At age 12, early use with substances including tobacco, alcohol, marijuana, and other illicit drug categories was assessed with questions from the Monitoring the Future questionnaire (Johnston, O’Malley, & Bachman, 1999). Each question regarding frequency of use was recoded to a binary variable, coded 0 for never used and 1 for used at least one time. At age 17, substance use was assessed with the Composite International Diagnostic Instrument—Substance Abuse Module (CIDI–SAM; Cottler, Robins, & Helzer, 1989). The CIDI–SAM is a structured, face-to-face interview designed for administration by highly trained lay interviewers. The dependent variable for substance use at age 17 was the number of substances used repeatedly; the CIDI–SAM defines repeated use as smoking at least 20 cigarettes or the use of alcohol or illicit drugs on more than five occasions. The substances queried included tobacco, alcohol, marijuana, and eight other illicit drug categories.
