First Author:   Matthew Jackson Stanford University jacksonm@stanford.edu   Economics, Stanford University Stanford, 94305-6072, USA       http://www.stanford.edu/~jacksonm/ Second Author: Alison Watts Southern Illinois University wattsa@siu.edu   Department of Economics, MC 4515, Southern Illinois University Carbondale, 62901-4515, USA       http://www.siu.edu/~econ/watts.html