Subjects were 744 adolescent twins (48.0% female; age at first assessment: M ± SD = 12.5 ± 0.21 years) participating in a longitudinal twin study of the genetics of brain function and behavior. The sample ascertained at age 12 included 169 MZ pairs (82 male, 87 female) and 203 DZ pairs (71 male, 49 female, and 83 opposite sex). Twins were recruited from the general population through a twin registry and represented three consecutive birth-year cohorts. The study was approved by Washington University Institutional Review Board, and written informed assent and consent were obtained from adolescent participants and their parents, respectively, after complete description of the study to the subjects and their parents. The subjects were retested at age 14 (M ± SD = 14.6 ± 0.24 years; n = 606, or 81.5% of the original sample). It is important to note that because the collection of follow-up data was still ongoing at the time of the analysis, the smaller number of subjects at age 14 reflects not only dropouts, but also incomplete follow-up.
