1. Department of Economics, The Pennsylvania State University, 609 Kern Building, University Park, PA 16802, and NBER (e-mail: )
2. Department of Economics, Yale University, Box 208264, New Haven, CT 06520, and NBER (e-mail: )
3. Booth School of Business, University of Chicago, 5807 S. Woodlawn Avenue, Chicago, IL 60637, and NBER (e-mail: )
4. School of Economics, The University of Sydney, Room 370, Merewether Building (H04), NSW 2006 Australia, and NBER (e-mail: )