1. University of California, Berkeley, Department of Agricultural and Resource Economics, 207 Giannini Hall, Berkeley, CA 94720.
2. North Carolina State University, Department of Agricultural and Resource Economics, Box 8109, Raleigh, NC 27695.
3. Columbia University, Department of Economics and School of International and Public Affairs, 420 W 118th St., New York, NY 10027 and NBER.