1. Program for Evolutionary Dynamics, Harvard University, Cambridge, MA 02138, USA.
2. Berkman Center for Internet and Society, Harvard University, Cambridge, MA 02138, USA.
3. Department of Economics, Stockholm School of Economics, 11358 Stockholm, Sweden.
4. Department of Economics, Harvard University, Cambridge, MA 02138, USA.
5. Department of Mathematics, Harvard University, Cambridge, MA 02138, USA.
6. Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.