Cooperating Without Looking Moshe Hoffman, 1,2∗† Erez Yoeli, 2,3∗ Martin Nowak 2 1 Department of Computer Science and Engineering, University of California at San Diego, La Jolla, CA 92093 2 Program for Evolutionary Dynamics, Harvard University, Cambridge, MA 02138 2 Federal Trade Commission, 600 Pennsylvania Ave. NW, Washington, DC 20004