The spatial evolution of economic activities and the emergence of cities

This paper studies the spatial agglomeration of workers and income in a continuous space and time framework. Production and consumption are decided in local markets, characterized by the presence of spatial spillovers and amenities. Workers move across locations maximizing their instantaneous utility, subject to mobility costs. We prove the existence of a short-run Cournot-Nash equilibrium, and that, in the limit of an infinite number of workers, the sequence of short-run equilibria can be expressed by a partial differential equation. We characterize the conditions under which the long-run equilibrium displays spatial agglomerations. Social welfare is non-decreasing over time, and in the long-run equilibrium the expected utility of a representative worker is equalized over space and, therefore, the spatial allocation is efficient. The model can reproduce several stylized effects, such as the emergence of spatial agglomerations (cities) with different sizes and shapes; the dependence by history of spatial pattern of economic activities; a non-linear out-of-equilibrium dynamics; and finally, the phenomenon of metastability, where a long period of apparent stability in the spatial distribution is followed by a sharp transition to a new equilibrium.