1. Harvard School of Public Health, 677 Huntington Avenue, Boston, MA 02115 (e-mail: )
2. Department of Economics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, E17-228, Cambridge, MA 02139 (e-mail: )
3. Social Security Administration, Office of Disability Adjudication & Review, 5107 Leesburg Pike, Suite 1400, Falls Church, VA 22041 (e-mail: )
4. National Bureau of Economic Research, 1050 Massachusetts Avenue, Cambridge, MA 02138 (e-mail: )