The MTFS sample consists of 1197 monozygotic (MZ) and 684 like-sex dizygotic (DZ) twin pairs (including 5 additional members from triplet sets). All pairs were ascertained from Minnesota state birth records between 1971–1985 and 1988–1994. The sample includes two cohorts: one initially assessed at age 11 years (the younger cohort) and one initially assessed at age 17 years (the older cohort). Intake and follow-up assessments of the twins were scheduled to coincide with major transitions in the lives of these adolescents and young adults. Target ages (rate of follow-up participation) are 11, 14 (92.9%), 17 (87.3%), 20 (88.6%), 24 (89.1%), and 29 (in progress) for the younger cohort and 17, 20 (88.7%), 24 (93.8%) and 29 (94.2%) for the older cohort. At intake, approximately 18% of the recruited MCTFR families refused our invitation to participate, and, based on a brief survey with non-participants, we found that non-participating families differed minimally in parental education, parental occupational status, or parental mental health from participating families (Iacono et al., 1999). The sample is thus broadly representative of the population of the state of Minnesota.
