To date, four published adoption studies have investigated whether AUD risk is higher among adoptees with presumed genetic liability who are also exposed to risky adoptive-family environments. In two independent samples of Swedish male adoptees (Cloninger, Bohman, & Sigvardsson, 1981; Sigvardsson, Bohman, & Cloninger, 1996), environmental risk (indicated by adoptive father occupational status) moderated genetic risk for a severe form of alcoholism with later onset and lower heritability (Type I), but not for a form characterized by earlier onset and externalizing comorbidity (Type II). Rates of severe Type I alcoholism among males with both high genetic and environmental risk were (11.4–11.5%), substantially higher than those with low environmental risk and those with low genetic risk (regardless of environment) whose rates ranged from 2.3% to 6.7%. In contrast, Bohman, Sigvardsson, and Cloninger (1981) found no evidence for GxE on lifetime alcoholism in a parallel study of Swedish female adoptees.
