District magnitude, electoral formula, and the number of parties

Duverger's propositions concerning the psychological and mechanical consequences of electoral rules have previously been examined mainly through the lens of district magnitude, comparing the properties of single-member district plurality elections with those of multimember proportional representation elections. The empirical consequences of multi-member plurality (MMP) rules, on the other hand, have received scant attention. Theory suggests that the effect of district magnitude on the number and concentration of parties will differ with regard to whether the allocation rules are plurality-based or proportional. I test this theory by drawing on a uniquely large-sample dataset where district magnitude and electoral formula vary but the basic universe of political parties is held constant, applying regression analysis to data from several thousand Hungarian local bodies elected in 1994 consisting of municipal councils, county councils, and mayors. The results indicate that omitting the variable of electoral formula has the potential to cause significant bias in estimates of Duvergerian consequences of district magnitude. In addition, the analysis of multi-member plurality elections from the local election dataset reveals counter-intuitively that candidate and party entry may increase with district magnitude under MMP, suggesting important directions for future investigation of MMP rules.