Comment on the Minnesota Transracial Adoption Study

Data from the Minnesota transracial adoption study reported in Weinberg. Scarr, and Waldman (1992) is reanalyzed and shown to be consistent with the hypothesis that the mean black/white difference in IQ is significantly genetic in origin. Weinberg, Scarr, and Waldman (1992) presents follow-up data on their study of black, mixed-race, and white children adopted by white families. In retrospect, they summarize their conclusions from their initial assessment of the performance of the adoptees as indicating that "putative genetic racial differences do not account for a major portion of the IQ performance difference between racial groups" (p. 118) and they seem inclined to interpret their follow-up as consistent with that conclusion. Weinberg, in a relatively recent article, although one published before the appearance of the follow-up, takes the study to have "disputed the hypothesis that IQ differences between blacks and white are due to genetic differences" (Weinberg, 1989, p. 101), although he does allow that h 2, the proportion of variance between individuals in IQ explained by genetic variation, is .5. (To be completely explicit, we may use H 2 to denote the proportion of the variance between races in IQ that is explained by between-race genetic variation.) There is an ambiguity in this remark cited from Weinberg: It is not clear whether he means to deny that all between-race variance is explained by genetic differences between the races, which is a position no one holds, or to deny that any of the between-race variance is explained by such genetic differences. If Weinberg is disputing the latter claim, he is implicitly making the very adventurous assertion that all mean race differences are due to differences in the environments in which members of different races find themselves. What to call these various positions is more significant than most definitional questions because of the emotions surrounding the issue of the size of H 2. It may be considered unfair to define environmentalism as the hypothesis that H 2 = 0 and The author wishes to thank Arthur Jensen and an anonymous referee for help, criticisms, and suggestions