The present sample was drawn from the Vietnam Era Twin Registry (VET-R), a national register of 7375 male-male twin pairs in which both members served in the military during the Vietnam-era (i.e., between 1965-1975). The construction of the Registry and the method of determining zygosity have been described in detail elsewhere (Eisen, Neuman, Goldberg, Rice, & True, 1989; Eisen, True, Goldberg, Henderson, & Robinette, 1987; Henderson et al., 1990). In 1992, 8169 individuals in the Registry completed a telephone diagnostic interview that included lifetime assessments for a number of psychiatric disorders, including abuse of/dependence on alcohol and illegal drugs (Tsuang et al., 1996, 1998). Telephone interviews were conducted by experienced interviewers from the Institute for Survey Research at Temple University, and interviewers obtained verbal informed consent as approved by the relevant Institutional Review Boards before conducting interviews. Of the 3934 individuals who had used at least one illegal drug, 95% used cannabis before any other illegal drug, 4% used cannabis only after using another illegal substance, and only 1% never used cannabis. Because early cannabis initiation was used as a
