1. MIT Sloan School of Management, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142;
2. National Bureau of Economic Research, Cambridge, Massachusetts 02138;
3. Max Planck Institute for Innovation and Competition, 80539 München, Germany;
4. University of California, Merced, Merced, California 95343;
5. Center for Economic and Policy Research, Washington, District of Columbia 20009;
6. University of Bath, Bath BA2 7AY, United Kingdom;
7. IZA Institute of Labor Economics, 53113 Bonn, Germany