Exercise contagion in a global social network : 10% booststrong contagion effects: on the same day, on average, an additional kilometre run by friends influences ego to run an additional 3/10th of a kilometre (Fig. 1a); an additional kilometre per minute run by friends influences ego to run an additional 3/10th of a kilometre per minute faster (Fig. 1b); an additional 10 min run by friends influences ego to run 3 min longer (Fig. 1c); and an additional 10 calories burned by friends influences ego to burn three and a half additional calories (Fig. 1d). This peer influence diminishes over time, with friends’ running today influencing ego less tomorrow and the day after for every measure.