"Both Lynn (1997, this issue) and Rushton (1997, this issue) dispute the task force's conclusion that there is no direct evidence for a genetic interpretation of the Black—White IQ difference. Lynn's succinct comment cites two lines of evidence that he finds particularly persuasive: (a) the Minnesota adoption study and its 10-year follow-up and (b) studies relating head or brain size to intelligence test scores. I respond to these two points in some detail and then comment briefly on other issues raised by Rushton.
The original Minnesota study ( Scarr & Weinberg, 1976 ) included both the adopted and the biological children of 101 middle-class families (each with two White parents), tested at an average age of about 7 years. The mean IQ of the adopted Black children was 106.3, well below the 111.5 of the adopted White children and the 116.7 of the biological children but a full standard deviation above the expected IQ mean of Blacks in Minnesota. Adoptees with one Black and one White birth parent scored higher than those with two Black birth parents, but even the latter averaged 96.8. These and other findings led Scarr and Weinberg to conclude that "the social environment plays a dominant role in determining the average IQ level of Black children" (p. 739). But follow-up testing when the children were about 17 years of age had quite a different result: The mean IQ of the retested Black adoptees was only 96.8, and those with two Black birth parents averaged 89.4 ( Weinberg, Scarr, & Waldman, 1992 ). That is why Lynn (1997) says, "Black babies adopted by White parents registered no IQ gains" (p. 73), a point he has elaborated elsewhere ( Lynn, 1994 ).
As Waldman, Weinberg, and Scarr (1994) made clear in their response to Lynn (1994) , this conclusion is misleading. Everyone involved in this debate is well-aware that such comparisons must be corrected for the Flynn effect: Mean scores on all standard IQ tests seem to rise steadily at about 0.3 points per year. In the Minnesota study, where the tests used in the follow-up were generally not the same as those that had been given the first time, these corrections are complex and must be made on an individual basis. Until they have been made–Waldman et al. reported that they are in progress–raw figures like those above are relatively meaningless.
A further complication is that race and preadoptive experience were strongly confounded in the Minnesota study ( Scarr & Weinberg, 1976 ). At the time they joined their new families, for example, the Black adoptees had had more prior placements, rated of poorer quality, than their White counterparts. This was especially the case for the children with two Black birth parents, who were not adopted until they were, on average, about 32 months old. Because any later IQ differences between these groups may have resulted from differences in preadoptive experience, the Minnesota data provide no clear evidence for the genetic hypothesis. But it is only fair to say that they do argue against certain versions of the environmental hypothesis (pending the necessary Flynn effect corrections): The mere fact of growing up in a middle-class home apparently does not, by itself, raise one's score on intelligence tests given at adolescence."