Abstract
District magnitude is regarded by many as the principal dimension that spans the classification of electoral systems. It is believed that larger parties prefer smaller district magnitudes and vice versa. Problems arise when one tries to be exact: how large must a party be for the single-member district system to be its most favorable choice? Will any party find a particular magnitude most preferable? This article extends existing theories of effective thresholds and proposes a seat–vote equation different from the cube law. With reasonable assumptions, I demonstrate that a certain district magnitude maximizes the expected seat share of a particular median-sized party in elections using the d’Hondt PR or SNTV formulae. The validity of this threshold model is verified by an empirical study on recent elections in Finland and Taiwan.
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