Five-hundred ES families visited the university to complete our day-long assessment. A single twin pair was later excluded due to intellectual limitations evidenced at the visit, resulting in a final sample of 499 twin pairs, with 48% drawn from the screened sample (101 unscreened male and 156 unscreened female pairs; 138 screened male and 104 screened female pairs). At the time of their intake visit, the mean age was 11.9 for both boys (SD=.41) and girls (SD=.44); ages ranged from 10.9 to 13.0 years. Ninety-one percent of the twins are Caucasian, which is representative of Minnesota-born children from the relevant birth years. Zygosity was determined through comparison of three estimates: 1) parental reports of physical similarity on a zygosity questionnaire, 2) staff evaluation of physical similarity, including similarity of the eyes and ears, and 3) zygosity diagnosis obtained by an algorithm using ponderal index, cephalic index, and fingerprint ridge count. When these estimates were inconsistent, DNA-based confirmation of zygosity was obtained using a panel of highly polymorphic short tandem repeat markers. The sample includes 300 monozygotic (MZ; 162 unscreened and 138 screened pairs) and 199 dizygotic (DZ; 95 unscreened and 104 screened pairs) twin pairs.
