Duverger's Law is a principle which asserts that a first-past-the-post election system naturally leads to a two-party system.

This was originally asserted by Maurice Duverger, a French sociologist who observed this effect in several papers published in the 1950s and 1960s. It eventually became referred to as a "law" in works by other political scientists, who applied further research to the proposition, and devised alternative voting systems which do not appear to be subject to the same characteristics as first-past-the-post.

A frequent consequence of Duverger's law is the spoiler effect, where a third-party candidate takes votes away from one of the two leading candiates.

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