The current study utilized DSM-IV-TR cocaine, alcohol, nicotine, and marijuana dependence symptom counts as measures of substance use severity. These counts were derived from the Semi-Structured Assessment for the Genetics of Alcoholism interview (SSAGA-II), which has demonstrated validity and reliability (Bucholz et al., 1994; Hesselbrock et al., 1999). To obtain approximately normally distributed regression residuals, the symptom counts for marijuana dependence were log-transformed [i.e., ln(symptoms+1)]. Symptom counts for all other substances (i.e., cocaine, alcohol, and nicotine) remained untransformed.
