The sample was drawn from the Australian Twin Registry. From a pool of twin pairs born between 1972 – 1979, 3348 MZ and DZ twins and 476 of their siblings (mean age at time of interview = 32.1, SD 3.04, range 21–46) completed the interview component of a study of cannabis and other drug misuse. A full description of the study methodology and of the characteristics of participants has been published previously (Lynskey et al., 2012).
