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 are also not likely to be due purely to our exclusive focus on females or to the wider range of traumas represented in our sample, as the sample used by Stein et al. (2002) was nonmilitary and 75 % female. However, given that Stein and colleagues did not test for gender differences in heritability estimates, it is possible that modest distinctions by gender in the relative contribution of genetic factors were present but not identified. Another factor to consider is that the prevalence of PTSD in our sample is lower than that reported for women in previous studies (Resnick et al. 1993; Kessler et al. 1995; Breslau et al. 1998). If this reflects under-reporting, we may have identified only the most severe cases of PTSD. To the extent that severity is associated with heritability, categorizing only cases at the high end of the severity continuum as positive for PTSD could inflate the heritability estimate. Importantly, although we found a greater
