Obesity is contagious.

One person's obesity can significantly increase the chance that his or her friends, siblings and spouse also will become heavy, according to the first study ever done on how weight gain spreads through social networks. And if a person slims down, the people around him or her also may lose.

"Both obesity and thinness are socially contagious," says study co-author James Fowler, an associate professor of political science at the University of California-San Diego.

At the heart of the matter is the sharing of acceptable norms for weight, not just sharing the same eating-and-exercise habits, says internist Nicholas Christakis, also a study author and a professor of medical sociology at Harvard Medical School. If someone you care about gains weight, your notion of an acceptable body size may change. You may decide it's OK to go up a couple of sizes, he says.

About a third of Americans are obese, about 30 or more pounds over a healthy weight, which increases their risk of heart disease, diabetes and other problems.

In the study, which was funded by the National Institute on Aging, researchers examined social ties among 12,067 people in the Framingham (Mass.) Heart Study, a multigeneration collection of data covering 32 years.

Findings are reported in Thursday's New England Journal of Medicine:

— When people become obese, the risk of their closest friends becoming obese over the next two to four years increases by 171 percent; the risk for their casual friends increases by 57 percent; their siblings' risk, 40 percent; their spouse, 37 percent.

— The reverse also is true. When one person sheds pounds, it has a ripple effect and increases the chances by similar percentages that their friends, siblings and spouse will trim down, Fowler says.

— A man's weight has more impact on the weight of his male buddies and brothers than on his sisters or female friends. And a woman's weight has more impact on her girlfriends and sisters than her brothers or guy friends.

"Men look to men. Women look to women," Fowler says.

Surprisingly, researchers found the obesity risk is not affected by geographic distance. "If you have a close friend or a sibling who lives a mile or a thousand miles away, that person's weight gain can have the same affect on your weight," Christakis says.

John Foreyt, an obesity researcher at Baylor College of Medicine in Houston, suggests, "If you are trying to lose or control your weight, pick your friends carefully. You may not want to be around people who are gaining weight or who are too heavy."

This study may implications for treatments. "If we can get even a small number of obese people to lose weight, it might have a ripple effect, and we could contribute to reversing the obesity epidemic," Fowler says.