• By
  • Justin Lahart

People who are in shape tend to be friends with other active people, and people who aren’t in such good shape tend to hang out together as well.

There are all kinds of reasons for this: The activities that people engage in — bicycling, say, or playing video games — draw them together. People who are friends generally have similar education and income levels (both important health factors). Some people live in neighborhoods with lots of fast food restaurants.

All of which makes figuring how much just being friends with someone might affect your fitness level difficult, but in a new paper economists Scott Carrell, Mark Hoekstra and James West came across a way do that.

The U.S. Air Force Academy randomly assigns freshmen and sophomores to 30-student squadrons who live, eat, compete in intramural sports and study together. It also requires that students take physical-education courses and fitness tests. Together, that creates a nice experiment on how friends influence friends.

The economists found strong evidence that students’ friends’ fitness affect their own fitness as well. What’s more, they found that less-fit students had far more of an effect on a squadron’s fitness level than more-fit students did. “Individuals appear to either compare their own fitness to the least fit among them, or adopt the diet and exercise of the least fit,” they write.

So with the holidays around the corner, be forewarned: If you put on weight, your friends’ waistlines might start to balloon. Call it the Hindenburg uncertainty principle.