Just as Facebook, MySpace and other Web sites have unleashed a potent new phenomenon of social networking in cyberspace, a growing body of evidence is suggesting that traditional social networks play a surprisingly powerful role in influencing how people behave - and how, as a result, our social connections impact our health.
The latest research comes from two academics on opposite coasts. The pair got widespread notice last summer, when they reported that obesity appeared to spread from person to person through social networks, almost like a virus or a fad.
In a follow-up to that provocative research, the team has produced similar findings about another major health issue: smoking.
In a study last month in the New England Journal of Medicine, they found that people's decision to kick the habit is strongly affected by whether others in their social network quit - even people they do not know. Indeed, entire networks appear to quit virtually at once.
Taken together, these studies and others are fueling a growing recognition that many behaviors are swayed by social networks in ways that have not been fully understood. And it may be possible, the researchers say, to harness the power of these networks for many purposes: encouraging safe sex, getting more people to exercise, even fighting crime.
"What all these studies do is force us to start to kind of rethink our mental model of how we behave," says Duncan Watts, a Columbia University sociologist.
"Public policy in general treats people as if they are sort of atomized individuals and puts policies in place to try to get them to stop smoking, eat right, start exercising or make better decisions about retirement, et cetera. What we see in this research is that we are missing a lot of what is happening if we think only that way."
The recent research was conducted by Nicholas A. Christakis, a medical sociologist at Harvard Medical School, and James H. Fowler, a political scientist at the University of California at San Diego. For both studies, they took advantage of detailed records kept between 1971 and 2003 about 5,124 people who participated in the landmark Framingham Heart Study.
Because many of the participants had ties to the Boston suburb of Framingham, Mass., many of them were connected somehow - through spouses, neighbors, friends, co-workers. This enabled the researchers to study a network that totaled 12,067 people.
When they analyzed the patterns of those who managed to quit smoking over the 32-year period, the researchers found that the decision appeared to be highly influenced by whether someone close to them stopped. A person whose spouse quit was 67 percent more likely to kick the habit. If your friend gave it up, you were 36 percent more likely to do so. If a sibling quit, your own chances increased by 25 percent.
A co-worker had an influence - 34 percent - only if the smoker worked at a small firm. The effects were stronger among the more educated, and among those who were casual or moderate smokers. Neighbors did not seem to influence each other - but friends did, even if they lived far away.
"You appear to have to have a close relationship with the person for it to be influential," Fowler says.
Still, the impact of one person quitting seemed to cascade through three degrees of separation, boosting the chance of quitting by nearly a third for people two degrees removed from one another.
"It could be your coworker's spouse's friend or your brother's spouse's coworker or a friend of a friend of a friend. The point is, your behavior depends on people you don't even know," says Christakis.
"Your actions are partially affected by the actions of people who are beyond your social horizon" - but in the broader network.
Interestingly, the researchers found that the size of smokers' own networks did not change over time, even though the overall number of smokers plummeted, from 45 percent to 21 percent of the population. So what happened?
"People quit in droves - whole groups of people quit together at roughly the same time," Christakis says. "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 a 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 says.
Another intriguing, and disturbing, finding: As more smokers quit, the remainder 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 says.
Social isolation itself is known 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," says Fowler.
He worries the same thing could happen in the fight against obesity.