The relative contributions of genetic and environmental factors to AD observed in our sample are very similar to those reported in earlier studies, with additive genetic factors accounting for just over half of the variance in the disorder and individual-specific, but not shared, environmental influences accounting for just under half (Kendler et al. 1994; van den Bree, 1998; Prescott et al. 1999). By contrast, the heritability estimate for PTSD in our sample is approximately twice that reported in earlier investigations. Xian et al.’s (2000) study using the VETR sample, which is among the few to examine the full lifetime PTSD diagnosis, reported the heritability of PTSD to be 35%. Although not as directly comparable to our findings, similar values have been reported for PTSD symptoms. Stein et al. (2002), for example, found that 38 % of variance in PTSD symptoms was accounted for by genetic sources. Discrepancies between our findings and earlier reports cannot be attributed to the possible confounding effects of genetic contributions to trauma exposure itself, as we modeled trauma as a separate phenotype in our analyses. They
