1. Department of Economics, Stanford University, 579 Serra Mall, Stanford, CA 94305, and NBER (email: )
2. Department of Economics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building E52, Room 442, Cambridge MA 02139, and NBER (email: )
3. Department of Health Research and Policy, Stanford University, Redwood Building T111, 150 Governor’s Lane, Stanford, CA 94305, and NBER (email: )