1. School of Mathematics and Statistics, University of St Andrews, St Andrews KY16 9SS, United Kingdom
2. División de Economía, Center for Research and Teaching in Economics, Centro de Investigación y Docencia Económicas, Aguascalientes, MX 20314
3. Sloan School of Management, Massachusetts Institute of Technology, Cambridge, MA 02139
4. Department of Biology, University of Pennsylvania, Philadelphia, PA 19104