processes that would produce or index IQ differences can be derived from genetic theory or evolutionary biology, and we can look for Requests for reprints should be sent to Brian Mackenzie, Department of Psychology, University of Tasmania, Hobart, Tasmania, Australia 7001. evidence that these processes occur differentially in the two racial groups. Alternatively, we can hypothesize that the difference is genetic (in whole or part), propose candidates for genetic variables that might produce or index the IQ differences, measure the black-white difference in these, and test for the relationship of these variables to IQ. The second is environmental evidence: Environmental processes that would produce IQ differences can be derived from psychological or sociological theory, and we can look for evidence that these processes occur differentially in the two racial groups. Alternatively, we can hypothesize that the difference is environmental, propose candidates for the relevant environmental variables, measure the black-white differences in these areas, and test for the relationship of these variables to IQ. In each of these two cases, the second strategy is theoretically weaker than the first. In the first, strong form, we actually derive the mechanisms or processes from the genetic or environmental theory. In the second, weaker form, we try merely to ensure that the proposed processes do not contradict the theory; we actually propose them on the basis of our intuitions, hunches, or wide clinical or experimental experience. The distinction is worth making here, because it is sometimes suggested that genetics is in a stronger position than psychology or sociology to account for race differences in IQ. The reason is that genetics has a formal, fully elaborated, and general theory, whereas the other two have only a bewildering variety of often incompatible microtheories. I do not dispute the stronger theoretical status of genetics here, not because it is indisputable but because it is irrelevant. Genetic hypotheses to account for the observed IQ differences are, at least as much as environmental hypotheses, based on the second, weaker strategy of proposing possibly relevant variables and effects that at best do not contradict the theories of their respective disciplines. No one has ever both derived and tested a mechanism that would produce or index the mean black-white IQ difference from genetic theory or evolutionary biology as such. Those that have been derived from the theories have not been testable. Those that can be tested have not been derived from the theories. I will return to this point in reviewing the genetic evidence. The third class is jointly genetic/environmental evidence. We acquire this kind of evidence by focusing on those situations in which genetic and environmental hypotheses lead to clearly opposite predictions. In doing so we empirically oppose the two hypotheses to each o