1. Department of Economics, University of British Columbia, Vancouver School of Economics, Faculty of Arts, Vancouver V6T 1L4, BC Canada;
2. Department of Economics, University of Pennsylvania, Perelman Center for Political Science and Economics, Philadelphia, Pennsylvania 19104;
3. Marketing Department, Wharton School of the University of Pennsylvania, Philadelphia, Pennsylvania 19104;
4. Marketing Division, Columbia Business School, Columbia University, New York, New York 10027