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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.”

Next page: Can coworkers make you merry?