Epidemiologic studies have demonstrated that the prevalence of drug use and abuse increases with age during adolescence and peaks in young adulthood (Johnston et al., 2008; Substance Abuse and Mental Health Services Administration (SAMHSA), 2006, 2008). Substance experimentation is common in adolescence and substantially elevates the risk for persistent substance use, substance use disorders (SUDs; i.e., abuse/dependence) and other comorbid disorders in later life stages (Bauman and Phongsavan, 1999; Brook et al., 1999; Gould et al., 1977; Kapusta et al., 2007; Riggs et al., 2007;Winters and Lee, 2008). Additionally, involvement with multiple substances and the risk for substance use problems appear to be driven by either a common risk factor or correlated risk factors (Grant and Dawson, 1998; Gil et al., 2004; Grant et al., 2006; Kendler et al., 2007, 2008; Rhee et al., 2003, 2006; Young et al., 2006). Despite this wealth of information, our understanding of substance use and related problems in youth remains limited by the fact that most data are cross-sectional. This permits investigation of cohort differences or temporal change, but does not allow for developmental models to be explicitly tested.
