Incumbency as a source of spillover effects in mixed electoral systems: Evidence from a regression-discontinuity design

In this paper we demonstrate empirically that incumbency is a source of spillover effects in Germany's mixed electoral system. Using a quasi-experimental research design that allows for causal inferences under a weaker set of assumptions than the regression models commonly used in the electoral systems literature, we find that incumbency causes a gain of 1.4–1.7 percentage points in PR vote shares. We also present simulations of Bundestag seat distributions to show that spillover effects caused by incumbency are sufficiently large to trigger significant shifts in parliamentary majorities.