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

Rein Taagepera and Matthew Soberg Shugart
The American Political Science Review
Vol. 87, No. 2 (Jun., 1993), pp. 455-464
Stable URL: http://www.jstor.org/stable/2939053
Page Count: 10
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Predicting the Number of Parties: A Quantitative Model of Duverger's Mechanical Effect
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Abstract

The mechanical effect of electoral systems, identified by Maurice Duverger, can be estimated by means of a quantitative model. The model predicts the range within which the effective number of parties in a district should fall for a given magnitude (number of seats) of the district. At the notational 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.

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