1 Institut d'Economie Industrielle (IDEI), University of Toulouse, 21 Allee de Brienne, Toulouse, 31000 France, GREMAQ, and Institut Universitaire de France. · 2 Genuity, Inc., 3 Van de Graaff Drive, P.O. Box 3073, Burlington, MA 01803. · 3 IDEI, GREMAQ, and University of Toulouse I. · 4 IDEI, GREMAQ (UMR 5603 CNRS), Toulouse, CERAS (URA 2036 CNRS), Paris, and Massachusetts Institute of Technology.