1. Duke University, 219B Social Siences Building, Durham, NC 27708, and National Bureau of Economic Research (NBER) (email: )
2. Massachusetts Institute of Technology, 50 Memorial Drive, E52-426, Cambridge, MA 02139, and NBER (email: )
3. University of California, Berkeley, 530 Evans Hall #3880, Berkeley, CA 94720, and NBER (email: )