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 index of generalized, or non-specific, substance dependence vulnerability (DV) best met their criteria for a phenotype that was clinically valid, familial, and heritable. DV was calculated by taking a total lifetime symptom count of DSM dependence criteria endorsed across all classes of substances, and then dividing by the number of substances used. Use was defined by the CIDI-SAM as using almost daily for at least 30 days for tobacco, having six or more drinks during one’s lifetime for alcohol, and using more than five times during one’s lifetime for illegal drugs (Corley et al., 2001; Stallings et al., 2003). Participants who did not meet the definitions of use were assigned a DV score of zero.
