From Hollywood movies to front-page headlines, the tormented veteran unable to transition from war zone to home front is a well-known concept. But post-traumatic stress disorder (PTSD) in fact hits more civilians than soldiers, and more women than men. And it manifests with a dizzying range of symptoms, from flashbacks, nightmares and aggression, to draining depression, numbness and avoidance. But as scientists search for an effective treatment, they’re still trying to understand a perplexing question. When so many people experience the trauma of sudden loss, near-death, and violence, what makes some more vulnerable to PTSD than others? Unfolding discoveries raise key questions about the fault-lines of fear and memory, and the roles geography and childhood development all may play in predicting personal responses to trauma.