While Christakis could show an obesity epidemic spreading through friend networks, he could only make an educated guess why.
"One possibility is that you start doing things -- certain behaviors that I copy," said Christakis.
So if one friend starts serving beer and cookies all the time, perhaps another friend will pick up the habit. Or if one friend joins a running club, perhaps another friend will join it too.
Christakis said another possibility is that "What's spreading between people is an idea, or a norm."
For example, if most people a person associates with are overweight, then that person's idea of "normal weight" is likely to be bigger than what is actually healthy.
Ayoob says this finding rings true with studies and his experience counseling families.
"I've had parents request evaluations because they think their children are too thin. But they're at the 50 percentile of the estimated height and weight for their age," said Ayoob. The children were smack in the middle of weight for their age, but they are "only thin compared to other children in the household who are obese."
Large or small portion sizes may also spread as social "norm," Ayoob said. If mom always served three scoops instead of two, chances are the children will grow up reaching for more.
"Social networking does not cause obesity," said Ayoob. "But I think it's a marker for a lifestyle."