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Researchers found that runners connected through a social network ran farther and faster on the same days their friends did.

April 19, 2017

Can your friends affect how many miles you cover—or the pace you set—each time you head out for a run? The answer seems to be yes, according to a new study published in Nature CommunicationsResearchers from MIT's Sloan School of Management found that runners connected through a social network influenced each other's training in surprising ways.

For their study, the researchers examined five years worth of data from a global social network in which people share data from their fitness trackers with virtual friends. The study authors focused on the daily patterns of about 1.1 million runners, and analyzed roughly 2.1 million virtual ties. They also collected precipitation and temperature data, so they would know the weather conditions on every run.

The results were impressive. The researchers found that if one person ran an extra kilometer on any given day, his or her friends would run an additional 0.3 kilometers, on average, that same day—even if their local weather wasn't great. Similarly, if one runner was a little faster than usual on a particular day, his or friends would pick up their speed as well. More calories burned for one runner would mean more calories burned for his or her friends. Same for the amount of time spent running.

The effects were more noticeable among certain groups. For example, runners were more likely to be influenced by peers who were slightly better or slightly worse runners than friends who were significantly better or significantly worse runners. 

And perhaps not surprisingly, the researchers noted a stronger influence among same sex friends than mixed sex friends. Men were affected by both their male and female friends, but more so by their male friends. And women weren't at all affected by the training routines of their male friends.

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The idea that our friends’ habits affect our own (and vice versa) is nothing new. Research suggests that hanging out with slimmer pals can help you lose weight; having a lonely relative, neighbor, or friend increases your own risk of loneliness; and happiness spreads among strangers.

But there are problems with some of the studies on so-called behavioral contagions. For starters, many of the studies are survey-based and rely on accurate self-reporting, which is something we are notoriously bad at. There is also the fact that we are subconsciously drawn to people who are similar to us, which can make it hard to work out how much we influence each other and what we would do naturally on our own. Plus, people who are geographically close are likely to be impacted by the same outside influences.

To avoid these pitfalls, the researchers only looked at relationships between runners who lived in different cities. And instead of using self-reported data, they looked at information that was automatically uploaded via the fitness trackers.

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While the findings are intriguing, the researchers note that they may not be relevant to all runners. "Our network sample is reasonably representative of the one in five Americans who owns a wearable device and the over 100M people who use fitness trackers worldwide," they wrote in the study. But the same effects may not hold true for running buddies who don't use trackers, they point out.