Few people mark the events as a milestone.
Mental-health experts feared unprecedented psychological damage from the Sept. 11, 2001, attacks. But five years later, mounting evidence from the most-studied emotional trauma the nation has faced suggests that people were more resilient than expected, though thousands who were closest to the disaster still need help.
Despite widespread concerns among mental-health professionals that the constant exposure to sights and sounds on television and other media in the days after the attacks would widen their impact, surveys and other studies over the past five years suggest that damage to the national psyche was limited and short-lived.
"There was a lot of concern about the prevalence of serious emotional responses, such as post-traumatic stress disorder [PTSD], after the attacks. It looks as though it was widespread, but not prolonged," said Dr. David Spiegel, a professor of psychology and behavioral sciences at Stanford University who tracked the emotional fallout of 9/11 through an Internet-based survey.
At least one national study done two months after the attacks found that those who spent more hours watching coverage of the assaults were more prone to have symptoms of PTSD: flashbacks of the event; avoidance of things that recall the trauma, or irritability and sleep disturbances from being in a state of constant alert.
But within six months after 9/11, studies showed that outside New York and the Pentagon community, symptom levels that had been three or four times greater than normal around the country had returned to "background" levels of 4 percent or 5 percent. Other surveys suggest that recovery for those with the greatest personal exposure has been slower.
Experts also note that traumatic events can contribute to mental disorders related to anxiety, depression or phobias beyond those classified as PTSD.
Survivors of 9/11 may be among the most intensely scrutinized group of mental-health subjects in history, with their thoughts and reactions plumbed in personal interviews and phone and Internet surveys, among other methods.
So frenzied was the surveying in the late fall of 2001 that the New York Academy of Medicine cautioned the research community about overwhelming people in lower Manhattan.
Still, the broad outreach and ongoing monitoring promise to give behavioral scientists many new insights into both psychological recovery from disaster and how people remember and process events.
"Not only did it affect so many people, but we have so many more tools available to study their reactions," said Elizabeth Phelps, a New York University researcher. She is part of a consortium that has done detailed surveys about what people in New York, Washington and elsewhere remember from that day, beginning just a week after the attacks.
"We're also doing brain-imaging studies that we'll be publishing soon that show very different responses in people who actually witnessed the events of 9/11 compared with those who were more distant," she said.
Surprisingly few people, even those who were in Manhattan, mark the events of 9/11 as a particular milestone in their lives, another study shows.
"Just because there's a shocking and dramatic news story doesn't mean everything is different," said Dr. Norman Brown, a psychology professor at the University of Alberta in Canada. He found that few people, even in New York, mark dates like weddings and births in context of the attacks. "People feel bad for a few days, maybe a few weeks, they may feel insecure, but on the other hand, almost everybody continues to go on with their own lives."
Dr. Charles Goodstein, a psychiatrist at the NYU Medical Center, argues, however, that a sense of increased vulnerability still affects most Americans to some extent, although it may be more immediate for New Yorkers.
"PTSD always requires firsthand experience of a trauma, but with 9/11, repeated exposure to the news, pictures and proximity made everyone feel part of the trauma, leading to feelings that reawaken with reports of every new and foiled terrorist attack," he said.