Why do some people develop post-traumatic stress disorder after a trauma and others remain symptom free? We are still trying to understand resilience, one characteristic of good mental health.

Researchers studying survivors of the September 11, 2001 World Trade Center attack have been surprised to find many of them to be free of PTSD symptoms. Resilience was less common among people who were actually in the buildings or in the immediate area, but the researchers report that even for "highly exposed individuals" the frequency of resilience never fell below one third.

Read more about resilience and the World Trade Center research.