The phrase "peer pressure" has profound meaning for the human species.

We are, after all, social animals that spend most of our time interacting with others.

In fact, this compelling need for interpersonal interaction is fundamental to our nature, something we share with other primates such as monkeys and apes. Sure, other animals form groups—zebras, for example, stand strip-to-stripe all day—but herding animals cluster together for protection against predators, not for friendship.

Primates, on the other hand, spend all their time touching each other, keeping track of everyone in the group and building specific friendships.

A primate's day is all about everyone else.

Presumably, our über-sociality has been an evolutionary advantage; all that energy spent on interpersonal relationships must be one reason primates have been a relatively successful order.

But that advantage is also balanced by the trouble we get into by being so close to others.

Last week, Nicholas Christakis and James Fowler reported in The New England Journal of Medicine that the growing rate of obesity in this country is a peer issue. According to their analysis of 12,067 people, you are more likely to be obese if your best friend is obese. You are also at high risk of obesity if your siblings or spouse is obese, but really, it's fat friends that make all the difference.

And it's not that overweight people are seeking out their own kind. This study tracked increases in weight over time; you and your friends are gaining weight in tandem despite different genes.

Obesity apparently is a social disease; it spreads from person to person without viruses or parasites. And it's not the only human disorder traveling by association.

Anthropologists have cataloged several contagious conditions in other cultures. For example, in South China, men sometimes believe their penis is shrinking inwards and that once the penis vanishes, they will die. There is, of course, no evidence that their genitalia are going anywhere.

But as they lie in wait for the end, this "disease," called koro, can spread like wildfire though a village; even women start believing their labia or breasts are disappearing Then, poof, everyone recovers and goes about their business.

In our own culture, we also have socially contagious disorders that wax and wane. During Victorian times, fainting on a couch was all the rage for women. Today, anorexia and bulimia are contagious; depression, high anxiety and paranoia are also catching.

In the same way, we become obese because our circle of friends is obese. We are peers in every sense, including body size, and that feels right, no matter the health risks.

But by harnessing that same peer pressure we could also presumably reverse the obesity trend. Perhaps by hanging out more often with different, slimmer friends, an overweight person might lose weight.

Unless, of course, the plan backfires and the new skinny friends respond to peer pressure and join the obese at the table.

Meredith F. Small is an anthropologist at Cornell University. She is also the author of "Our Babies, Ourselves; How Biology and Culture Shape the Way We Parent" (link) and "The Culture of Our Discontent; Beyond the Medical Model of Mental Illness" (link).