In this paper, we have presented preliminary results from an interview study of 3,824 Australian-born twins and their nontwin siblings which included comprehensive assessments of cannabis-related phenotypes and related measures of substance use and psychiatric disorders. A striking find-ing from the current study was the high levels of cannabis involvement: some lifetime use of the drug was normative with 64.7% of females and 75.2% of males reporting ever having used the drug. These rates of use modestly exceeded rates from an independent cohort of Australian twins born between 1964 and 1971 (Lynskey et al., 2002) indicating increasing use of cannabis in Australia for cohorts born before 1990, consistent with prior findings from the Aus-tralian Household Survey (Degenhardt et al., 2000). While the majority of those reporting cannabis use had used the drug only infrequently, there were substantial minorities who reported frequent use or experiencing problems as a consequence of their cannabis use. Strikingly, 11.8% of females and 24.5% of males met criteria for DSM-IV cannabis abuse and/or dependence.
