Gregory Clark is Danish National Research Foundation Professor in the Department of Economics and a member of the Danish Institute for Advanced Study, University of Southern Denmark. He is also a visiting professor at the Department of Economic History, LSE, and Distinguished Professor Emeritus, University of California, Davis. His two earlier books were A Farewell to Alms: A Brief Economic History of the World (Princeton University Press, 2007) and The Son Also Rises: Surnames and the History of Social Mobility (Princeton University Press, 2014). His email address is gclark@ucdavis.edu.