Estimates from the full bivariate factor models for men and women are depicted in Figures 4 and 5, respectively. The genetic correlations between the age of substance use initiation and substance use disorder were significant in both men (rG=.55 [.38, .72]) and women (rG=.53 [.29, .76]). Further, although shared environmental influences explained only a modest proportion (19%) of the variation in disorder in women, these influences were very highly correlated with the shared environmental influences on initiation (rC=.83 [.45, 1.00]). Notably, after accounting for overlap in the familial influences on initiation and disorder, significant unique environmental correlations were observed (men: rE=.42 [.06, .77], women: rE=.23 [.04, .42]). Descriptively, this suggests that a member of an identical twin pair who starts using substances earlier than their co-twin will be more likely to develop a substance use disorder than their co-twin. This is consistent with the hypothesis that early substance use uptake causally influences risk for disorder: if the relation were fully mediated by familial influences, no remaining association would be observed among identical twins after accounting for genetic and family environmental factors.
