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<p>When single member plurality electoral rule is used, Duverger’s “law” posits prevalence of a “two-party system”. This system often has been construed to mean that Laakso-Taagepera effective number of parties is close to 2.0. But this can be misleading because <i>N</i> = 2.0 can result not only from 50–50-0 but also from 66.6–16.7–16.7. A safer index of “two-partyness” has been developed for district votes, but the issue is even more complex at the nationwide level. Here the Duvergerian ideal for seats in a single election is not stalemate (50–50-0) but a tilted balance: a comfortable majority, yet also vigorous opposition. Also needed are alternation in power and long-term parity in number of elections won. This study develops indices of closeness to these four ideals and applies them to a dataset. Results confirm a Duverger’s <i>tendency</i>, but it is too early to proclaim a Duverger’s law.</p>

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Duverger’s tilted balance: “Two-party system” operationalized

  • 期刊论文

  • 发表日期：

    2026年5月11日

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  • Rein Taagepera

摘要

When single member plurality electoral rule is used, Duverger’s “law” posits prevalence of a “two-party system”. This system often has been construed to mean that Laakso-Taagepera effective number of parties is close to 2.0. But this can be misleading because N = 2.0 can result not only from 50–50-0 but also from 66.6–16.7–16.7. A safer index of “two-partyness” has been developed for district votes, but the issue is even more complex at the nationwide level. Here the Duvergerian ideal for seats in a single election is not stalemate (50–50-0) but a tilted balance: a comfortable majority, yet also vigorous opposition. Also needed are alternation in power and long-term parity in number of elections won. This study develops indices of closeness to these four ideals and applies them to a dataset. Results confirm a Duverger’s tendency, but it is too early to proclaim a Duverger’s law.