Participants in this study derived from two interrelated studies of Caucasian same-sex twin pairs participated in VATSPSUD.21 All subjects were ascertained from the population-based Virginia Twin Registry formed from a systematic review of birth certificates in the Commonwealth of Virginia. Female-female (FF) twin pairs, born from 1934–1974, were eligible if both members responded to a mailed questionnaire in 1987–1988. Reports on symptoms of lifetime AD used in this report were collected at the fourth interview wave (FF4), conducted in 1995–1997 with cooperation in earlier waves ranging from 88 to 92%.21 For this wave, we succeeded in interviewing 85% of eligible twins. Data on the male-male and male-female pairs (MMMF) came from a sample (birth years 1940–1974) with a 72% cooperation rate, initially ascertained directly from registry records containing all twin births. The first interview (MMMF1) was completed largely through phone in 1993–1996 and was followed by a second wave of interviews (MMMF2), conducted in 1994–1998, with a response rate of 83%. Data on symptoms of AD were used from the MMMF2 wave.
