Participants were young adults in the sibling sample of the National Longitudinal Study of Adolescent Health. Detailed explanation of the study design and sampling strategy are available elsewhere (Harris et al. 2006; Add-Health 2008). Participants were respondents to an in-school survey with a twin or a sibling (age 12–20 years) from the same biological parents and were randomly selected for inclusion in an in-home interview sample. Within the twin and full-sibling sample, a total of 3,988 individuals participated in this initial in-home interview, with 3,640 participating again 1 year later at wave II, and 3,297 participating again 5 years later at wave III. Mean ages at each of the three interviews were 16.2, 17.1, and 22.5, respectively. Of the 306 monozygotic twin pairs, 447 dizygotic twin pairs, and 1,248 full sibling pairs included in the present study, 47.4% were male. All participants were part of a twin or full sibling pair. The mean age difference between sibling pairs was 2.24 years (SD = 1.03). The ethnic composition, based on self-nomination, was 48.9% Caucasian, 21.1% African–American, 12.9% Hispanic, 6.4% Asian American, and 2.8% Native-American, and 7.8% other.
