Participants in this study derive from 2 interrelated studies of Caucasian same-sex twin pairs who participated in Virginia Adult Twin Study of Psychiatric and Substance Use Disorders (VATSPSUD) (Kendler, 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 Commonwealth of Virginia. Female–female twin pairs (FF), 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%. Zygosity was determined by discriminate function analyses using standard twin questions validated against DNA genotyping in 496 pairs (Kendler and Prescott, 1999). All female–female data on AC and alcohol dependence used in this report were collected at the fourth wave of interviews (FF4), conducted in 1995 to 1997. For this wave, we succeeded in interviewing 85% of the sample who had responded to the previous questionnaire. Data on the male–male (MM) pairs, birth years 1940 to 1974, came from a sample initially ascertained directly from registry records, which contained all twin
