After doing this same kind of analysis over thousands of connections among the million users, the researchers report a causal relationship between social influence and running, says study author Christos Nicolaides, a Postdoctoral Fellow at the MIT Sloan School of Management. If fair weather pushed Boston to run more than usual, Chicago was more likely to push themselves too—even without a particularly lovely day to influence them. Men tended to be more susceptible, and likely to be influenced by both male and female peers, while women only tended to feel peer pressure when other women ran more. And intriguingly, the effect was strongest when less active friends outdid themselves. In other words, Chicago would feel more pressure from Boston if Boston usually ran less than their friend in the windy city.