1. Department of Agricultural and Resource Economics, University of California, Berkeley, 207 Giannini Hall, MC 3310, Berkeley, CA 94720.
2. Department of Economics, University of California, Santa Cruz, 415 Engineering 2, 1156 High Street, Santa Cruz, CA 95064.
3. Mailman School of Public Health, Columbia University, 600 West 168th Street, Sixth Floor, New York, NY 10032.