1. Department of Economics, University of California at Davis, 1 Shields Ave., Davis, CA 95616, and NBER (e-mail: )
2. Department of Economics, University of North Carolina Greensboro, Bryan 462, PO Box 26170, Greensboro, NC 27402, and NBER (e-mail: )
3. Department of Economics, University of Colorado at Boulder, 256 UCB, Boulder, CO 80309 (e-mail: )
4. George P. Shultz Professor, Sloan School of Management and Director, Center for Energy and Environmental Policy Research, Massachusetts Institute of Technology, 100 Main St., Cambridge, MA 02142, and NBER (e-mail: )