To address these issues we used an adoption design, first elucidated by Scarr and Weinberg12 in 1983, which offers an especially sensitive and precise test for the presence of family-level environmental influences. That is, the correlation between two individuals who are not biologically related, but who have been reared together, provides a direct estimate of the shared environmental effect. Genetic influences are inferred only to the extent that larger effects are demonstrated in biologically related families than adoptive families.
