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By Kate Stinchfield

THURSDAY, Dec. 4, 2008 (Health.com) — If your friends are happy, you're often happy too. That's a no-brainer. But what about your next-door neighbor—can his happiness rub off on you? A new study says yes.

Using data from the comprehensive Framingham Heart Study, James Fowler of the University of California at San Diego and Nicholas Christakis of Harvard Medical School analyzed the happiness and social networks of 4,739 people from 1983 to 2003.

After figuring out how everyone was related, the researchers looked at participants' answers to four questions on a commonly used depression index. They found patterns of happiness among groups of friends, spouses, family members, and neighbors.

Not surprisingly, an individual's happiness was influenced by that of the people around her. But the study, published Thursday in the British Medical Journal, also revealed that the subjects were affected by the happiness of people who were as many as three degrees removed from them.

The findings show that if you have a happy friend, you're 15% more likely to be happy. But if a friend of a friend of a friend is happy, your chances of being happy are still about 6% greater than they would be otherwise. In other words, your friend's buddy—even if he's a stranger to you—may have a significant effect on your outlook on life.

"We've known for some time that social relationships are the best predictor of human happiness, and this paper shows that the effect is much more powerful than anyone realized," says Daniel Gilbert, PhD, a professor of psychology at Harvard University.

Distance plays a role

Physical proximity was a strong predictor of how happiness spread in social networks. Nearby friends—those who lived within a mile of the subjects—who were happy increased a subject's chance of being happy by 25%. Distant friends—those living more than a mile away—had no significant effect on someone's happiness.

So the faraway folks we keep in touch with on our iPhones and Facebook pages may not have as great an influence on our emotional life as the neighbor we wave to when we take out the trash.

"This means that life is much more local than people think it is," says John F. Helliwell, a fellow at the Canadian Institute for Advanced Research. "Modern technologies allow you to maintain these very long-distance networks, but it's the local ones that really count."

Sitting next to a jolly cubicle mate does not mean you'll catch the bliss bug. The study found no relation between the happiness of coworkers. This suggests, the authors write, “that the social context might moderate the flow of happiness from one person to another"—meaning that the office environment may not be fertile ground for gleefulness to grow.

The researchers didn't investigate why your downstairs neighbor may influence your mental state more than your sister three hours away. And because it was conducted in one New England town, the results may not translate to other communities, wrote Peter Sainsbury, the director of the Division of Population Health in the Central Sydney Area Health Service, in an editorial accompanying the study. "Perhaps Framingham [Massachusetts] was unique in some way," he surmised. "Don't drop your unhappy friends yet."

Why the study matters

The discovery adds to a growing body of research that proves social networks are powerful predictors of behavior. Two recent studies found that you're more likely to be obese or a smoker if those close to you are. The authors of the BMJ study note that those findings could have significant implications on how we approach public health. Better care for people who are ill, for example, may end up making lots of people—some of them strangers—healthier or happier.

The results are also fuel for scientists studying the importance of positive states. By understanding the conditions in which happiness spreads, they argue, we may be able to foster well-being, rather than trying to restore it once it has been lost.

"Traditionally, we've waited until something goes wrong and then we've tried to fix it," says Helliwell. "We ought to be thinking of how to improve people's well-being, because the things that make people happy transmit positively to their neighbors."