May 21 (Bloomberg) -- The same peer pressure that leads people to try their first cigarette can work in reverse, pushing members of social circles to quit smoking together, a study says.

The analysis published today in the New England Journal of Medicine says groups of friends, relatives and co-workers often stop smoking in clusters.

The percentage of Americans who smoke dropped to 20.8 percent from 41.9 percent between 1965 and 2006, according to the U.S. Centers for Disease Control and Prevention. Many quit at the same time as other people they knew, the study found.

``People quit in droves,'' said Nicholas Christakis, a Harvard University researcher and co-author of the study. ``In a very fundamental way, decisions to quit smoking in humans are like decisions to fly to the left or right in birds in a flock. The individual bird doesn't decide alone.''

The researchers at Harvard in Cambridge, Massachusetts, and the University of California, San Diego, tracked people who were involved in a long-running health study and recorded the smoking status of their friends, spouses, neighbors and colleagues. The researchers found that people linked in social networks tended to quit around the same time.

According to the study, a smoker was 67 percent more likely to quit if a spouse did so, 36 percent more likely if a friend did so, and 25 percent more likely if a sibling did so.

It Takes a Village

Christakis said in a telephone interview today that he wasn't surprised at the influence of spouses or close friends. What impressed him, he said, was the effect of less direct relationships.

``We showed that people are more likely to quit not only when their friends quit, but also when their friends' friends and their friends' friends' friends quit,'' Christakis said. ``Decisions to quit are made not only by isolated individuals but also reflect collective decisions.''

Though groups often seemed to stop smoking in concert, Christakis said the process appears largely unplanned.

``It's spontaneous organization. There's no head fish,'' he said. ``We think what's happening is a change in norms within the networks. We think for the norm to be transmitted, people have to be connected. You don't affect the habits of people to whom you have no connection.''

The study shows that social networks can be an important tool in public health campaigns aimed at ending smoking or other unhealthy habits, said Richard Suzman, director of the U.S. National Institute on Aging's division of behavioral and social research.

``The culture of individualism is so strong that we sometimes forget how powerfully and silently social networks and those around us influence our health,'' Suzman said in an e- mailed statement.

To contact the reporter on this story: Rob Waters in San Francisco at rwaters5@bloomberg.net.

Last Updated: May 21, 2008 17:02 EDT