1. Department of Economics and Department of Agricultural, Food, and Resource Economics, Michigan State University, and NBER, 18C Marshall-Adams Hall, Michigan State University, East Lansing, MI 48824-1038.
2. Department of Economics, University of Michigan, and NBER, 238 Lorch Hall, 611 Tappan St., Ann Arbor, MI 48109-1220.
3. The Harris School, University of Chicago, and NBER, 1155 E. 60th Street, Chicago, IL 60637.
4. Institute for Social Research, University of Michigan. P.O. Box 1248, Ann Arbor, MI 48106-1248.