Post-traumatic stress disorder, or P.T.S.D., occurs when an experience is so disturbing that it disrupts the information processing system of the brain. This system has as one of its main functions the transformation of disturbing experiences into mental adaptation. That is, it takes a disturbing event and processes it in such a way that appropriate neural connections are made within the memory networks, which eliminate those aspects of the event (for example, negative thoughts, unpleasant emotions and physical sensations) that are no longer useful.
Sometimes, however, the event is so disturbing that the system is unable to perform these natural functions. The result is that the memory of the incident is stored along with the psychological and physical aspects of the event, including the negative beliefs that it engendered. Such an unprocessed traumatic memory may be stimulated by a current experience, and the encoded negative emotions, thoughts and sensations can emerge and color the perception of the present.
The reason that some people are affected more than others depends on genetics, the intensity of the experience, length of exposure and earlier life experiences. Some people have had positive experiences that contribute to greater resilience. Others have had negative experiences that can make them susceptible to later problems. For instance, an official diagnosis of P.T.S.D. requires that the individual experience a major trauma, like a rape, accident or battlefield experience. However, recent research indicates that in many cases, P.T.S.D. symptoms can occur as the result of less dramatic events. Some examples are hurtful childhood experiences with parents and peers, which can have a very negative effect on a person’s sense of self-worth. These events can set the groundwork for a wide range of symptoms, including a vulnerability to P.T.S.D.
Source: Francine Shapiro, Ph.D (creator of the EMDR technique) from an interview in The New York Times, March 16,2012