Drug dependence was assessed using the Composite International Diagnostic Interview-Substance Abuse Module (CIDI-SAM: Cottler et al., 1989), a structured diagnostic interview that assesses DSM-IV abuse and dependence criteria for tobacco, alcohol, marijuana, cocaine, amphetamines, sedatives, inhalants, hallucinogens, opiates, and phencyclidine (PCP). For the purpose of these analyses, we examined nonspecific substance dependence (Stallings et al., 2003), given the evidence that use of multiple substances, rather than specializing in any single substance, is typical in adolescents, especially among those with substance dependence (Glantz and Leshner, 2000; Johnston et al., 2001; Young et al., 2002). Moreover, there is increasing empirical evidence that common genetic influences are responsible for the comorbidity across use of and dependence on different substances, and even across substance use and other externalizing problems (Button et al., 2006; Kendler et al., 2003; Krueger et al., 2002; Slutske et al., 1998; Stallings et al., 2003; Stallings et al., 2005; Young et al., 2000; Young et al., 2006). Stallings et al. (2003) considered 10 alternative phenotypes that might quantify an adolescent’s vulnerability to develop substance dependence. Of these 10 alternatives, an
