1. Stockholm University, Department of Economics, Stockholm, SE-106 91, Sweden.
2. Harvard University, Department of Economics, 1805 Cambridge Street, Cambridge, MA 02138 and CIFAR.
3. Princeton University, Department of Economics, Princeton, NJ 08544 and WWS.
4. UC San Diego, Department of Economics, 9500 Gilman Dr. MC 0508, La Jolla, CA 92093.