Participants were offspring of male twins from the Vietnam Era Twin Registry (VETR), which is a national registry of monozygotic (MZ) and dizygotic (DZ) twin pairs who served in the military during the Vietnam Era (965– 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). Data used to select families for the present project came from 1987 and 1992 surveys of the twin fathers. A mailed questionnaire sent to the twins in 1987 was used to verify family composition including number and birth dates of their children (Henderson et al; 1990). The twin fathers' DSM-III-R nicotine dependence and other psychiatric diagnoses were obtained from responses to a 1992 computer assisted telephone administration of the Diagnostic Interview Schedule (Robins LN et al. 1988; Robins, et al. 1981)
