LONDON • When you’re smiling, the whole world really does smile with you.

A paper being published today in a British medical journal concludes that happiness is contagious – and that people pass on their good cheer even to total strangers.

American researchers who tracked more than 4,700 people in Framingham, Mass., as part of a 20-year heart study also found the transferred happiness is good for up to a year.

“Happiness is like a stampede,” said Nicholas Christakis, a professor in Harvard University’s sociology department and co-author of the study. “Whether you’re happy depends not just on your own actions and behaviors and thoughts, but on those of people you don’t even know.”

While the study is another sign of the power of social networks, it ran through 2003, just before the rise of social networking Web sites such as Friendster, MySpace and Facebook. Christiakis couldn’t say for sure whether the effect works online.

“This type of technology enhances your contact with friends, so it should support the kind of emotional contagion we observed,” he said.

Christakis and co-author James Fowler of the University of California in San Diego are old hands at studying social networks. They previously found that obesity and smoking habits spread socially as well.

For this study, published in the British journal BMJ, they examined questionnaires that asked people to measure their happiness. They found distinct happy and unhappy clusters significantly bigger than would be expected by chance.

Happy people tended to be at the center of social networks and had many friends who were also happy. Having friends or siblings nearby increased people’s chances of being upbeat. Happiness spread outward by three degrees, to the friends of friends of friends.

The power of social networks

Increase in the chance that you will be happy if the following people are happy:• 34 percent, next-door neighbor• 25 percent, nearby friend• 14 percent, nearby sibling• 8 percent, spouse

James H. Fowler, The Washington PostHappy spouses helped, too, but not as much as happy friends of the same gender. Experts think women in particular take emotional cues from people who look like them.

More on the study at www.bmj.com

Denver Broncos linebacker Wesley Woodyard celebrates their win over the Atlanta Falcons during second half action at the Georgia Dome in Atlanta on Sunday, Nov. 16, 2008. Denver came back to win the game 24-20. Photo by The Gazette, Bryan Oller