Compositional effects, internal migration, and electoral outcomes

• Department of Political and Social Sciences

This session of the Political Behaviour Colloquium features a presentation by Moritz Marbach, Associate Professor in Data Science & Public Policy at the University College London.

Internal migration, a common phenomenon in all countries, reshapes political geography by altering both the composition and preferences of local electorates, with significant implications for electoral outcomes. Despite increasing research on the political consequences of internal migration, there is little guidance on how to disentangle compositional from exposure effects when analysing the causal effect of internal migration on district-level outcomes. In this paper, we define compositional effects within a standard potential outcome model, and we demonstrate that compositional and exposure effects jointly constitute the total causal effect of internal migration. We discuss potential avenues to identify, bound, and estimate compositional effects, leveraging additional data and assumptions. To illustrate the importance of disentangling compositional from exposure effects, we analyse the exodus of East Germans to West Germany shortly after the fall of the Berlin Wall, demonstrating how out-migration to West Germany shaped electoral outcomes in East Germany through compositional effects.

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