Participants were members of the national Australian Twin Registry (ATR). Details concerning participant recruitment are reported elsewhere (Lynskey et al., 2002; Slutske et al., 2009). In 1996-2000, members of the ATR born between 1964-1971 were contacted to participate in a study involving a telephone administration of a structured psychiatric interview in which CD was assessed. The participation rate for this interview was 84.2%, with a total of N=6,265 individuals participating. There was no evidence that participants and non-participants in this interview differed in terms of their levels of CD symptomotology.1 Of the 6,265 twins who participated in the 1996-2000 interview, 2% (n=109) were excluded from the current analyses due to missing data. At the time of this interview, the twins were between 24-36 years of age (M=30, SD=2.44).
