1. Department of Psychology, University of Pennsylvania, 3720 Walnut Street, Philadelphia, PA 19104-6241 (e-mail: )
2. The Wharton School of the University of Pennsylvania, 3620 Locust Walk, SHDH 1455, Philadelphia, PA 19104 (e-mail: )
3. Departments of Sociology, Ecology and Evolutionary Biology, and Medicine, Yale University, Yale Institute for Network Science, PO Box 208263, New Haven, CT 06520-8263 (e-mail: )
4. Political Science Department and Medical Genetics Department, University of California, San Diego, Social Science Building 392, 9500 Gilman Drive #0521, La Jolla, CA 92093-0521 (e-mail: )