1. Tuck School of Business, Dartmouth College, 100 Tuck Hall, Hanover, NH 03755, and NBER.
2. McDonough School of Business, Georgetown University, 411 Old North, Washington, D.C. 20057, and NBER.
3. London School of Economics, Houghton Street, London WC2A 2AE, and CEPR.
4. Yale School of Management, 135 Prospect Street, New Haven, CT 06520, and NBER.