The Minnesota Adoption Studies: genetic differences and malleability.

In 1974 we launched 2 large adoption studies for 2 quite different purposes. The Transracial Adoption Study was designed to test the hypothesis that black and interracial children reared by white families perform on IQ and school achievement tests as well as other adoptees because they are reared in the culture of the tests and the schools. In addition, transracial families provided a sample with large numbers of adopted and natural children in the same families. Sources of individual differences among siblings could be studied without fear of possible differences between adoptive families and those with their own children. The Adolescent Adoption Study was designed to assess the cumulative impact of differences among family environments at the end of the child-rearing period. All of the children were adopted in the first year of life and averaged 18.5 years at the time of the study. A comparison sample of families with their own adolescents was also studied. Black and interracial children scored as well on IQ tests as adoptees in other studies. Individual differences among them, however, were more related to differences among their biological than adoptive parents, whether they lived together or not. Young siblings were found to be intellectually quite similar, whether genetically related or not. Adolescents' IQ test scores were similar to those of their parents and siblings only if they were biologically related. Our interpretation of these results is that younger children are more influenced by differences among their family environments than older adolescents, who are freer to seek their own niches.