et al., 2007), particularly among men of European Ancestry (Grant et al., 2012) as is the case for these twin fathers. It is not possible to measure all environmental influences on offspring. For instance the present study modeled perceptions of sibling substance use but not self reported measures from all siblings. Expansion of the shared environment assessment may reveal key parent, sibling and peer level variables that have not been adequately measured. Maternal characteristics, here included as covariates, might be better treated in the context of analyses that model assortative mating, which is beyond the scope of the present report. Finally, temporal relationships cannot be established by these cross sectional data. For instance, parental behavior may be a response to, as well as a predictor of childhood psychopathology, and peer substance use may signify either deviant socialization or peer selection. Future research is warranted to determine if patterns of comorbidity hold as offspring age and to assess the relative contributions of familial and non-familial factors to psychiatric comorbidity in a longitudinal perspective.
