This is scientific American 60 second science. I'm Christopher in Tagayata.

If your friends are happy, it turns out you're more likely to be happy too.

And 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 analyze 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, then 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.

He says since different cities have different weather patterns, 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 through peer

influence of the friends who live between New York and San Diego.

And that is exactly what he and his colleagues saw, that the behavior of one city's

runners could indeed affect the behavior of runners in another socially connected city.

The studies in the journal Nature Communications.

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.

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