Exercising in Herds : Treatment Size and Status Specific Peer Effects in a Randomized Exercise Intervention

In a field experiment using university students, we find that subjects who have been incentivized to exercise increase their recreation center usage more if they have more friends who have been incentivized, and less if they have more friends in the control group. Controls, however, are not influenced by their peers. Findings highlight subtle effects of randomization, indicating that the fraction treated in an experiment of this kind has a large influence on outcomes, and that spillovers may vary greatly by treatment status. The methodology is applicable to other settings and quantifies spillovers that previous approaches do not detect. We thank Kelly Bedarad, Eric Bettinger, Olivier Deschenes, Peter Kuhn, Heather Royer, Bruce Sacerdote, Jon Sonstelie, and Cathy Weinberger for helpful comments. We are grateful for funding by the Hellman Family Foundation, and excellent research assistance by Jennifer Carnan, Jennifer Schulte, Allison Nuovo, Christy Helvestine, Bonnie Queen, Jessica Evans, Natalie Brechtel, and Randi Golde. We are also grateful for assistance from Chris Clontz at the Recreation Center at the University of California, Santa Barbara.