In summary, we find that PTSD—a disorder that by definition requires an environmental exposure, trauma—is also partly genetic in origin. This result comes as a foregone conclusion for researchers steeped in the behavioral genetics literature, but for those unfamiliar with twin studies or skeptical of their results, heritability based on molecular genetic data is compelling. Reassuringly, these molecular genetic heritability results are consistent with twin studies. All available evidence suggests that PTSD heritability among females is higher than males. Overall, PTSD heritability is comparable to that of MDD, but female heritability is close to that of SCZ and BIP, two of the most genetically influenced psychiatric disorders. Male PTSD SNP-chip heritability, in contrast, was lower than any recorded for a major psychiatric disorder and not distinguishable from zero with current sample sizes. Although large in comparison with previous reports, the current sample size of over 20 000 was still a limiting factor in single variant analyses. The strong evidence of shared genetic effects between PTSD and SCZ is worth further investigation and consistent with data from birth cohort studies that
