Duverger's Law is a principle which asserts that a first-past-the-post election system naturally leads to a two-party system. The discovery of this principle is attributed to Maurice Duverger, a French sociologist who observed the effect and recorded it in several papers published in the 1950s and 1960s. In the course of further research, other political scientists began calling the effect a “law”. It is important to realise Duverger's law suggests a nexus or synthesis between a party system and an electoral system - with a proportional representation (PR) system creating the electoral conditions necessary to foster party development and a FPTP system marginalising many smaller - single issue - political parties.

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How and why it occurs

Two-party systems often develop spontaneously when the voting system used for elections discriminates against third or smaller parties, because the number of votes received for a party in a whole country is not directly related to the proportion of seats it receives in the country's assembly/assemblies. While there is sometimes a coincidental relationship between votes cast and seats received in these systems, voters are not assured that their one vote will directly count toward an additional seat. The most widely-used system to have this effect, the simple plurality system (first past the post) often appears to pull systems into encouraging the survival of only two major parties: a third force can break in on the scene (the Labour Party in 20th-century Britain, or the Republican Party in the 19th-century United States, for example) but only at the ultimate expense of a former major party (the Liberal Party in Britain, the Whigs in the USA respectively). The overall system re-stabilizes into two-party mode after a three-party interlude.

Some representation systems - such as those involving a single elected president or a mayor dominating the government - may encourage two-party systems, since ultimately the contest will pit the two most popular candidates against each other.

When constituencies (districts) vote for candidates on the basis of a geographical constituency, all votes for candidates other than the winner count for nothing. This reflects another factor that encourages a two party system: smaller parties often cannot win enough votes in a constituency because they have smaller support and sometimes more scattered support than larger parties. Often a first-past-the-post electoral system and the election of candidates from geographical constituencies (districts) appear together in a single political system: this means that some smaller parties can garner a significant proportional of votes nationally, but receive few constituency seats and thus cannot realistically expect to compete overall on an equal footing with larger country-wide parties.

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Spoiler effect

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 candidates. The common desire among voters (including a third-party candidate oneself) to avoid this problem frequently leads to the distortion of tactical voting, in which a race can be won by a candidate who is not comparatively (and proportionally) the most popular.

My very strong feeling is, if liberals or moderates wish to try to force a "three-party interlude", the time to do so is clearly not when an oppressive and authoritarian party is in control of all the major organs of government and has the mainstream media behind it. Under those conditions, such an effort is all but doomed to fail, and can only help the ruling party to remain in power.