All data were collected over the telephone by trained interviewers. An initial screening to determine zygosity of the twin pair was conducted in Wave 1 with one of the twin’s parents. Diagnostic interviews were then scheduled with parents who agreed to take part in the study. Interviews with the twins were scheduled after obtaining verbal consent (and, for those under the age of 18, the consent of parents). In Waves 4 and 5, verbal consent was also obtained from all participants prior to the start of interviews, consistent with procedures approved by the Human Research Protection Office at Washington University.
