Subjects for the current study were offspring of male twins from the Vietnam Era Twin Registry (VETR; Eisen et al., 1987, Henderson et al., 1990). The VETR comprises male-male twin pairs identified from military discharge files, each of whom served in the military during the Vietnam Era (1965–1975). Twin zygosity, monozygotic (MZ) or dizygotic (DZ), was determined using questionnaires and blood group typing (Eisen et al., 1989). Twin pairs from the VETR were selected for participation in a subsequent offspring-of-twins (OOT) study designed to examine the intergenerational transmission of illicit drug dependence based on differing levels of genetic and environmental risk to offspring. Twins (i.e., biological fathers of the subjects in the OOT study) completed a 1987 health survey (Henderson et al., 1990) in which the birth dates of offspring were reported and a 1992 telephone administration of the Diagnostic Interview Schedule (DIS-III-R; Robins, 1989) in which DSM-III-R alcohol abuse and dependence, drug abuse and dependence and psychiatric disorders were assessed (Tsuang et al., 1996, 1998). Beginning in 2002, twins were recruited into the OOT study based on 3 selection
