1. Stanford Medical School, 117 Encina Commons, Stanford, CA 94305 and National Bureau of Economic Research.
2. University of Heidelberg, Bergheimer Strasse 20, Heidelberg, Germany, 69115, and CESifo and Institute for the Study of Labor (IZA).
3. Stanford Medical School, 117 Encina Commons, Stanford, CA 94305 and and National Bureau of Economic Research.