1. Stanford University, 655 Knight Way, Stanford, CA 94305, and the Hoover Institution (e-mail: )
2. Department of Economics, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218 (e-mail: )
3. Department of Economics, MS-22, Rice University, PO Box 1892, Houston, TX 77251-1892 (e-mail: ).