Interaction Effects in Mixed-Member Electoral Systems

Abstract

The past decade has witnessed a surprising growth in the popularity of mixed-member electoral systems. Under these systems, voters choose representatives simultaneously under both proportional representation (PR) and single-member district plurality (SMDP) rules. It is widely accepted that SMDP rules tend to winnow competition down toward two large parties, and evidence from mixed systems suggests that this Duvergerian “gravity” reduces the number of parties surviving SMDP competition under mixed systems as well. Nevertheless, we argue, simultaneous balloting under PR rules softens this winnowing effect, operating as a “centrifugal force” that prevents Duvergerian gravity from reducing competition to the degree it does under pure SMDP systems. Thus, these newsystems produce effects unanticipated by their designers. To test for the presence of this centrifugal force, we examine elite-level electoral strategies in Germany, Japan, and Italy and compare district-level SMDP election results from pure systems with those of mixed-member systems.