1 Division of International Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, Bethesda, MD 20892; viboudc@mail.nih.gov.
2 Laboratory for the Modeling of Biological and Socio-technical Systems, Northeastern University, Boston, MA 02116.
3 Institute for Quantitative Social Science, Harvard University, Cambridge, MA 02138.
4 Institute for Scientific Interchange Foundation, 10126 Turin, Italy.