Second, female and male sex may be viewed as environmental variables, in that genetic variation is expressed in the context of a particular individual, and there are differences in female and male biology (for example, sex hormone levels). To the extent that the female biological ‘environment’ is more conducive to the expression of genetic variation, and the male biological ‘environment’ tends to dampen genetically influenced variability in PTSD liability, sex itself may account for different heritability estimates in females vs males. Biological differences have been reported between sexes regarding responses to trauma51 and responses to environmental variables thought to mediate the later development of PTSD.53 Thus, there may be sex-based biological differences in trauma liability. In order for such effects to explain the observed greater heritability in females compared with males, the specified biological processes would need to lead to greater expression of genetically influenced liability to PTSD in females compared with males.
