Participants were drawn from ongoing studies of Australian children of twins selected from two broadly representative Australian volunteer twin panels maintained by the Australian National Health and Medical Research Council (NHMRC). The older panel (Cohort I) was born between 1893 and 1964 and ascertained as adults (Heath, Bucholz, Madden, Dinwiddie, Slutske et al., 1997). The younger panel (Cohort II) was born between 1964 and 1971 and ascertained as children through their parents in response to flyers distributed throughout Australian schools (Heath, Howells, Kirk, Madden, Bucholz et al., 2001; Knopik, Heath, Madden, Bucholz, Slutske et al., 2004). Twins in both cohorts are of primarily European decent reflecting the predominantly Caucasian Australian population from which they were ascertained. Detailed characterization of each panel is described elsewhere (see Heath et al., 1997; Heath et al., 2001).
