The findings, published in The New England Journal of Medicine, have nothing to do with bacteria or viruses.
Instead, the new data show that obesity is "socially contagious." That means that people tend to follow suit when their friends and family become obese or lose weight to ditch obesity.
"We find that a person's chances of becoming obese increase by 57% if they have a friend who becomes obese, 40% if they have a sibling who becomes obese, and 37% if a spouse becomes obese," say researchers Nicholas Christakis, MD, PhD, and James Fowler, PhD.
"Mutual friends more than triple the risk to each other," note Christakis and Fowler. "If one of the two [mutual friends] becomes obese, the chance for the other to follow suit goes up 171%."
Christakis is a professor of sociology at Harvard University and professor of medical sociology at Harvard Medical School in Boston. Fowler is an associate professor of political science at the University of California, San Diego.
Obesity, Family, and Friends
Christakis and Fowler analyzed 32 years of obesity data on more than 12,000 people who participated in the Framingham Heart Study.
Every two to four years, participants got weighed during checkups. The researchers used participants' height and weight to calculate their BMI (body mass index). BMI of at least 30 is considered obese.
At their checkups, participants named family and friends that the Framingham Heart Study researchers could contact in case they lost touch with any participants.
Christakis and Fowler computerized that contact information and mapped each participant's social network -- the array of the friends and family they named over the years.
Christakis and Fowler then traced obesity trends through participants' social networks. "Because people are connected, their health may be interconnected," Christakis explains.
The study included many spouses, siblings, and friends. Mutual friends named each other as friends on their contact sheets.
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.
Curbing Obesity With Family, Friends
Working on your weight? Enlist your social network.
"If you're going on a diet, then you want to convince them to go on a diet. If you want to start to run or to change your exercise behavior, you also want to encourage your friends to engage in those behaviors," says Fowler.
"You want to act in concert with your friends," Christakis says.
"We are not suggesting that people should sever their ties with overweight friends," Christakis adds. "But we are suggesting that people are influenced by the behaviors of those around them, and if they're interested in losing weight, forming ties with people who are the proper weight is likely to be beneficial."
Don't forget to consult your doctor before starting a new exercise program.
The study adds "a new public health perspective on obesity" and is "one of the most exciting studies in medical sociology that I have seen in decades," says Richard Suzman, PhD.
Suzman directs the behavioral and social research program at the National Institute on Aging of the National Institutes of Health, which funded the study. He predicts that the social network theory will be pursued in research on other health issues.
Social norms aren't the only influence on obesity. Genetics also play a role, notes Matthew Gillman, PhD.
Gillman directs Harvard Medical School's obesity prevention program and is an associate professor at Harvard Medical School.
"What's new is that your friends, even if they live 500 miles away from you, might have an impact on your risk of developing obesity," Gillman says.
That may partly stem from early childhood influences among kids who are friends and later move far away from each other, Gillman suggests.