We assessed several measures of early use and misuse in addition to SUDs. Because early substance use predicts a variety of mental health problems (Grant & Dawson, 1997), we created binary measures reflecting whether adolescents had used alcohol (without permission), tobacco, or illicit drugs before age 15 (cf. McGue & Iacono, 2005). We also included two measures of early misuse: whether one had been intoxicated by this age and a count of the number of different substance classes used by age 15. To improve stability of estimates, we combined responses of four or more (different categories of drugs), none of which individually represented as much as 1% of the sample, into one category. We also assessed the adolescents' own lifetime maximum alcohol consumption in 24 hours at the time of the age-17 assessment.
