The heritability of individual differences in intelligence within the white population (European and North American Caucasians) is so well established by a number of independent studies - to the effect that genetic factors are about twice as important as environmental factors as a cause of individual differences in IQ - that this conclusion is now generally accepted by scientists who are familiar with the evidence. The situation regarding mean differences in intelligence between subpopulation groups is quite another matter. Not only does there prevail a marked a priori preference for environmental explanations of group differences - particularly if different racial groups are involved - but in most discussions even the possibility of genetic differences is never raised. The investigator's task is assumed to be solely that of hypothesizing or identifying the environmental factors responsible for the mean intelligence difference between the two groups in question. Usually any and all environmental differences found to exist between the groups, in whatever degree, are deemed adequate to explain the IQ difference, whatever its magnitude. In many instances this results in attributing quite large differences to very weak causes as judged from the correlation between the hypothesized environmental effect and IQ variance within either of the population groups being compared. But, logically, unless a direct causal relationship (rather than just a correlation) between an environmental factor and IQ is established, there is no more basis for preferring an explanation in tenns of some visible environmental difference than in terms of some invisible genetic difference. And in the case of race
differences in IQ, there are even v£sible genetic differences (e.g., skin color, hair texture, etc.) between the groups, the purely logical status of which, in relation to IQ, is not different from the visible environmental differences between the groups. In both cases, the visible differences mayor may not make a causal difference in IQ. The visible environmental differences and the visible physical genetic differences between two racial groups may have no causal connection with IQ; both may be merely correlated with some other factors which directly influence IQ. Since we know from studies of the heritability of individual differences in IQ that genetic factors have comparatively powerful effects and environmental factors have comparatively weak effects, is there probabilistically more reason to hypothesize environmental factors as of greater importance than genetic factors in explaining group differences in IQ? The a priori preference for strictly or preponderantly environmental explanations seems to stem more from ideological than from any logical or scientific considerations. Thus, Jencks (1969, p. 29) writes, 'While a significant number of black children may well suffer serious prenatal damage, Jensen's evidence suggests that we should probably look elsewhere to explain racial differences in IQ scores. But it hardly follows that we must look to genes. We might do equally well to look at patterns of child rearing.' This clearly expresses a preference which could determine one's research strategy, but the preference would seem to run counter to the probabilities suggested by already established evidence. Genes have already been established as having powerful effects on IQ; individual genetic differences correlate about 0·85 to 0·90 (the square root of the heritability) with IQ. Correlations between child-rearing practices and lQ within racial groups are minute by comparison, and the extent of their causal connection with lQ differences between racial groups has not been determined. (If the broad heritability, including GE covariance, of 1Q is 0·75, for example, the maximum correlation between lQ and all environmental effects combined would be only J1 -0·75 = 0·50 within the population in question.) Pointing to some environmental difference whose causal relationship to IQ is not established is logically no more plausible as an environmental explanation of a mean lQ difference than is pointing to some clearly genetic difference, such as skin color, as a genetic explanation of the difference.