First Author:   Yves Zenou University of Southampton, GAINS, Universite du Maine and CEPR yz@soton.ac.uk   Department of Economics, University of Southampton Southampton, SO17 1BJ, UK   (44) 23 80 59 32 64(Phone) (44) 23 80 59 32 64(Fax) http://www.economics.soton.ac.uk/staff/zenou Second Author: Sebastien Steinmetz INRA-Grignon, Laboratoire d'Econometrie de l''Ecole Polytechnique steinmet@poly.polytechnique.fr   Laboratoire dEconometrie de l'Ecole Polytechnique, 1 rue Descartes Paris, 75005, France