Vote-splitting

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.

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.

Different electoral systems have different levels of vulnerability to vote splitting.

Vote splitting most easily occurs in plurality voting because the ballots don't gather any information about the secondary preferences of the voters.

Approval voting reduces the vote-splitting effect compared to plurality voting, but vote splitting can still occur because the two-level means full preferences of voters are not collected.

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