1. Tian Zheng is Assistant Professor, Department of Statistics , Andrew Gelman is Professor, Department of Statistics and Department of Political Science , and Matthew J. Salganik is a Doctoral Student, Department of Sociology and Institute for Social and Economic Research and Policy , Columbia University, New York, NY 10027. The authors thank Peter Killworth and Chris McCarty for the survey data on which this study was based, and Francis Tuerlinckx, Tom Snijders, Peter Bearman, Michael Sobel, Tom DiPrete,...