There are several possible reasons why female heritability may be higher than male heritability, including differences in trauma exposure47, 48 that itself is heritable,49, 50 sex-based biology and sex-based symptom differences. Given that heritability estimates convey the relative importance of genetic and environmental influences, sex-based differences in either would affect heritability. We can assume comparable genetic variation in female and male subjects because we only used genetic variants that conformed to Hardy–Weinberg equilibrium expectations (thus implying no selective removal of particular alleles). This leaves sex-based differences in relevant environmental influences as one explanation and, indeed, rates of exposure to various types of trauma are known to vary by sex.51 As well, particular trauma types vary in the degree to which they are associated with PTSD.52 Greater variability in the profile of PTSD-inducing environmental influences in males, as compared with females, would lead to greater relative importance of environmental factors in the development of PTSD in males, and this could explain observed lower heritability estimates in males. This possibility could be tested via detailed study of trauma and environmental histories of
