Are your Facebook friends making you fat?

In fact, while the average student in our sample has over 110 Facebook friends, they have only six “picture friends,” a number very similar to the number of “close” friends people list when asked in sociology studies.

It turned out that when we restricted our analysis to “picture friends,” we found evidence that overweight people cluster on Facebook.

The results suggest that if one of your picture friends is obese, it increases the likelihood that you are obese by 11 percent. And, even more remarkably, if a picture friend of your picture friend is overweight, it increases the likelihood you’ll be overweight by 4 percent.

In other words, we find evidence that clusters of overweight friends extend two degrees of separation on Facebook.

Many processes could underlie this clustering online, just as was the case in the offline world. Perhaps people are more likely to befriend others who resemble them in terms of body size. Perhaps groups of online friends share exposures to things that make all of them gain — or lose — weight in synchrony.

Or perhaps when one of your close friends online gains weight, you follow suit.

It is not yet clear which of these processes is occurring, but this is the first evidence we’ve seen that suggests the online world may be like the offline world when it comes to body size.