Participants were 499 male youths (Mean age = 17.5 years, SD = 0.4) 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. A comprehensive description of the MTFS is found in Iacono & McGue (2002). All participants and their parents gave written informed assent or consent as appropriate.
