Rethinking Duverger’s Law: Predicting the Effective Number of Parties in Plurality and PR Systems

Attempts to predict the number of political parties emerging in democracies have usually been based on one of two seemingly incompatible approaches: ( 1 ) the ‘institutional’ approach (e.g.. Duverger’s Law and Hypothesis) focuses o n the nature of the electoral system and also on the number of seats per district: (2) the ‘ideological’ approach stresses the nature and extent of social cleavages. This article attempts a synthesis by showing that election system and cleavage type interact to affect the number of parties. with the former factor determined in part by the latter. Our most striking finding. however. is that the effective number of parties tends to be obtained by adding ‘one’ to the number of issue dimensions. -Nithin this broader framework, Duverger’s Law emerges as a special case for polities with a single issue dimension. and Duverger’s Hypothesis is replaced by a much more quantitative prediction as to the effective number o f parties.