Participants were biological (genetic) offspring of male twins from the Vietnam Era Twin Registry (VETR), a national registry of monozygotic and dizygotic twin pairs who served in the military during the Vietnam era (1965–1975). Construction of the registry and method of determining zygosity have been previously reported (Eisen et al., 1987; Eisen et al., 1989; Henderson et al., 1990). In the present study, we analyze data collected from twin-families participating in two complementary OOT projects initiated in 2001 and 2004, respectively: offspring of twin fathers concordant or discordant for alcohol dependence (AD; Project 1; for details see Jacob et al., 2003) and offspring of twin fathers concordant or discordant for illicit drug dependence (DD; Project 2; for details see Scherrer et al., 2008). Both studies included offspring of unaffected twin pairs (as controls) as well as the biological or custodial mothers of the offspring. Offspring were eligible to participate if the twin gave permission to contact them (Project 1, but not Project 2, also required permission from the mother before contacting offspring). Experienced staff from the Institute for Survey Research
