The sample consisted of twin pairs born in Minnesota who participated in the longitudinal Minnesota Twin and Family Study (MTFS). Participants were initially identified through birth records from 1977 to 1982 for males and 1981 to 1985 for females. Families were contacted via telephone and invited to begin their participation when the twins were approximately 11-years-old. Of the study-eligible twin pairs (living within one day’s drive from Minneapolis, MN and without physical or mental handicap which would interfere with assessment) identified from birth records, 90% were able to be located and fewer than 20% declined participation. Overall, 376 male and 380 female twin pairs participated at intake, at which point project staff obtained written informed consent. Zygosity was initially assessed by a questionnaire filled out by the twins’ parents; trained project staff diagnosis based on physical similarity; and ponderal index, cephalic index, and fingerprint ridge count. If there was any discrepancy among these three methods, zygosity was determined by evaluating 12 blood group antigens from blood samples. In confirmatory analysis of 50 twin pairs, project staff and physical similarity diagnoses
