The heritability of alcohol dependence (AD) is 50–60% (Reed et al. 1996; True et al. 1996; van den Bree et al. 1998), with the remainder of variance accounted for by individual-specific (i.e. non-shared) environmental influences. Twin studies of PTSD indicate that PTSD is attributable to a greater extent to individual-specific environmental sources of influence (with no substantial shared environmental influences), but genetic contributions are consistently found, typically accounting for about 30 % of variance in risk for the disorder (True et al. 1993; Stein et al. 2002; Koenen et al. 2008). Unlike AD, which has been studied extensively in females, producing nearly identical heritability estimates to those found for males (Kendler et al. 1994; Heath et al. 1997; Prescott et al. 1999), genetic influences on PTSD in women have been examined in only one known twin study (Stein et al. 2002). The vast majority of genetically informative research on PTSD is based on twins from the all-male Vietnam Era Twin Registry (VETR).
