1. Department of Economics, Dartmouth College, 6106 Rockefeller Hall, Hanover, NH 03755, and NBER (e-mail: )
2. Department of Economics, Wellesley College, Pendleton East, 106 Central Street, Wellesley, MA 02481, and NBER (e-mail: )
3. Institute for Policy Research, Northwestern University, 2040 Sheridan Road, Evanston, IL 60208, and NBER (e-mail: )