Participants were adolescent male and female twins who participated in 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). Two age cohorts compose the MTFS sample: a younger cohort initially recruited the year the twins turned 11 years old, and an older cohort recruited the year the twins turned 17 years old. Participants were identified through public birth records of twin pairs born in the years 1972–1978 in the case of the older male cohort, 1975–1979 for the older female cohort, 1978–1982 for the younger male cohort, and 1981–1984 for the younger female cohort. Zygosity (MZ=monozygotic; DZ=dizygotic) was determined by the agreement of three estimates: (1) a standard zygosity questionnaire completed by parents, (2) evaluation by MTFS staff of physical similarity based on eye, hair, face, and ear characteristics, and (3) similarity on fingerprint ridge count, cephalic index, and ponderal index. A serologic analysis was conducted if the 3 estimates did not agree. Twins return at roughly 3-year intervals for follow-up assessments.
