In addition to the possible overlap at times in the processes underlying the internalizing and externalizing pathways, we also recognize the potential for externalizing symptoms themselves to mediate the risk between early internalizing symptoms and substance involvement in adolescence. Several theories account for the development of externalizing symptoms secondary to internalizing symptoms, with the failure to form healthy relationships and deficits in social development serving to increase this comorbidity (Oland & Shaw, 2005). Moreover, both social context theory (Dishion et al., 1994) and the self-derogation model (Kaplan, 1980) suggest that the same social forces posited to propel youth with internalizing symptoms toward substance use (i.e., gaining acceptance but also peer support for deviance in a marginalized peer group) may act to increase deviant behavior more generally. As such, we also predict that for some youth externalizing symptoms may serve as a final common pathway to substance involvement, mediating the relation between childhood internalizing symptoms and substance use in later adolescence (Zucker, 2006). Thus, progression along the internalizing pathway to SUDs does not preclude the presence of externalizing symptomatology, particularly during adolescence. Rather, the internalizing pathway instead emphasizes emotion-based risk processes as underlying the emergence of externalizing symptoms and SUDs.
