Results have implications for prevention efforts. We obtained notable overlap in the liability for early initiation and substance use disorder, suggesting that appropriately targeted interventions may reduce the risk for early uptake and problematic use of multiple substances. Significant genetic overlap was observed among both men and women, indicating the importance of addressing heritable, trait-level factors that confer vulnerability to substance misuse broadly (e.g., behavioral disinhibition and childhood externalizing behavior (McGue, Iacono, & Krueger, 2006)). Further, the high shared environmental correlation obtained among women suggests that interventions targeting parent and peer factors may be particularly relevant for this group. Although there was no overlap in the family environmental influences on initiation and disorder among men, this does not suggest that environmental factors are unimportant. Traits such as conduct disorder may confer risk for substance misuse through active genotype-environment correlation (Scarr & McCartney, 1983), in which individuals seek out environments (e.g., substance-using peers) that facilitate expression of this vulnerability. Furthermore, gene-environment interaction may influence risk for substance misuse, such that heritable influences on adolescent involvement are increased in more “facilitative” environments
