Like an Ink Blot on Paper: Testing the Diffusion Hypothesis of Mass Migration, Italy 1876 -1920

Why were the poorer countries of the European periphery latecomers to the Age of Mass Migration? We test the diffusion hypothesis, which argues that mass emigration was delayed because it was primarily governed by a gradual process of spatial diffusion of migration networks. We propose a model of migration within a spatial network to formalize this hypothesis and to derive its testable predictions. Focusing on post-unification Italy, we construct a comprehensive municipality- and district-level panel of emigration data over four decades, and use it to show that the testable predictions of the diffusion hypothesis are validated by the data. The emerging picture is that Italian mass migration began in a few separate epicenters from which it expanded over time in an orderly pattern of spatial expansion, and that the epidemiological characteristics of this expansion match those underlying our model. These findings strongly support the diffusion hypothesis and call for a revision of our understanding of one of the most important features of the Age of Mass Migration - the delayed migration puzzle.