Participants derived from two inter-related studies of Caucasian same-sex twin pairs from the Virginia Adult Twin Study of Psychiatric and Substance. Use Disorders (VATSPSUD; Kendler & Prescott, 2006). Subjects were ascertined from the population-based Virginia Twin Registry formed from a review of all birth certificates in the Commonwealth of Virginia. Female–female (FF) twin pairs, born 1934–1974, were eligible if both members responded to a mailed questionnaire in 1987–1988. Nearly all the variables used in these analyses were collected at the fourth interview wave (FF4), conducted in 1995–1997, with cooperation in earlier waves ranging from 88% to 92% (Kendler & Prescott, 2006). For this wave, we succeeded in interviewing 85% of eligible twins.
