Participants were members of the young adult cohort of the Australian Twin Registry, a volunteer twin panel maintained by the Australian National Health and Medical Research Council (the older cohort of the registry, born from 1944 to 1963, provided data for an earlier analysis of the heritability of alcohol dependence; see Heath et al. 1997). All twins in the current study were born in Australia between 1964 and 1971 and were recruited into the Australian Twin Registry through mass media and school system appeals to their parents between 1980 and 1982 (Lynskey et al. 2003). Twins participated in a telephone interview during 1996–2000, when their mean age was 30 years (range 24–36 years). Data from 2711 pairs of twins and 761 singletons with complete data on alcohol abuse and dependence symptoms were used in this analysis (55% female).
