Colloquium: Beyond Causal Effects: Compositional Effects, Internal Migration and Electoral Outcomes

Internal migration, a common phenomenon in all countries, shifts the spatial distribution of political preferences, leading to downstream consequences for electoral outcomes. This paper provides a formal framework to analyze the compositional effects of internal migration on the average political preferences in a district. We decompose changes in aggregate preferences into variation due to compositional change from movers and preference change among non-movers. Using the potential outcome framework, we define the compositional effect of internal migration as the variation in the total causal effect of internal migration that remains after accounting for all constituent causal effects. We discuss potential avenues to bound and point-identify such compositional effects. To illustrate the empirical significance of disentangling causal and compositional effects, we examine the exodus of East Germans to West Germany shortly after unification and its consequences for electoral outcomes.