1. Department of Economics, College de France, 52 Rue du Cardinal Lemoine, Paris, 75231 France (email: )
2. Department of Economics, Stanford University, 450 Serra Mall, Stanford, CA 94305, and NBER (email: )
3. Department of Economics, Stanford University, 450 Serra Mall, Stanford, CA 94305 (email: )
4. Harvard Business School, Harvard University, Boston, MA 02163, and NBER (email: )
5. Department of Economics, London School of Economics and Political Science (email: )