The above studies have established a foundation for examining common vulnerability to the onset of licit and illicit drug use. Two key issues that remain relatively under-explored are addressed in the current study. First, we examine genetic and environmental contributions to the timing of alcohol, cigarette, and cannabis use onset. Despite the well-known risks associated with early initiation, very few genetically informative studies have addressed age at initiation; even fewer have simultaneously examined timing of first use for more than one substance. Second, we focus on African-American females, who are under-represented in genetically informative studies of substance use initiation in general and virtually absent from the limited literature on cross-substance risk for onset of use. The lower prevalence of cigarette and alcohol use (Ellickson et al., 2004a; Grucza et al., 2008; Heath et al., 1999; Scarinci et al., 2002; Vega et al., 2007) and differences in sequence of licit and illicit substance initiation (Guerra et al., 2000) in African-Americans compared with Caucasians hint at distinctions in pathways of risk that cannot easily be detected in studies where African-Americans comprise only
