1. Graduate School of Business, Harvard University, 214 Rock Center, Soldiers Field, Boston, MA 02163,and NBER.
2. Institut d'Economíe Industrielle, University of Toulouse, 21 Allee de Brienne, Toulouse 31000, France, GREMAQ (UMR 5604 CNRS), CERAS (URA 2036 CNRS), Paris, and MIT.