individual-specific environmental influences were evident for PTSD and AD, shared environmental factors were also found to play a role in trauma exposure, accounting for 33% of variance. Of greatest relevance to our primary research question, we found that the genetic factors that contribute to trauma exposure and PTSD were correlated at 0.54 with those that contribute to AD, thus accounting for just under 30 % of the genetic variance in AD. Although we did not test a causal model of the PTSD–AD association (Chilcoat & Breslau, 1998), our findings have implications for such models, specifically that they need to take into account the contribution of common genetic influences to the two disorders.
