Respondents were drawn from two broadly representative Australian volunteer twin panels maintained by the Australian National Health and Medical Research Council (NHMRC). Reflecting the predominantly Caucasian Australian population from which an older and young twin cohort were ascertained, respondents are of primarily European decent. While well-educated individuals are over-represented, this bias is more pronounced in the older cohort ascertained as adults than in the younger cohort, who were recruited as children and volunteered by their parents. However, any impact of differential participation rates as a function of educational attainment in the older cohort appears minimal (Heath et al., 1997; Heath, Howells, Kirk, Madden, Bucholz et al., 2001), as does potential bias with respect to drinking behavior (Jardine & Martin, 1984).
