Details on response rates of twins, biological mothers and offspring invited to participate are available from previous studies (Duncan et al., 2008; Scherrer et al., 2008). In brief, 80% (excluding twins who were too ill or untraceable) of the twin fathers participated, 71% of the mothers were interviewed from 2002–2003, and fathers gave consent to contact 950 offspring of whom 839 (88%) were interviewed initially from 2003–2004. Response rates did not significantly (chi-square=3.34; p=0.34) differ by genetic and environmental risk group. We observed 81.5% of HG-HE twins, 76.3% of HG-LE twins, 78.5% of MG-LE twins and 75.8% of LG-LE twins responded. As well, no differences in response rate among offspring were observed. Of those contacted for interview, 86.6% of HG-HE offspring completed interviews, 94.0% of HG-LE, 90.3% of MG-LE and 88.9% of LG-LE offspring (chi-square=7.53, p=0.44).
