Potential common genetic contributions to PTSD and AD have been tested in studies using the VETR sample. Xian et al. (2000) examined PTSD in combination with AD and illicit drug dependence (DD) and reported that 15.3 % of variance in PTSD was accounted for by heritable influences common to AD and DD and 55.7 % of variance in AD was attributable to genetic sources of influence shared by PTSD and DD. In Scherrer et al.’s (2008) study of combat exposure, PTSD and AD in VETR twins, a link found between combat exposure and AD was accounted for by genetic influences shared with PTSD. By contrast, in Koenen et al.’s (2003) co-twin control design study, rates of AD in monozygotic (MZ) co-twins of PTSD-positive twins did not differ from MZ co-twins of combat-exposed twins who did not develop PTSD, indicating that genetic liability to PTSD did not confer additional risk for AD.
