Vote splitting is an electoral effect in which the distribution of votes among multiple similar candidates reduces the chance of winning for any of the similar candidates, and increases the chance of winning for a dissimilar candidate. The aspect of vote splitting remains controversial, as some may argue that the notion of vote splitting discourages minor parties.
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Vote splitting most easily occurs in plurality voting (also called first-past-the-post) in which each voter indicates a single choice and the candidate with the most votes wins, even if the winner does not have majority support. For example, if candidate A1 receives 30% of the votes, similar candidate A2 receives another 30% of the votes, and dissimilar candidate B receives the remaining 40% of the votes, plurality voting declares candidate B as the winner, even though 60% of the voters prefer either candidate A1 or A2.
Runoff voting methods are less vulnerable to vote splitting compared to plurality voting. Pairwise-counting Condorcet methods minimize vote splitting effects.
A well-known effect of vote splitting is the spoiler effect, in which a popular candidate loses an election by a small margin because a less-popular similar candidate attracts votes away from the popular candidate, allowing a dissimilar candidate to win.
Strategic nomination takes advantage of vote splitting to defeat a popular candidate by supporting another similar candidate.
Vote splitting is one possible cause for an electoral system failing the independence of clones or independence of irrelevant alternatives fairness criteria.
Vote splitting and electoral systems
Different electoral systems have different levels of vulnerability to vote splitting.
Vote splitting most easily occurs in plurality voting because ranked ballots are not used, so there is no information about the secondary preferences of the voters.
Approval voting reduces the vote-splitting effect compared to plurality voting, but vote splitting still occurs because the full preferences of voters are not collected.
Runoff voting is less vulnerable to vote splitting, yet vote splitting can occur in any round of runoff voting. Although instant runoff voting (IRV) uses ranked ballots, secondary preferences are considered in the same sequence as in multiple rounds of voting, so this method does not reduce the vote-splitting effect (compared to runoff voting).
Vote splitting rarely occurs when the chosen electoral system uses ranked ballots and a pairwise-counting method, such as a Condorcet method. Vote splitting rarely occurs when using pairwise counting methods because they do not involve distributing each voter's vote among the candidates. Instead, pairwise counting methods separately consider each possible pair of candidates, for all possible pairs. For each pair of candidates there is a count for how many voters prefer the first candidate (in the pair) to the second candidate, and how many voters have the opposite preference. The resulting table of pairwise counts eliminates the step-by-step distribution of votes that facilitates vote splitting in other voting methods.
When ranked ballots are used, a voter can vote for a minor party candidate as their first choice, and also indicate their order of preference for the remaining candidates, without regard for whether a candidate is in a major political party. For example, voters who support a very liberal candidate can select a somewhat liberal candidate as their second choice, thus minimising the chance that their vote will result in the election of a conservative candidate.
Voting methods that are vulnerable to strategic nomination, especially methods that fail independence of clones, are vulnerable to vote splitting. Vote splitting also can occur in situations that do not involve strategic nomination, such as talent contests (such as American Idol) where earlier rounds of voting determine the current contestants.
In the United States vote splitting commonly occurs in primary elections. The purpose of primary elections is to eliminate vote splitting among candidates in the same party. If primary elections or party nominations are not used to identify a single candidate from each party, the party that has more candidates is more likely to lose because of vote splitting among the candidates from the same party. Primary elections only occur within each party, so vote splitting can still occur between parties in the secondary election.
In addition to applying to single-winner voting systems (such as used in the United Kingdom, the United States and Canada), a split vote can occur in proportional representation methods that use election thresholds, such as in Germany, New Zealand and Turkey. In these cases, "fringe" parties that do not meet the threshold can take away votes from larger parties with similar ideologies.