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.