Are your friends making you fat? Or keeping you slender?

According to new research from Harvard and the University of California, San Diego, the short answer on both counts is “yes.”

The research says that obesity is “socially contagious,” spreading from person to person in a social network. The study — the first to examine this phenomenon — finds that if one person becomes obese, those closely connected to them have a greater chance of becoming obese themselves.

Surprisingly, the greatest effect is seen not among people sharing the same genes or the same household but among friends. If a person an individual  considera a friend becomes obese, the researchers found that this individual’s own chances of becoming obese go up 57 percent. Among mutual friends, the effect is even stronger, with chances increasing 171 percent.

Researchers also looked at the influence of siblings, spouses and neighbors. Among siblings, if one becomes obese, the likelihood for the other to become obese increases 40 percent; among spouses, 37 percent. There was no effect among neighbors, unless they were also friends. “It’s not that obese or non-obese people simply find other similar people to hang out with,” “Rather, there is a direct, causal relationship.”

Further analysis also suggested that people’s influence on each other’s obesity status could not be put down just to similarities in lifestyle and environment, like for example, people eating the same foods together or engaging in the same physical activities. Not only do siblings and spouses have less influence than friends, but also geography doesn’t play a role. The striking impact of friends seems to be independent of whether or not the friends live in the same region.

In part because the study also identifies a larger effect among people of the same sex, the researchers believe that people affect not only each other’s behaviors but also, more subtly, norms. “What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size. People come to think that it is okay to be bigger since those around them are bigger, and this sensibility spreads”. ” This is about people’s ideas about their bodies and their health. Consciously or unconsciously, people look to others when they are deciding how much to eat, how much to exercise and how much weight is too much.

“Social effects, are much stronger than people realized. There’s been an intensive effort to find genes that are responsible for obesity and physical processes that are responsible for obesity and you really should spend time looking at the social side of life as well”.

The policy implications of the study, the researchers say, are profound. The social-network effects extend three degrees of separation — to your friends’ friends’ friends — so any public-health intervention aimed at reducing obesity should consider this in its cost-benefit analysis. When we help one person lose weight, we’re not just helping one person, we’re helping many. That needs to be taken into account by policy analysts and also by politicians who are trying to decide what the best measures are for making society healthier.