respondent twins at this assessment was 24.4 years (SD = 1.50, range 22.8 to 27.2), with a response rate of 88.1%. For ease of presentation, this assessment is referred to as age 25 throughout this paper. Parallel to current practice in gene identification efforts for alcohol dependence, only individuals who had evidence of alcohol exposure were included in twin analyses, so that genetic and environmental influences on the decision to initiate alcohol are not confounded with genetic and environmental influences on AC or problems. After exclusion of individuals who had not been exposed to alcohol, data were available for 685 complete pairs of twin brothers (287 monozygotic [MZ] and 398 dizygotic [DZ]) and 693 complete pairs of twin sisters (378 MZ and 315 DZ).
