David Lazer1, Ryan Kennedy2, Gary King3, Alessandro Vespignani4. 1. Lazer Laboratory, Northeastern University, Boston, MA 02115, USA. Harvard Kennedy School, Harvard University, Cambridge, MA 02138, USA. d.lazer@neu.edu. 2. Lazer Laboratory, Northeastern University, Boston, MA 02115, USA. Institute for Quantitative Social Science, Harvard University, Cambridge, MA 02138, USA. University of Houston, Houston, TX 77204, USA. 3. Institute for Quantitative Social Science, Harvard University, Cambridge, MA 02138, USA. 4. Laboratory for the Modeling of Biological and Sociotechnical Systems, Northeastern University, Boston, MA 02115, USA. Institute for Scientific Interchange Foundation, Turin, Italy. Institute for Quantitative Social Science, Harvard University, Cambridge, MA 02138, USA.