Duverger's law

Winner-takes-all voting systems tend to result in only two viable parties

In political science, Duverger's law holds that in political systems with single-member districts and the plurality voting system,, two main parties tend to emerge. In this case, votes for minor parties can potentially be regarded splitting votes away from the most similar major party. In contrast, systems with proportional representation usually have more representation of minor parties in government.

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