Overall, our findings converge on a central position regarding the nature of the liability for the development of SUDs in young adulthood in cases where substance use begins in adolescence. That is, early users do not appear to limit their use to a single substance, and this broad sampling of substances generally increases the risk for developing problems of abuse and dependence across a number of substances. Moreover, a SUD diagnosis on any given substance in young adulthood is not entirely predicted from involvement with the same substance during adolescence but also with other substances. These observations suggest that there is either a generalized risk factor or correlated risk factors for early onset substance use and subsequent development of substance use disorders. These patterns seen in our longitudinal analyses are consistent with findings from previous epidemiological and biometrical studies that have inferred a generalized liability to SUDs (Goldman and Bergen, 1998; Kendler et al., 2007, 2008; McGue and Iacono, 2008; Petraitis et al., 1995; Rhee et al., 2003, 2006; True et al., 1999; Vanyukov et al., 2003; Young et al.,
