The Australian Twin Registry (ATR) includes a total of 4131 twin pairs born between 1972 and 1979. The ATR approached 7850 twin individuals of which 3876 (49%) consented to participate, 1971 declined and the remainder gave passive refusals. Attempts to recruit these individuals to participate in the present study were made using a two tiered process, as required by the ATR’s ethics committee: first, the ATR contacted twins (by mail and subsequently by telephone) and asked if they were willing to have their name and contact details forwarded to the Queensland Institute of Medical Research (QIMR) for potential participation in an interview based study of substance use and mental health. Contact details of those consenting were then forwarded to QIMR who recontacted potential subjects to explain the purposes of the study and enrol them in the study. Further details regarding the recruitment procedure and study characteristics can be found in Lynskey et al. (2012).
