1. School of Business Administration, University of Michigan, Ann Arbor, MI 48109.
2. Department of Anthropology, University of California at Los Angeles, Los Angeles, CA 90095.
3. Department of Economics, University of Massachusetts, Amherst, MA 01003, and Santa Fe Institute.
4. Department of Economics, California Institute of Technology, Pasadena, CA 91125.
5. Institute for Empirical Research in Economics, University of Zürich, Blümlisalpstrasse 10, CH-8006, Zurich, Switzerland.
6. University of Massachusetts, Amherst, MA 01003.