Analyses of twin data for vulnerability to develop dependence on a substance fit with large additive genetic components (a2), large components for nonshared environmental influences (e2) and small components for c2 terms that represents familial or other environmental influences that are shared between members of the twin pair [47, 50–56]. What about the possibility that there could be large interactions between these genetic and environmental terms (G × E interactions), invalidating additive models for genetic and environmental contributions? G × E correlations of three types have been described [86, 87]. In one terminology, “passive” G × E correlation occurs when parents transmit both genes and environmental influences that are relevant for a trait [88, 89], “active” G × E correlation occurs where subjects of a certain genotype actively select environments that are correlated with that genotype and “reactive” G × E correlation occurs when an individual’s genotype provides different reactions from the environment. Small values for c2 influences of common environments shared by members of sibpairs appear to provide evidence against “passive” G × E correlations. On these bases, “active”
