1. Department of Finance, The Wharton School, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA;
2. Sloan School of Management, Laboratory for Financial Engineering, and Computer Science and Artificial Intelligence Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142, USA;
3. Santa Fe Institute, Santa Fe, New Mexico 87501, USA
4. Department of Economics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142, USA;
5. Department of Economics, University of Chicago, Chicago, Illinois 60637, USA;