Your friends' weight gain can raise your own risk, even three friends away

If a friend gains enough weight to become obese during a given period, your own chance of becoming obese in that same period rises roughly 57 percent.

That change over time is the finding from Nicholas Christakis, who cared for dying patients in Chicago, and his collaborator, James Fowler. They tracked 2,200 people, mapped in 2000, and followed the network with cuts across about 30 years to watch weight change spread.

A second set of numbers comes from a snapshot of that network. It measures how much more likely you are to have obesity at that time if someone at a given distance has it. If a friend has obesity, your likelihood is 45 percent higher. If a friend of a friend has it, your likelihood is 25 percent higher. If a friend of a friend of a friend has it, someone you probably do not even know, your likelihood is still 10 percent higher. Only at four steps out does the link fade.

Three things could create that clustering. One is spread, what researchers call induction, where one person's weight gain helps cause another's. The second is similarity in friendship, what researchers call homophily, the tendency for similar people to seek each other out as friends, where people who already share a body size form ties. The third is shared surroundings, what researchers call confounding, where something in common affects both people at once, like joining the same health club and losing weight together.

Christakis and his team found evidence for all three, including spread from person to person.

Spread can work in two ways. One is copying habits, such as friends falling into a routine of getting muffins and beer together, and then gaining weight together. The other is more subtle. As friends gain weight, your idea of what counts as a normal or acceptable body size shifts.

The result is that weight is partly social, shaped by habits and expectations that can move from friend to friend to reach someone you probably do not even know.