The primary sample was drawn from a large prospective cohort study (the Missouri Adolescent Female Twin Study (MOAFTS)) of female same-sex twin pairs born between July 1st 1975 – June 30th 1985 who were identified from birth records24. Twins were eligible to participate if both members of the twin pair had survived past infancy, were not adopted and if their biological parents were Missouri residents at the time of the twins’ birth. Using a cohort-sequential sampling design, twins and at least one biological parent (typically the biological mother), were invited to participate in the baseline interviews during 1994–1999, when the twins were 13, 15, 17 or 19 years old. Recruitment of additional 13 year olds continued over a two-year period as twins became age-eligible. A telephone diagnostic interview was administered, first to the parents, and subsequently, after obtaining parental permission, to the twins (minors). Further details regarding sample recruitment and characteristics of this first wave of interview data, which are not utilized in the current study, are given elsewhere25. Subsequently, participants were invited to participate in several full-length and mini follow-up interviews and respond to mailed questionnaires.
