The answer is far from clear, but some factors seem to increase one's risk of developing PTSD. Traumas of great severity, or that occur in a series, are more likely to cause PTSD. (For example, prolonged combat with sniping, machine guns, artillery and air bombardment produces PTSD more often than brief combat with less weaponry.) Traumas between people (such as sexual assault or mugging) are more likely to cause PTSD than natural disasters like earthquakes.

Experts aren't sure why some people develop PTSD after a comparatively minor trauma while others exposed to great trauma do not. Those who are very young or old are more vulnerable, as are those with poor social support. Individuals with other anxiety disorders, depression and some personality disorders also seem more likely to get PTSD after extreme trauma.