A new research (performed by the University of California) has shown that some people could be more susceptible to develop a Post-Traumatic Stress Disorder (PTSD) after experiencing a life-threatening event than others.
This susceptability seems to be inherited: there are specific variants of two genes involved (TPH1 and TPH2). Probably these the specific gene variants produce less serotonin, which could explain why these people could be more vulnerable to develop PTSD.
More research about this needs to be done in the future, but for now you can read more at http://newsroom.ucla.edu/portal/ucla/ucla-study-identifies-first-genes-231248.aspx