There was evidence for overlap in the familial influences on the timing of substance use initiation and symptoms of substance use disorder. Genetic influences were significantly correlated in both men and women. Further, although shared family environmental influences only explained a modest (and non-significant) proportion of the variation in disorder among women, these influences were highly correlated with the family environmental influences on initiation. The overlap in familial influences on first use and disorder is in accord with previous studies (e.g., Agrawal et al., 2008; Prescott & Kendler, 1999; Sartor et al., 2009).
