Participants were twin pairs from two community based twin samples participating in the NIDA-funded Center on Antisocial Drug Dependence (CADD; DA-11015): the Colorado Longitudinal Twin Sample (LTS, Emde and Hewitt, 2001; Rhea et al., 2006) and the Community Twin Sample (CTS) of the Colorado Twin Registry (CTR) (Young et al., 2002). The LTS twins were recruited through the Colorado Department of Health’s Division of Vital Statistics (Young et al., 2000) and were included in the CADD sample as they reached their 12th birthday. Twins from the CTR were recruited through the Department of Health and 170 (of 176 eligible) school districts in Colorado (Young et al., 2000). Written informed consent from parents or guardians of participants and written informed assent from the minor participants was obtained. The full twin sample comprised 1377 twin pairs (data for only 1 twin was available for 5 of these “pairs”), between 12 and 18 years of age (mean = 14.52, SD = 2.10). Of these, 657 were identical or monozygotic (MZ) pairs (362 female and 295 male) and 720 were fraternal or dizygotic (DZ) pairs (222 female, 223 male, and 275 opposite sex).
