A dynamic Duverger’s law
Electoral systems promote strategic voting and affect party systems. Duverger (Les partis politiques, 1951) proposed that plurality rule leads to bi-partyism and proportional representation leads to multi-partyism. We show that in a dynamic setting, these static effects also lead to a higher option value for existing minor parties under plurality rule, so their incentive to exit the party system is mitigated by their future benefits from continued participation. The predictions of our model are consistent with multiple cross-sectional predictions on the comparative number of parties under plurality rule and proportional representation. In particular, there could be more parties under plurality rule than under proportional representation at any point in time. However, our model makes a unique time-series prediction: the number of parties under plurality rule should be less variable than under proportional representation. We provide extensive empirical evidence in support of these results.
KeywordsDuverger’s law Electoral rules Party entry and exit
JEL ClassificationD72 C73
We thank Catherine Bobtcheff, Daniel Diermeier, Carlo Prato, seminar audiences at Carleton, UQAM and Ryerson, conference participants at CPEG 2013, Elections and Electoral Institutions IAST 2014, MPSA 2014, Formal Theory and Comparative Politics 2015, and Scott Legree for excellent research assistance. Finally, the editors and two referees provided excellent comments and suggestions.
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