Obesity Spreads Socially
A person's chances of becoming obese were influenced by his or her family and friends, even if they were hundreds of miles away.
"We were stunned" by that finding, Fowler says. He notes that immediate neighbors didn't affect a person's chances of becoming obese, which suggests that the findings weren't strongly tied to social class.
The researchers considered many other factors, including gender, natural weight gain with aging, and the tendency of people to associate with people similar to them.
The results held. In other words, the findings weren't about thin people favoring thin people as their friends.
How Does Obesity Spread?
The data don't show how obesity spread through the social networks. But social norms appear to play a role, Christakis notes.
For instance, he says someone might see their faraway brother or friend once a year at Thanksgiving and notice their weight gain. "You might say, 'It's OK to be heavier,' and then go back home" and perhaps emulate that heavier weight, Christakis says.
That likelihood was strongest for same-sex pairs -- among brothers, for instance, or among friends of the same gender. That may be why friends were more influential than spouses.
"Although spouses are presumably friends, they also are opposite gender, and so those two effects tend to work against one another," says Fowler.
While overweight people were especially likely to become obese if their friend or relative did, the same pattern also applied to leaner people. And it generally wasn't a tiny bit of weight gain that nudged participants into the obese category, the researchers note.
Is Thinness Socially Contagious?
Christakis and Fowler also found that when someone lost weight and was no longer obese, their friends and family tended to lose weight, too.
"What we're looking at is how much your friend's weight change affects your own weight change. And it can be up or it can be down. It can be becoming obese or becoming thin," Fowler says.
That suggests that your weight isn't just about you.
"Other people are going to be looking to you, and so your health behaviors don't just affect you. They affect your friends as well," Fowler says.