Subjects were 506 male youths (228 twin pairs and 50 unmatched twins; Mean age = 17.5 years, SD = 0.4; range 16.6 – 18.3 years) from the older cohort of the Minnesota Twin Family Study (MTFS), a longitudinal and epidemiological study investigating the development of substance use disorders and related psychopathology. Subjects were identified from birth records as twins born between January 1, 1972 and December 31, 1978. A comprehensive description of the MTFS is found in Iacono & McGue (2002). Consistent with demographics of the state of Minnesota at the time the twins were born, nearly all (99%) were Caucasian. All subjects and their parents gave written informed assent or consent as appropriate.
