More than meets the eye: Disproportionality and design in Mixed-Member majoritarian systems

Mixed-member majoritarianism is an increasingly common electoral system, yet scholars consistently view these systems as subpar, producing disproportionality, over-concentrated party systems, and generally poor electoral outcomes. In this study, we argue that these views are a function of our generally insufficient knowledge of how institutional variation between these systems impacts outcomes. For instance, we provide evidence that disproportionality in these systems is at least in part a function of the number seats in the majoritarian electoral tier, though this relationship is not linear as electoral theory might lead us to suspect. Going beyond description, we use simulations to test a deductive theory of the mixed incentives to nominate district candidates in these systems. We argue that these mixed incentives often lead smaller parties to over-nominate candidates, producing higher levels of disproportionality in systems where ambiguity is higher, as more candidates from parties that cannot win single-member contests will mechanically produce more disproportionality. More broadly, we argue that our evidence suggests a better understanding of the impacts of institutional variation will lead scholars to better evaluations of the outcomes associated with mixed-member majoritarianism.