**2. Sex differences in PTSD prevalence**

Males are exposed to more potentially traumatic events (PTE's) than females (Tolin & Foa, 2008), particularly in adolescence and young adulthood (Norris et al., 2007). In the National Comorbidity Survey, 60.7% of males and 51.2% of females reported at least one PTE, and significantly more males than females reported exposure to more than two trauma types (Kessler et al., 1995). Despite the finding that males are more likely to experience a PTE and experience more types of PTE's than females, the female-male ratio in the prevalence of PTSD is approximately 2:1 (Tolin & Foa, 2008) with females reporting higher levels of both re-experiencing, avoidance, and arousal (Ditlevsen & Elklit, 2010). The lifetime prevalence of PTSD is 10.4% for females and 5.0% for males and the conditional risk across trauma types is 20.4% for females and 8.2% for males (Kessler et al., 1995). Sex differences in the prevalence of PTSD become evident early in life, peak in early adulthood, and become weakened with increased age (Ditlevsen & Elklit, 2010; Norris et al., 2002). Across studies, the increased prevalence of PTSD in females compared to males appears to be particularly evident for lifetime PTSD (Tolin & Foa, 2008), indicating that PTSD tends to be of longer duration in females than in males, and in the Detroit Area Survey of Trauma the median time from onset of PTSD to remission was four years for females compared to one year for males (Breslau et al., 1998).

Despite findings that sex differences in PTSD have been found across cultures and thus appear to be culturally persistent, variations regarding how pronounced such sex differences are have been reported. Norris et al. (2001) compared sex differences in a Mexican, an African-American, and an Anglo-American sample and found that sex differences in the prevalence of PTSD were amplified in the Mexican sample and attenuated in the African-American sample, with the Anglo-American sample falling in between (Norris et al., 2001). It has been suggested that sex differences in PTSD are particularly evident in communities that emphasise traditional gender roles (Norris et al., 2007). This suggests that social gender and biological sex are both important in making up such differences. Interestingly, the cross-cultural variations on sex differences appear to be more pronounced for intrusion and avoidance than for arousal symptoms, which are thought to be rooted in biological processes. It is thus possible that sex differences in arousal are primarily related to biological sex, whereas sex differences in avoidance and intrusion are also affected by social gender.
