Large interactions between genetic and environmental components would be likely to lead to 1) differences in estimates of heritability from samples obtained in different environments, and 2) differences in molecular genetic findings in individuals from different environments. Data from studies of twins who were sampled from a number of different environments is nevertheless largely convergent. Such convergence supports relatively modest upper limits on (G × E) interactions between genetic and environmental influences on addiction vulnerability. Modest G × E influences are also consistent with molecular genetic results that identify substantial overlaps between molecular genetics of vulnerability to dependence on illegal substances in samples from substantially different environments, such as the United States and Asia (see below).
