1. ICREA, Universitat Pompeu Fabra, Ramon Trias Fargas 25-27, 08005 Barcelona, Spain, Barcelona GSE, CREI, and CEPR (e-mail: )
2. University of Michigan, 611 Tappan Street, Ann Arbor, MI 48105, NBER, and CEPR (e-mail: )
3. Ecole Polytechnique, 91128 Palaiseau Cedex, France, and CEPR (e-mail: )