A brief initial interview, using standard questions for zygosity assignment (Nichols and Bilbro, 1966), was conducted with a parent to determine zygosity of the twins. Comprehensive structured diagnostic telephone interviews were scheduled with parents of the twins and with the twin pairs. Verbal consent, or assent if minors, was obtained from all participants prior to their participation in the interview, as well as parental consent for the participation of their minor children in the study. All procedures were approved by the Institutional Review Board at Washington University, St Louis.
