1. University of Pennsylvania, 3720 Walnut Street, Philadelphia, PA 19104 (e-mail: )
2. Interdisciplinary Center for Economic Science, George Mason University, 4400 University Drive, Fairfax, VA 22030 (e-mail: )
3. Humboldt University and German Institute for Economic Research (DIW), Mohrenstrasse 58, Berlin, Germany, and Research Institute for Industrial Economics (IFN) (e-mail: )