Family studies report that children of parents with high-risk alcohol dependence, or that are from families where one member is diagnosed with an SUD, are at much greater risk for developing alcohol problems (Chassin, Rogosch, & Barrera, 1991). Consequently, family studies have demonstrated that SUD clusters within families, implicating a role for both genetic and environmental influences. In comparsion to family studies providing estimates of familial clustering, twin studies have estimated specific sources of variance in the etiology of SUD. Twin studies use monozygotic and dizygotic twin pair variances and covariances to estimate the proportion of total phenotypic variance of a trait due to additive genetic (additive genetic effects of alleles at every locus), shared environmental (environmental influences common to both twins), and unique environmental influences (environmental influences not shared by members of the twin pair; (Cherny, 2009). Twin studies of SUD consistently report that substance initiation is significantly influenced by genetic as well as shared and unique environmental factors. This is consistent across populations that initiate tobacco, alcohol, or cannabis use (Agrawal et al., 2010; Huizink et al., 2010).
