Excerpt: "Posttraumatic stress disorder (PTSD) is a psychological disorder precipitated by exposure to a traumatic event or a series of events. This event is usually experienced by an individual. However, PTSD can also develop as a result of observing or hearing of a traumatic event occurring in someone else's life (such as a relative or a close friend). PTSD was introduced as a disorder in 1980 in the DSM-III. The DSM-Ill-R's condition for diagnosis was the experiencing of a traumatic event that was "outside the range of usual human experience." However, in DSM-IV, the focus is not so much on the nature of the event as it is on the individual's response to the event and his or her vulnerability to developing the characteristic symptoms."
Seegobin, Winston, "Posttraumatic Stress Disorder" (1999). Faculty Publications - Grad School of Clinical Psychology. 309.