But researchers later discovered that the standard approach at the time, in which the therapist urges a distressed person to talk through the experience and emotions, backfires for many people. They plunge even deeper into anxiety and depression when forced to relive the mayhem.

Crisis response teams now take a much less intense approach called psychological first aid, teaching basic coping skills and having victims recount experiences only if it seems helpful.

One of the biggest lessons of Sept. 11, said Richard McNally, a psychologist at Harvard who did not contribute to the new report, was that it “brought attention to the limitations of this debriefing.”

Another, he said, was that it drove home the fact that people are far more resilient than experts thought. No one disputes that thousands of Americans who lost loved ones or fled from the collapsing skyscrapers are still living with deep emotional wounds. Yet estimates after the attack projected epidemic levels of post-traumatic stress, afflicting perhaps 100,000 people, or 35 percent of those exposed to the attack in one way or another.

Later studies found rates closer to 10 percent for first responders, and lower for other New Yorkers. (The prevalence in children was slightly higher.)

“Some of us were making this case about resilience well before 9/11, but what the attack did was bring a lot more attention to it,” said George A. Bonanno, a psychology professor at Columbia.

It also stirred a debate that may soon change the definition of post-traumatic stress. In the breathless weeks and months after the attack, experts and news articles warned that people who had no direct connection to the tragedy would also develop diagnosable symptoms merely from seeing the images on a television screen.