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A plurality (or "relative majority") is the largest share of something, which may or may not be a majority. For example, if an election had three candidates, who received 40%, 25%, and 35% of the vote, the candidate with 40% would have a plurality, but not a majority. Some elections or voting methods require merely a plurality; the best-known is the First Past The Post system, sometimes called plurality voting, but other systems such as Borda count are also plurality-based. Other systems require a majority, using methods such as runoff voting if a majority is not initially produced.

Plurality-based voting systems can be very simple and transparent compared to ones that require majorities; the simplest case, First Past The Post, simply allows each voter to select one candidate and awards the election to the candidate with the most votes. However, they are also the most prone to tactical voting, and can elect a candidate even though there is another candidate who would be preferred over them by the majority of voters.

For example, in a vote like this

  • Candidate A, Democratic Communist Party, 30%
  • Candidate B, Socialist Communist Party, 30%
  • Candidate C, Conservative Party, 40%

it seems clear that a majority of voters - 60% - want a communist candidate to be elected. However, because their vote is split between two candidates, the conservative candidate will be elected with a plurality even though conservatives are in a minority. This effect means that jurisdictions with plurality-based systems tend to have small numbers of active parties - often, as in the United States, only two parties with any real chance of election. In these systems it can be very difficult for any view not within the two established parties to be heard, as switching a vote from your favourite of the two most popular parties to a smaller party increases the chances that, with the vote split, the voter's less favoured of the two most popular parties will be elected.

It is even possible for a party with majority support to fail to be elected under these systems, if it is not one of the two traditional biggest parties; for example, in a First Past The Post vote:

  • 40 voters (A) prefer party 1, followed by party 3
  • 50 voters (B) prefer party 2, followed by party 3
  • 60 voters (C) prefer party 3, followed by party 1
  • 70 voters (D) prefer party 3, followed by party 2

In this case, party 3 is by far the preferred party, with 130 of the 220 voters preferring it to the other parties. However, if people think that only party 1 and party 2 (the traditionally established parties) have a chance of being elected, then these voters are likely to vote tactically for the party they prefer of the ones they consider electable; so in fact party 2 will be elected, with 120 votes, followed by party 1 with 100 votes. Party 3 will gain few votes, reinforcing the perception that it is unelectable and encouraging its supporters to continue to vote tactically in further elections between those parties. It has been suggested that the Liberal Democrats (UK) in the United Kingdom may have at various points in the 1990s and 2000s been the party preferred by the highest number of voters, while remaining in third place in General Election votes.

This problem does not exist in voting systems requiring a majority. For example, in the Alternative Vote system (the single-place version of Single Transferable Vote), the candidate with the fewest votes is eliminated, and their votes transferred to the voter's second-preferred (or in later rounds third- or fourth-preferred) party, successively until one candidate has over 50% of the vote. This should mean that voters are free to vote for their genuine first preference, knowing that if that party really is in a small minority, their vote will be transferred to their favourite of the two top parties. However, all such systems are complex when compared with the extremely simple First Past The Post system, and may introduce their own undesirable effects; so plurality systems are still used in the major national elections of the majority of jurisdictions.

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