感冒会传染，吸烟会传染，可您知道吗？戒烟也可以传染。研究人员发现戒烟也是一种社会活动，也会“传染”。

Smoking has always been a social habit, but researchers now believe that quitting may be a social activity too.

It's no surprise that you're more likely to light up if your close friends do. But Nicholas Christakis of Harvard Medical School and James Fowler at University of California San Diego report that quitting smoking may be contagious as well. Even people who don't necessarily know each other, but are connected in some distant way, tend to stop smoking at the same time. "People tend to quit smoking in droves, and this coordinated quitting is literally like a flock of birds changing direction," says Christakis. "So smoking is not an individual behavior, but rather a collective process."

Here's how it works. Christakis and Fowler traced the social network of 5,000 individuals who were enrolled in the large, federally funded Framingham Heart Study over a period of 32 years. The authors carefully worked out the relationships among the subjects, many of whom were related by family, social or professional ties. Then, they layered onto this network the number of cigarettes each person smoked a day, from zero upward.

Back in 1971, when the Framingham study began, smokers and nonsmokers were equally likely to be at the center of their social-relationship "nodes." By 2000, however, nonsmokers not only outnumbered the smokers, in all age groups, but they had pushed smokers to the edges of any networks they belonged to - smokers were no longer connected to as many other people. Such marginalization, says Christakis, reflects the new perception by the network as a whole that smoking isn't as desirable any more. "This shows that our health behaviors are not just affected by our friends, but by our friend's friend's friend, because behaviors in a network cascade throughout the network," he says.

The idea is that people pay it forward - with health. When one person (we'll call him the index case) quits smoking, his closest contacts, such as friends and family members, become 36% less likely to be smokers too. These folks then influence their social circles, and so forth, until people several degrees removed from the index case also become nonsmokers. In the study, even people who did not mutually identify themselves as friends, but were in the same social network, were affected by each other's behavior: people who labeled themselves as friends of the index case, for example, but were not similarly identified as friends by the index case, were still 20% less likely to smoke if the index case decided to quit.

Such ripple effects among social groups may seem pretty obvious - people naturally look to their friends to figure out what behaviors are socially acceptable - but Christakis notes that the scope and size of the networks in which these effects operate is much larger than previously thought. His research, for example, shows that geographical distance between individuals in the network doesn't seem to weaken behavioral influences. That means that prevention and treatment programs for health-related behaviors such as quitting smoking, losing weight and exercising could become more efficient by taking advantage of the network effect. "The wonderful property of social networks is that they augment what you seed them with, so if you can seed a network with a smoking cessation program, you will get multiplicative power in getting results." When it comes to getting healthier as a nation, it may take a village after all - a well-connected one.