Data were derived from two community-based samples from the Australian Twin Registry (ATR). Sample 1 included 6,257 individuals (55.2% female) aged 24–36 (mean age 29.9, SD = 2.5) who were interviewed between 1996 and 2000 (Lynskey et al., 2002). Sample 2 included 3,326 twins (64.8% female), aged 27–40 (mean age 31.9, SD = 2.5), who were interviewed between 2005 and 2009 (Lynskey et al., 2012). Additionally, 476 nontwin siblings were interviewed. However, as the age range was broad (21–46 years) and some of them may not have been past the age of risk for CUD symptoms (Wagner and Anthony, 2002), we excluded the nontwin siblings from analyses. Despite different birth years and different years at interview, the twins from both samples were approximately (within two years) the same age at the time of the interview. The total sample consisted of 9,583 individuals (58.5% female), with a mean age of 30.6 (SD = 2.6). There were 2,472 female identical (monozygotic; MZ) twins, 1,630 male MZ twins, 1,877 female non-identical (dizygotic; DZ) twins, 1,314 male DZ twins, and 2,290 opposite-sex DZ twins.
