The present sample was drawn from the Vietnam Era Twin Registry (VETR), a national register of 7,375 male-male twin pairs in which both members served in the military during the Vietnam-era (i.e., between 1965 and 1975). Registry creation and the method of determining zygosity have been described in detail elsewhere (Eisen et al., 1987, 1989; Henderson et al., 1990). In 1987, 10,981 veterans completed a mailed questionnaire for the Survey of Health (SOH) assessing education, service in Vietnam, and general health status. In 1992, 8,169 individuals in the Registry completed a telephone administration of the diagnostic interview schedule (DIS-III-R; Robins et al., 1989) for the Harvard Drug Study (HDS) that included lifetime assessments for psychiatric disorders, including use of, abuse of, and dependence on alcohol, nicotine and illegal drugs (Tsuang et al., 1996, 1998).
