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Social Networks' Sway May Be Underestimated

"People quit in droves -- whole groups of people quit together at roughly the same time," Christakis said. "You can see it ripple through a network. It's sort of like an ant colony or a flock of birds. A single bird doesn't decide to turn to the right or the left; the whole flock has mind of its own."

The study did not examine why this occurs, but it is probably the result of a shift in social norms within each group -- smoking becoming unattractive or disparaged.

"Something changes in the zeitgeist that makes smoking unacceptable, and all these people move together in lockstep," Christakis said.

Another intriguing -- and disturbing -- finding was that as more people quit, the remaining smokers tended to wind up on the edges of society, with fewer and fewer social connections.

"In 1971, you have this crowd of people, and smokers are dispersed among them. But eventually by 2003, the smokers have been pushed to the periphery of the crowd," Christakis said.

That indicates that the remaining hard-core smokers are more socially isolated, which by itself has been shown by other research to have negative health consequences.

"So at the same time we are trying to help smokers to quit, we have unintentionally been hurting them by wreaking havoc on their social lives," Fowler said. "One of the implications is it's harder to reach smokers. Increasingly, they are huddled together in groups that are not connected to other people who don't smoke."

The findings could also have implications for the obesity epidemic.

"If we use these norms to fight the obesity epidemic, we may, in the process of stigmatizing the state of being overweight, further stigmatize obese people," Fowler said. "Smoking is an example of how we can create problems at the same time we solve others."

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