Kick the habit: It's a chain reaction Credit: Corbis

“Tell me who your friends are and I'll tell you who you are.” In the context of recent scientific findings, the old Russian proverb could be revised to also include your friends' friends, and their friends, too.

Researchers have found increasing evidence that certain health-related behaviors ripple through social circles like fashions coming into style. Smokers are more likely to kick the habit when family, friends and colleagues are doing it, according to a study by Nicholas Christakis of Harvard Medical School and James Fowler from the University of California in San Diego (N. Engl. J. Med. 358, 2249–2258; 2008).

By analyzing 32 years of data on thousands of participants in a cardiovascular study, the scientists discovered that a person's likelihood of smoking decreases by 67% when a husband or wife kicks the habit, 36% when a friend quits and 34% when a co-worker quits.

Notably, the effect does not stop with close relationships but reverberates across large social webs, spreading as far as three degrees of separation. “Whenever you experience a change, that change will ripple to your friend, your friend's friend, and your friend's friend's friend,” says Fowler.

With every degree of separation, the influence diminishes by at least half, but the collective impact is powerful. “People were quitting in droves,” Christakis says of the smoking study findings.

Previously, Christakis and Fowler had published research suggesting that obesity is socially contagious up to three degrees of social separation (N. Engl. J. Med. 357, 370–379; 2007). They are now probing the effect of social networks on a variety of other phenomena, including alcohol consumption and prescription drug use.