According to a new article in the New England Journal of Medicine, obesity is socially contagious, spreading from person to person within a social network. If your friend becomes obese, your odds of becoming obese go up 57 percent. And if your portly friend considers you a friend as well, your odds of becoming equally plump rise 171 percent.

This social network effect extends (with progressively less impact) to three degrees of separation - to your friends' friends' friends. Not only that, but a faraway friend has as much impact on your obesity as a friend close at hand. A stranger, however, even one who lives next door, has no effect on your odds of obesity. In other words, increasing social distance decreases the effect, but increasing geographic distance does not.

To arrive at these conclusions, researchers analyzed the social networks of 12,067 people who were followed for 32 years as part of the Framingham Heart Study. After painstakingly mapping 38,611 social interconnections among all the subjects, they discovered distinct clusters of thin and heavy people.

Statistical analysis revealed that the clusters were not due to any tendency for people to congregate with others like them. Furthermore, people's influence on their friends' weight could not be attributed to eating the same foods as their friends or to engaging in similar activities.

The analysis revealed that the infectious effect is much greater among friends of the same sex. You have a 71 percent increased risk of obesity if your same-sex friend gains a lot of weight. If you are a woman and your sister becomes obese, your risk rises by 67 percent. If a man's brother becomes obese, his risk rises by 44 percent. And if you become obese, your spouse has a 37 percent increased risk of excess adiposity.

How could this possibly work? Given the dramatic effect of distant friends on your odds of obesity, common exposure to a particular environment cannot be the cause. The researchers hypothesize that your esteem for your friends may alter your conception of what constitutes normal and proper body size. They propose that this contagious social influence should be exploited to spread positive health behaviors from one person to that person's entire social network.

Source: New England Journal of Medicine, July 2007; EurekAlert