Together, the findings are consistent with behavior genetic studies reporting stronger environmental influences on alcohol use in adolescence (as reflected by significant peer socialization effects from adolescence to emerging adulthood), with the role of genetic factors increasing in adulthood (as evidenced by the G × E interaction not emerging before young adulthood; Kendler et al., 2008). The results also help reconcile previous findings of no DRD4 by environment interactions for substance use outcomes in adolescence (Creemers et al., 2011; Hiemstra et al., 2013; van der Zwaluw et al., 2012), but the presence of such interactions in samples containing adults (Hutchison et al., 2002; Larsen et al., 2010; Park et al., 2011; Perkins et al., 2008). Thus, our findings and the literature suggest that the presence of some G × E interactions is dependent on developmental timing and may not appear during all stages of development. For substance use outcomes specifically, G × E interactions that are found among adults may not replicate for adolescents. Future studies of G × E interactions in alcohol and other substance use outcomes should explicitly examine age as a possible moderator of these interactive effects to further elucidate the role of developmental timing.
