1. Division of Humanities and Social Sciences, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125 (e-mail: )
2. Department of Economics, Harvard University, Littauer Center, 1805 Cambridge St., Cambridge, MA 02138 (e-mail: )
3. Department of Economics, Stanford University, 579 Serra Mall, Stanford, CA 94305, the Santa Fe Institute, and CIFAR (e-mail: ).