The sample used in this research has been described in detail elsewhere (Iacono & McGue, 2002; Miller et al., in press). In short, it is composed of two studies of Minnesota families: a community-representative sample of twins and their parents, as well as a study of adoptive families. Sample sizes for the twin and adoptive families are provided in Table 1. Twin families are further divided into three prospective cohorts: (A) 17-year-old twins (N = 1139) first assessed between 1989–1996, followed regularly at ages 20, 24, and 29; (B) 11-year-old twins (N = 1167) assessed at ages 11, 14, 17, 20, 24, and 29, with their age-17 assessment occurring between 1996–2003; and (C) another sample (N = 571) of 11-year-old twins assessed at ages 11, 14, and 17, with their age-17 assessment occurring between 2005 and 2010. In the adoption study all families were composed of two families and two offspring. Some families had two biological offspring, some had two adopted offspring, and some had one biological offspring and one adopted offspring. Parents were typically assessed at intake, regardless of cohort. In total, the present study included 7188 Caucasian participants from 2300 families.
