1. Dyson School of Applied Economics and Management, Cornell University, 405 Warren Hall, Ithaca, NY 14853 (e-mail: )
2. Resources for the Future, 1616 P Street NW, Washington, DC 200367 (e-mail: )
3. Department of Economics, University of California—Davis, 1 Shields Avenue, Davis, CA 95616, and NBER (e-mail: ).