Study participants were enrolled in the Minnesota Twin Family Study (MTFS), a longitudinal population-based study of genetic and environmental influences on substance abuse and related psychopathology. All were identified from public birth records covering twin births in Minnesota between 1972 and 1984. The MTFS sample consists of two cohorts, one initially assessed when the twins turned 17 and the other when they turned 11. The 1,383 families whose twins completed an age-17 assessment, whether an intake assessment for the older cohort or a follow-up assessment for the younger one, and whose biological mothers completed a clinical assessment served as subjects for the current report. Twins and parents gave written assent or consent to participate, as appropriate, with parents also providing consent for minor children. Consistent with the demographic profile of Minnesota during this period, the vast majority of participants were white (97.1% in the combined sample). Family demographic characteristics are given in Table 1, while a more detailed description of the study aims, design, ascertainment procedure, and sample bias is available elsewhere (Iacono, Carlson, Taylor, Elkins, & McGue, 1999).
