1. College de France, 3 rue d'Ulm, 75006, Paris, France, London School of Economics, NBER, and CIFAR (e-mail: )
2. University of Chicago, 1126 E. 59th Street, Chicago, IL 60637, and NBER (e-mail: )
3. Princeton University, 361 Wallace Hall, Princeton, NJ 08544, and NBER (e-mail: )
4. Harvard University, Littauer Center, 1805 Cambridge Street, Cambridge, MA 02138 (e-mail: )