A substantial genetic contribution to the timing of initiation was found for alcohol (44%), cigarette (62%), and cannabis (77%) use. Shared environmental factors had a modest influence on the timing of first drink (10%), but we found no evidence that they play a role in shaping the onset of cigarette and cannabis use—a surprising finding given the support in the literature for the role of shared environment in substance use initiation, especially in girls (Han et al., 1999; Hopfer et al., 2001). A significant number of studies on alcohol (Fowler et al., 2007; Han et al., 1999), cigarette (Koopmans et al., 1999; Han et al., 1999), and cannabis use (Fowler et al., 2007; Maes et al., 1999; McGue et al., 2000) have reported that greater than half of the variance in initiation can be accounted for by shared environmental factors. Discrepancies between our findings and those reported elsewhere may be attributable to differences in the phenotypes under study. Whereas the current study addressed the timing of first use, the vast majority of genetically informative studies have operationalized initiation more broadly
