Body weight seems like an individual decision, doesn’t it? It’s something that’s under individual control, right?

Well, it is—and it isn’t. Our body weight is influenced by those around us. Could it be that body weight is contagious?

Americans are gaining weight, on average, though the desire to do something about it has not changed much over time. As we’ve discussed this week, the majority of Americans are concerned about body weight, some occupations are more susceptible to obesity than others, avoiding the dentist is a predictor of obesity, and healthy eating habits have declined during the course of 2013.

Body weight is contagious, argue researchers Nicholas A. Christakis and James H. Fowler. But a biological virus is not the agent; a social virus is the cause of contagion. We are influenced by those around us—members of our social network who influence our values and norms about appropriate body weight. This research was published in the prestigious New England Journal of Medicine.

Using data collected over three decades in Massachusetts, the researchers documented some startling facts. Here are a few:

Your risk of becoming obese increases by 171% if a close friend becomes obese. This effect is much stronger for men than for women.

If your brother or sister becomes obese, your risk increases by 40%.

If one spouse becomes obese, the other spouse is 37% more likely to become obese as well.

Now, all this works in reverse as well. If your close friend, or sibling, or spouse achieves ideal weight, your changes of doing the same are much greater, too.

Are you surprised by the social influences on body weight?

Is your body weight considerably different from the body weights of your close friends, your siblings, or your spouse?