Vote splitting

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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.

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.

Cardinal voting methods are immune to vote splitting, since each candidate is rated independently of each other. Pairwise-counting Condorcet methods minimize vote splitting effects. Plurality-runoff voting methods (like Exhaustive ballot, Two-round system/Top-two primary, Instant-runoff voting, Supplementary vote, and Contingent vote) still suffer from vote-splitting in each round, but can somewhat reduce its effects compared to single-round plurality voting.

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. As a result, the notion of vote splitting is controversial because it can discourage third party candidates.

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.

Plurality Voting

Vote splitting most easily occurs in plurality voting because the ballots only gather the "least worst" preference of the voter. 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 before the General Election. 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 open primaries, vote splitting occurs between all candidates.

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.

Ordinal 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.

Runoff voting is less vulnerable to vote splitting compared to plurality voting, 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 this method does not reduce the vote-splitting effect.

Vote splitting rarely occurs when the chosen electoral system uses ranked ballots and a pairwise-counting method, such as a Condorcet method. Pairwise counting methods 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.

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.

Cardinal Voting Methods

Cardinal voting methods require an independent score to be given to candidates, as opposed to a ranking. The three primary Cardinal Voting methods are Approval Voting, with a range between 0-1, Score Voting where there's an arbitrary range, and STAR voting.

All cardinal voting methods are immune to vote-splitting, as each candidate is evaluated independently of each other candidate.

Historical examples of vote splitting

• When the cities of Fort William and Port Arthur merged and (in 1969) voted on a name for the new town, the vote was split between the popular choices of "Lakehead" and "The Lakehead", allowing the third option to win, creating the town of Thunder Bay, Ontario.
• In 1987 Roh Tae-woo won the South Korean presidential election with just under 36% of the popular vote because his two main rivals split the vote.
• In the 2000, spoiler candidate Ralph Nader is believed to have split votes away from Democratic candidate Al Gore, contributing to the victory of Republican candidate George W. Bush.
• In the 2000 presidential election in Taiwan, James Soong left Kuomintang (KMT) party and ran as an independent against KMT's candidate Lien Chan. This caused vote-splitting among KMT voters and resulted in victory for Democratic Progressive Party's candidate, Chen Shui-bian. It is the first time in Taiwan history that KMT did not win in presidential election and became the opposition party.
• In the 2002 presidential election in France, the left-wing vote was fragmented among the Socialist Party and several smaller parties, relegating the most successful left-wing candidate, Lionel Jospin, to third place, and precipitating a runoff between two right-wing candidates, incumbent president and RPR candidate Jacques Chirac, and FN candidate Jean-Marie Le Pen. The total vote for the two candidates advancing to the runoff totaled less than forty percent of the votes cast in the first round.
• In the special 2003 California gubernatorial race won by Republican Arnold Schwarzenegger, which did not involve a primary election and which listed 135 candidates on the ballot, concerns about vote splitting caused the Democratic party to withdraw all but one of its major candidates, and caused the Republicans to withdraw most of their candidates. Likewise, any supporters of Republican Tom McClintock changed their mind at the last minute and voted for Schwarzenegger for fear of the Democratic candidate, Cruz Bustamante, winning.
• From 1993 to 2004, the conservative vote in Canada was split between the Progressive Conservatives and the Reform (later the Alliance) Party. This allowed the Liberal Party to win almost all the seats in Ontario during this period as well as win three successive majority governments.
• Similarly, in Quebec, it is argued that the success of the Bloc Québécois in elections from 1993 to 2008 was because of the federalist vote being split between the Liberals and the Conservatives.
• In the 2004 Philippine presidential election, those who were opposed to Gloria Macapagal-Arroyo's presidency had their vote split into the four candidates, thereby allowing Arroyo to win. The opposition had film actor Fernando Poe, Jr. as their candidate, but Panfilo Lacson refused to give way and ran as a candidate of a breakaway faction of the Laban ng Demokratikong Pilipino. Arroyo was later accused of vote-rigging.
• In the 2010 special election for the 1st congressional district of Hawaii, Republican Charles Djou won against Democratic candidates Colleen Hanabusa and Ed Case.
• In the 2012 Egyptian presidential election the two candidates who qualified for the runoff election, Freedom and Justice Party candidate Mohamed Morsi (24.8%) and Independent candidate Ahmed Shafik (23.7%), each received more votes than any other candidate, but they failed to get enough votes to prove that each winning candidate was actually more popular than the Dignity Party candidate Hamdeen Sabahi (20.7%), Independent candidate Abdel Moneim Aboul Fotouh (17.5%), or Independent candidate Amr Moussa (11.1%).
• In the 1994 European Elections, Richard Huggett stood as a "Literal Democrat" candidate for the Devon and East Plymouth seat, with the name playing on that of the much larger Liberal Democrats. Huggett took over 10,000 votes, and the Liberal Democrats lost by 700 votes to the Conservative Party. The Registration of Political Parties Act 1998, brought in after the election, introduced a register of political parties and ended the practice of deliberately confusing party descriptions.
• In Australia there have been the occurrences of three-cornered contest which are contests between the Australian Labor Party and the two parties in the Coalition, the Liberal and National Parties.

While the Liberal and National Parties tend to avoid three-cornered contests they do occur when there is a dispute of which party has claim to seats in question. Australia has the system of preferential voting and in three-cornered contests the Liberal and National Parties would preference each other while the Australian Labor Party would normally preference the Liberal Party ahead of the National Party as they deem the Liberal Party to be less conservative than the National Party.

See also

• Independence of clones
• Independence of irrelevant alternatives
• Spoiler effect
• Strategic nomination
• Tactical voting