1. Department of Economics, University of Washington, 331 Savery Hall, Seattle, WA 98195, and NBER (e-mail: )
2. Department of Economics, University of Virginia, 254 Monroe Hall, Charlottesville, VA 22904 (e-mail: )
3. Department of Economics, University of Texas at Austin, 2225 Speedway Stop C3100, BRB 3.134D, Austin, TX 78712, and NBER (e-mail: )
4. Department of Economics, University of Washington, 331 Savery Hall, Seattle, WA 98195 (e-mail: )