Background: Determinants of cross-national differences in the prevalence of mental illness are poorly understood. Aims: To test whether national post-traumatic stress disorder (PTSD) rates can be explained by (a) rates of exposure to trauma and (b) countries' overall cultural and socioeconomic vulnerability to adversity. Method: We collected general population studies on lifetime PTSD and trauma exposure, measured using the WHO Composite International Diagnostic Interview (DSM-IV). PTSD prevalence was identified for 24 countries (86 687 respondents) and exposure for 16 countries (53 038 respondents). PTSD was predicted using exposure and vulnerability data. Results: PTSD is related positively to exposure but negatively to country vulnerability. Together, exposure, vulnerability and their interaction explain approximately 75% of variance in the national prevalence of PTSD. Conclusions: Contrary to expectations based on individual risk factors, we identified a paradox whereby greater country vulnerability is associated with a decreased, rather than increased, risk of PTSD for its citizens.