1. University of Chicago, 1155 East 60th St., Chicago, IL 60637, and NORC (e-mail: )
2. Duke University, 419 Chapel Drive, Box 90097, Durham, NC 27708, and NORC (e-mail: )
3. University of Michigan, 238 Lorch Hall, 611 Tappan Avenue, Ann Arbor, MI 48109 (e-mail: )
4. Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15232, NORC, and NBER (e-mail: ).