"The spatial distribution of people exhibits clustering across a wide range of scales, from household (∼ 10−2 km) to continental (∼ 104 km) scales. Empirical data indicates simple power-law scalings for the size distribution of cities (known as Zipf’s law1 ), the geographic distribution of friends2 , and the population density fluctuations as a function of scale. We derive a simple statistical model that explains all of these scaling laws based on a single unifying principle involving the random spatial growth of clusters of people on all scales. The model makes important new predictions for the spread of diseases and other social phenomena. - Todd Hoff