Intellectual similarities within families of both adopted and biological children

The effects of genetic and environmental differences on intellectual differences among children were examined in a study of families with both biological and adopted children. IQ scores of all family members and education of natural parents were used to estimate intellectual similarities among related and unrelated persons, living together and apart. Comparisons of correlations between related and unrelated siblings produced negligible heritability values, whereas the parent-child data suggested moderate heritability for the children's IQ differences. The high mean values of the adopted children's IQ scores and the high degree of similarity among unrelated sibs suggest that IQ scores are more maleable than previously thought.