What do adoption studies reveal about the effects of heredity and environment on IQ?

Question

What will be an ideal response?

Answer 1

In adoption studies, researchers gather two types of information: 1. correlations of the IQs of adopted children with those of their biological and adoptive parents, for insight into genetic and environmental influences; and
2. changes in the absolute value of IQ as a result of growing up in an advantaged adoptive family, for evidence on the power of the environment.
Findings consistently reveal that when young children are adopted into caring, stimulating homes, their IQs rise substantially compared with the IQs of nonadopted children who remain in economically deprived families. But adopted children benefit to varying degrees. In one investigation, children of two extreme groups of biological mothers—those with IQs below 95 and those with IQs above 120—were adopted at birth by parents who were well above average in income and education. During the school years, the children of the low-IQ biological mothers scored above average in IQ but did less well than the children of high-IQ biological mothers placed in similar adoptive families. Parent–child correlations revealed that as the children grew older, they became more similar in IQ to their biological mothers and less similar to their adoptive parents.
Other investigations confirm that both environment and heredity contribute to IQ. In fact, children adopted in the early years attain IQs that, on average, match the scores of their adoptive parents' biological children and the scores of nonadopted peers in their schools and communities. These outcomes suggest a sizable role for environment in explaining SES variations in mental test scores. At the same time, adoption studies repeatedly reveal stronger correlations between the IQ scores of biological relatives than between those of adoptive relatives—clear evidence for a genetic contribution.