1. Department of Economics, MIT, 50 Memorial Drive, Cambridge, MA 02142, CEPR, and NBER (email: )
2. Economics, University of Chicago, 1126 E. 59th Street, Chicago, IL 60637, CEPR, and NBER (email: )
3. Department of Economics, University of Pennsylvania, 133 South 36th Street, Philadelphia, PA 19104 (email: )
4. Economics, Stanford University, 579 Serra Mall, Stanford, CA 94305, CEPR, and NBER (email: )
5. Harvard Business School, Rock Center 212, Boston, MA 02163, Bank of Finland, and NBER (email: )