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date: 26 March 2019

Abstract and Keywords

The question of how electoral institutions affect party systems has been central to the literature on elections. For a given electoral system configuration, how many parties earn votes and win seats? How large is the largest party’s share of all votes and seats? This varies from country to country, from election to election, and, inside the country, from district to district. Yet two institutional inputs—district magnitude and assembly size—determine the worldwide averages surprisingly well, and they do so for well-defined logical reasons. These worldwide averages supply benchmarks against which to compare individual countries, elections, and districts: given their two basic institutional inputs, do they have rather many or few parties, and by how much are they off, compared to logical average expectations? History, culture, and current politics account of course for which parties form and which of them is the largest, but institutions shape their number and sizes.

Keywords: electoral system, effective number of parties, disproportionality, district magnitude, assembly size, seat product, Duverger’s law

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