Not all people are equally traumatized by the same incident. What kinds of personal factors influence the risk of developing PTSD? Research suggests that people with a previous history of trauma, those with pre-existing psychiatric problems or personality disorders, those with less social support, and those with an external locus of control are more likely to develop PTSD in response to a traumatic event. People with an external locus of control believe that outside forces determine the events in their life. They do not believe they have much control over their life. In contrast, people with an internal locus of control believe they have significant control over their life. They tend to be more optimistic and more proactive about solving problems. Finally, people who react with dissociation at the time of event are shown to be likely to develop PTSD.