“It was appropriate to go after Bin Laden, just to try to cut the head off that serpent, but I don’t think it’s decent to celebrate a killing like that,” said George Horwitz, a retired meat cutter and Army veteran in Bynum, N.C.  Others were much more critical. “The worst kind of jingoistic hubris,” a University of Virginia student wrote in the college newspaper, The Cavalier Daily. In blogs and online forums, some people asked: Doesn’t taking revenge and glorying in it make us look just like the terrorists?  The answer is no, social scientists say: it makes us look like human beings. In an array of research, both inside laboratories and out in the world, psychologists have shown that the appetite for revenge is a sensitive measure of how a society perceives both the seriousness of a crime and any larger threat that its perpetrator may pose.  Revenge is most satisfying when there are strong reasons for exacting it, both practical and emotional.  “Revenge evolved as a deterrent, to impose a cost on people who threaten a community and to reach into the heads of others who may be contemplating similar behavior,” said Michael McCullough, a psychologist at the University of Miami and author of “Beyond Revenge: The Evolution of the Forgiveness Instinct” (Jossey-Bass, 2008). “In that sense it is a very natural response.”  Many of the sources of the joyous outburst were obvious: A clear victory after so many drawn-out conflicts. A demonstration of American competence, and of consequences delivered. The public relations value of delivering a public blow to a worldwide terror network. And, it needs to be said, the timing: The news hit just as many bars were starting to clear out for the night.  But this was much more than a simple excuse to party.  “Pure existential release,” said Tom Pyszczynski, a social psychologist at the University of Colorado at Colorado Springs, who has studied reactions to 9/11. “Whether or not the killing makes any difference in the effectiveness of Al Qaeda hardly matters; defeating an enemy who threatens your worldview, the very values you believe are most protective, is the quickest way to calm existential anxiety.”  After almost 10 years, the end was nothing if not final. “The emotions were so strong, I think, because the event was compacted: Bin Laden was found and killed, and it was done — done and over, just like that,” said Kevin Carlsmith, a social psychologist at Colgate University and the Center for Advanced Study in the Behavioral Sciences at Stanford. “We’re so used to people being brought in, held at Guantánamo, the trials, the appeals; it feels like justice is never done.”  As a rule, people are far more forgiving than they might guess, studies find. After most betrayals, like being dumped by a love interest or insulted, the urge for revenge erodes around the same rate that certain memories do: sharply in the first few weeks, and much more slowly afterward. The same kind of pattern can follow even physical assaults, depending on the circumstances and the personality of the victim.  “The intensity of the emotion falls off precipitously, simply because the body can’t carry such a giant load of outrage and function very well,” Dr. McCullough said.  But the urge for payback — especially for a crime like the Sept. 11, 2001, attacks, which killed almost 3,000 civilians — never goes to zero. “There is a stubborn part of the memory that hangs on to the urge, to a little piece of it,” and the pain is refreshed every time the memory is recalled, Dr. McCullough said.  It is easy to forget how much fear was in the air after the 9/11 attacks: the anthrax mailings, the airport lines, the color-coded terror alerts. Many of those celebrating late on Sunday and early Monday were teenagers during those years, young people who have lived much of their lives under the threat of terrorism — and this terrorist in particular — and who had the time and energy to hit the streets and share the moment.  “For them this was a chance to be a part of history,” Dr. Pyszczynski said.  In a long series of studies, psychologists have shown that when people are reminded that they will one day die, they fixate on attributes they consider central to their self worth. Those who are religious become more so; those who value strength or physical attractiveness intensify their focus on these qualities; and people generally become more patriotic, more supportive of aggressive military action.  “Even subtle reminders of 9/11 have the same effect,” Dr. Pyszczynski said.  The sight of Bin Laden’s face on television or a smartphone news feed might have been enough to move people from the sidelines into the streets, to cheer for the home team.  Finally, people everywhere have a strong belief in “just deserts” punishment. In a 2002 study, psychologists at Princeton University had more than 1,000 participants evaluate specific crimes and give sentencing recommendations for each. The subjects carefully tailored each recommended sentence to the details of the infraction, its brutality and the record of the perpetrator.  The drive to enforce those sentences varies widely from person to person. But in a crowd of like-minded people, the most intense drives for justice become the norm: People who may have felt a mix of emotions in response to the news can be swept up in the general revelry.  Thus the natural urge for revenge — satisfied so suddenly, releasing a decade of background anxiety, stoked by peers — feeds on itself. Delight turns to chanting turns to climbing on lamp posts.
Raucous rallies that broke out in New York and Washington after news of Osama bin Ladenâs killing were a matter of âpure existential release,â a social psychologist said.