Several limitations of the present study should be considered when interpreting these results. First, we cannot properly distinguish between non-additive and shared environmental effects without the inclusion of extended family data. The incorporation of other family types, such as adoptive sibling pairs and non-twin sibling pairs, would allow us to assess the extent of these possible effects. Second, the low frequency of alcohol, tobacco, and cannabis problem users limited our power to distinguish between additive genetic and shared environmental effects. The study’s low power also influenced our model selection; however, we compensated for this by using both the chi-square difference test and the Akaike Information Criterion to select the best-fitting model. In light of the power limitations, we are still confident that the latent trait exists in males and females and that there are common and specific familial effects operating in each gender. Third, covariates of substance addiction, such as attention deficit hyperactivity disorder (Disney et al., 1999; Elkins et al., 2007), delinquency, conduct disorder, depression, and antisocial tendencies (Fu et al., 2002) were not included in this study. Though
