Individual and group differences in adoption studies of IQ.

In the last 5 years, the Colorado and Texas adoption projects have replicated and elaborated the findings of the classical adoption studies of this century: IQs of adoptees are more strongly related to the IQs of their biological parents than to measurable characteristics of their adoptive environment. At the same time, several studies from France have demonstrated substantial IQ gains in children adopted from impoverished biological parents to middle-class adoptive homes. The apparent contradiction between these two findings is emblematic of the nature-nurture controversy. A common model for resolving the paradox, the two-realms hypothesis, is conceptually inadequate and encourages separate analysis of individual and group differences. Subsuming the two kinds of findings in a single model shows that they are less divergent than they seem and highlights the need for further research into why some contradictions remain. Despite periodic declarations of peace (Clarke, 1984b; Plomin, 1983; Scarr Wachs, 1983) or victory (Feldman Urbach, 1974), the nature-nurture debate has not been resolved. There are many reasons for its continuation, some scientific and some ideological. Among the most important are the persistently opposite conclusions of two varieties of the adoption design. In one (the individual-difference design), IQs of adopted children are related to the IQs of their biological parents and some measure of the adoptive environment. Positive statistical relationships between IQsofbiological parents and adopted-away children are indicative of a genetic effect; positive relationships between rearing environment and adopted children's IQs demonstrate environmental influences. The other (group-difference) design focuses analysis on differences between mean IQs of groups. Children adopted from unfortunate circumstances into more environmentally favorable homes are evaluated to see if the environmental improvement is accompanied by an increase in IQ. Beginning with the classic studies of Burks (1928) and Leahy (1935), individual-difference analyses of adopted children's IQs have usually shown stronger relationships with biological parents' IQs than with measures of adoptive environment and have thus supported genetically weighted conclusions. In contrast, children adopted from deprived environments into stable adoptive homes have often shown significant increments in IQ, consistent with an environmentally weighted explanation. Indeed, these two findings have sometimes been reported in the same study. The divergence of the individual- and group-difference traditions is now practically complete. Recent reviews of genetic