1. Department of Economics, University of California-Davis, One Shields Avenue, Davis, CA 95616, and NBER (email: )
2. Department of Economics, Texas A&M University, 3087 Allen Building, College Station, TX 77843, and NBER (email: )
3. Department of Economics, Southern Methodist University, 3300 Dyer Street, Dallas, TX 75275, and NBER (email: )