recruited. MATCH twins were selected from female same-sex pairs from both the 1989 cohort and an older “1981” cohort described elsewhere (see Heath et al., 1997; Waldron et al., 2009). PACER twins were selected from male same- and opposite-sex pairs from the 1989 cohort only. Assessment of twin parents by telephone interview began in 2000 and 2005 for MATCH and PACER studies, respectively. During the same period, offspring ages 11–24 were invited to complete an interview, also by telephone. From the 1989 cohort, 1,341 offspring completed MATCH or PACER interviews, of whom 23 (<2%) were excluded from analyses because of missing data on twin substance dependence, parental separation, and/or offspring substance involvement, resulting in a final sample of 1,318 offspring. Samples sizes by risk group(s) are shown in Table 2, with sample characteristics provided in Table 3.
