At the time this study began (2005), the volunteer Australian Twin Registry (ATR) had enrolled a total of 4,131 twin pairs born 1972–1979. Attempts to recruit these individuals to participate in the current cannabis study were made using 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 Queens-land Institute of Medical Research (QIMR) for potential participation in an interview-based study of substance use and mental health. Contact details of those consenting to such were then forwarded to QIMR who made separate attempts to (re)contact potential subjects, explain the purposes of the study to them, and enroll them in the study. Due to a lower than anticipated initial response from enrolled twins, efforts were made in the final phases of data collection to identify and recruit siblings of participating twins as there are well known increases in statistical power for genetically informative research design including three or more related individuals.
