What's more, smokers tend to quit in groups, and those who don't stop puffing increasingly find themselves pushed to the edge of their social circles, the researchers found.
"Your smoking behavior depends upon not just the smoking behavior of the people you know, but also the people who they know" and so on, said Dr. Nicholas Christakis, a medical sociologist at Harvard Medical School and lead author of the new report.
The findings back up previous studies showing that peer influence plays a key role in people's decision to stop lighting up and provide evidence that the "buddy system" used by smoking cessation, weight loss and alcoholism programs to change addictive behavior works.
"Anecdotally, we hear people say they quit smoking because their spouse or friend quit," said Jennifer Unger, a smoking prevention expert at the University of Southern California who had no role in the study. "If you influence a few people, those people might go on to help others to quit."
Last year, Christakis and his colleague James Fowler of the University of California, San Diego, published a study suggesting that obesity can spread among friends, much like an infectious disease. The duo mined data from a large social network of people who had been followed for three decades and found that when one person gained weight, close friends tended to pack on the pounds, too.
Their latest study, which appears in Thursday's New England Journal of Medicine and is funded by the National Institute on Aging, focused on people's smoking habits in the same social network.
The researchers examined the social lives of 12,067 people in the Framingham Heart Study, which has been tracking the health of residents of that Boston suburb from 1971 to 2003. They were able to reconstruct people's ties to one another since participants had to list contact information for their family, friends, co-workers and neighbors so researchers would not lose track of them over the years. The prevalence of smokers in the Framingham study over the years mirrored national trends.
Not surprisingly, the greatest influence was seen in close relationships. When a spouse stops smoking, the other partner is 67 percent less likely to smoke. Similarly, when a friend quits, the odds of the other continuing drops by 36 percent. The odds are similar among co-workers and siblings.
People who were connected to others by up to three degrees of separation were also influenced. If one person quits, the odds of a person two degrees apart stopping is 29 percent. In a three-degree separation, the chances are 11 percent.
"One person in the group gets the motivation to quit and it starts to cascade and ripple through the group," said Fowler.
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Jill Palmer, 28, was a one-pack-a-day smoker until she checked into a cessation program last year at the University of Wisconsin, Madison where she works. She took nicotine gum and worked with a counselor to set a "quit date."
Several days after Palmer went smoke-free, her husband threw away his last pack.
"It was spurred by my timing. He didn't want to be a smoker anymore," said Palmer, who credits her nonsmoking co-workers with persuading her to enroll in the cessation program.
The researchers also found, by analyzing random samples of smoking clusters, that whole groups became nonsmokers over time. People who remained smokers found themselves moving to the fringe of their social circles.
Cigarette smoking kills about 400,000 people in the United States every year, according to the Centers for Disease Control and Prevention. About 45 million U.S. adults are smokers, though the prevalence has fallen dramatically since the 1960s.
Stanley Wasserman, an Indiana University statistician who studies social networks, noted that while the study was cleverly done, it does have its limitations.
Wasserman said it's hard to tease out whether social influence is mainly responsible for a whole group kicking the habit. Other factors such as public bans on smoking or studies highlighting the harmful effects of smoking may also play a role.
"You can't prove it with this data," he said. "You have to go to people and ask,
'Why did you stop smoking?'"
On the Net:
New England Journal: http://www.nejm.org/
Framingham Heart Study: http://www.framinghamheartstudy.org/
Press; By ALICIA CHANG]
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