Data for the current investigation was collected as part of the age 17 assessment for both cohorts (i.e., intake assessment for the older cohort and the second follow-up assessment for the younger cohort). The total sample included 1315 twin pairs: 437 monozygotic (MZ) and 251 dizygotic (DZ) male twin pairs, and 418 MZ and 209 DZ female twin pairs. The mean age of the total sample at the time of assessment was 17.8 years (SD = .68 years). Zygosity was determined by the agreement of 3 separate reports: a standard zygosity questionnaire completed by parents, evaluation of the physical resemblance of the twin pairs including hair and eye color and face and ear shape completed by MTFS staff, and an algorithm assessing the similarity of twins on ponderal and cephalic indices and fingerprint ridge counts. A serological analysis was performed if these estimates did not agree.
