科学美国人60秒:Healthy Behavior Can Spread Like Illness

If your friends are happy—turns out you're more likely to be happy too. If your friends are overweight, that too ups the odds you'll pack on pounds. Those effects have been shown in studies. And now researchers have identified another seemingly contagious quality: exercise.

The investigators analyzed the running activity of more than a million individuals worldwide who used an exercise tracking device for five years. And they used weather patterns as a way to randomly examine different parts of that global network.

"If it happens to be a really nice day out, sunny and not too hot, not too cool, that will induce people to run more.” Sinan Aral, a computational social scientist at MIT. “If it's a rainy day and cold, that will induce people to stay in more on average." 

And since cities have different weather patterns, he says, this natural experiment allowed them to ask: Does a rainy day in New York affect running in San Diego? "If the weather in New York causes changes in the running behavior in San Diego, it can really only be happening thru peer influences of the friends who live between New York and San Diego."

And that is exactly what he and his colleague saw: that the behavior of one city's runners could indeed affect the behavior of runners in another socially connected city. The study is in the journal Nature Communications. [Sinan Aral & Christos Nicolaides, Exercise contagion in a global social network]

A few caveats: women tended to be influenced more by the female runners in their networks. And less active runners tended to influence more active runners to run more, but not so much the other way around. Still, this could be valuable intel for health professionals. "We have to start thinking about consumers and citizens as networked consumers and networked citizens. Where they are influenced by and influence their social network in very strong and dynamic ways, that will change the way a particular intervention succeeds or fails."

In other words—if your prescription is more exercise? The doctors might want to write a prescription for your friends and family, too.

—Christopher Intagliata