1. Department of Finance and Business Economics, University of Southern California Marshall School of Business, 3670 Trousdale Parkway, Los Angeles, CA 90089-0804 (e-mail: )
2. University of California, San Diego, 9500 Gilman Drive, MC 0519, La Jolla, CA 92093 and NBER (e-mail: )
3. Mailman School of Public Health, Columbia University, 722 W. 168th Street, New York, NY 10032 and NBER (e-mail: )