In support of this pathway, externalizing symptoms are highly correlated with substance use, particularly in adolescence (Hussong et al., 1998; King, Iacono, & McGue, 2004; Steele, Forehand, Armistead, & Brody, 1995). Moreover, recent behavioral genetics studies suggest that externalizing symptoms and substance use share a common genetic diathesis for disinhibited behavior (Iacono, 2008).1 Evidence for other aspects of this theoretical model has also begun to emerge (see Zucker et al., 2006). Thus, the externalizing pathway may well be a dominant pathway of risk for SUDs. This line of research has important implications for prevention, with accumulating evidence showing support for prevention and treatment programs that address deficits or introduce protective factors relevant to the externalizing pathway (e.g., Henggeler, Clingempeel, Brondino, & Pickrel, 2002; Lochman et al., 2007).
