Predicting the Number of Parties: A Quantitative Model of Duverger's Mechanical Effect

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Abstract
Themechanical effectof electoral systems, identified by Maurice Duverger, can be estimated by means of a quantitative model. The model predicts the range within which theeffective numberof parties in a district should fall for a given magnitude (number of seats) of the district. At the national level, a related model predicts the effective number of parties based on the effective magnitude and the number of seats in the national assembly. The institutional variables considered—magnitude and assembly size—define a great portion of the structural constraints within which a given country's politics must take place. The model developed provides a good fit to data in spite of its having been developed from outrageously simple starting assumptions.