Why are we unequal when it comes to post-traumatic stress disorder?

Post-traumatic stress disorder (PTSD) is an illness that occurs after a traumatic event. Of all people who experience trauma, only 25-35% develop PTSD. A team of scientists at the Swiss Federal Institute of Technology in Lausanne set out to understand what factors make some people more vulnerable, focusing in particular on glucocorticoids, hormones (e.g. cortisol) that our bodies release when we are stressed.