The sample consisted of 9,591 adult twins from the Australian Twin Registry assessed in two separate cohorts. The first cohort of twins was born between 1964 and 1971 and interviewed between 1996 and 2000. The second cohort was born between 1972 and 1979 and interviewed between 2005 and 2009. More information on interview procedures and participant demographics is documented elsewhere (Lynskey et al. 2003, for more information on interview procedures and participant demographics;Lynskey et al. 2012). In the current study, we used data only from the 7,334 same-sex monozygotic (MZ) and dizygotic (DZ) twins. Lifetime non-drinkers (N=125) and singletons (N=1,127) were removed for the discordant twin analyses leaving 6,082 twin individuals (3,464 MZ and 2,618 DZ), ages 23–40 (mean age=30.47), 59% of whom were female. For the bivariate twin modeling, singletons were included in the analyses (MZF: 1,080 pairs + 287 unpaired; MSM: 652 pairs + 295 unpaired; MZF: 800 pairs + 254 unpaired; DZF: 504 pairs + 292 unpaired)
