Sample 1 consisted of 6,265 individuals aged between 23 and 39 years (mean = 29.9±2.5) interviewed between 1996 and 2000. Participants were members of the young adult cohort, a volunteer panel of twins born between 1964 and 1971. The interview was based upon a modified version of the SSAGA (Semi-Structured Assessment of the Genetics of Alcoholism33). Detailed information about the sample recruitment, the study procedure, and the measures can be found elsewhere34. Sample 2 comprised 9,688 individuals aged between 18 and 91 years (mean = 46.3±11.3) interviewed between 2001 and 2005. Participants were members of the older and younger adult cohort of Australian twin pairs (born between 1895 and 1964, and between 1964 and 1971, respectively). A subset of this sample was ascertained based on large sibship size, or having a relative with nicotine or alcohol dependence. The interview used for this sample was also based upon a modified version of the SSAGA. Further details about the sample and assessment can be found in Heath et al. (2011)35.
