The Connected States of America: Quantifying Social Radii of Influence

Human dynamics are inextricably intertwined with the social, geographical and economic environment. The continuous flux of people communicating as well as migrating, commuting, and traveling inevitably spans acquaintances across geographic space that is far from random and exhibits regular patterns. For instance, it has been shown that the probability of being acquainted with someone is closely related to the inverse distance between them. In this paper we investigate aggregated mobile phone call detail records from a large US cellular operator and map them into space to characterize the social radius of influence at two different scales: communication and mobility. We discover that scaling properties with respect to population agglomeration are similar to those discovered for other indicators of cities. We also discover spatial community structures that are divorced from administrative boundaries, and use them to quantify the different social radii of influence discovered from the data.