Participants in this study derived from two inter-related studies of Caucasian same-sex twin pairs who participated in VATSPSUD (Kendler and Prescott, 2006). All subjects for the VATSPSUD were ascertained from the population-based Virginia Twin Registry formed from a systematic review of birth certificates in the Common-wealth of Virginia. Female–female (FF) twin pairs, from birth years 1934 to 1974, became eligible if both members previously responded to a mailed questionnaire in 1987 to 1988, the response rate to which was approximately 64%. Data on AC and AD used in this report were collected at the 4th wave of interviews (FF4), conducted in 1995 to 1997. For this wave, we succeeded in interviewing 85%of the eligible sample. Data on the male–male (MM) pairs came from a sample (birth years 1940 to 1974) initially ascertained directly from registry records, which contained all twin births. The first interview (MM1) was completed largely by phone in 1993 to 1996 and obtained a 72% response rate. This was followed by a second wave of interviews (MM2), conducted in 1994 to 1998, with a response rate of
