Tactical manipulation of runoff voting

Like virtually all other electoral systems, in the two-round system, there is potential for both tactical voting and strategic nomination.Tactical voting is where voters do not vote in accordance with their true preferences, but instead vote insincerely in an attempt to influence the result. Runoff voting is intended as a method that reduces tactical voting, but two tactics called compromising and pushover are still possible in many circumstances. In particular voters are strongly encouraged to compromise by voting for one of the three leading candidates in the first round of an election.

Strategic nomination is where candidates and political factions influence the result of an election by either nominating extra candidates or withdrawing a candidate who would otherwise have stood. Runoff is intended to reduce the spoiler effect, but it does not eliminate it. A famous example of the importance of both strategic nomination and tactical voting in runoff voting was the 2002 French presidential election.

For illustrative purposes, the following is a sample election that does not involve any tactical manipulation. Imagine an election to choose which food to eat for dessert. There are 100 people having dessert and three candidates: Ice Cream, Apple Pie and Fruit. Runoff voting is used to find the winner.

Round 1: In the first round of voting each diner votes for the one candidate they most prefer. The results are as follows:

Round 2: No candidate has an absolute majority of votes (in this election that would be 51) so the two candidates with the most votes, Ice Cream and Fruit, proceed to a second round, while Apple Pie is eliminated. Because their favourite candidate has been eliminated Apple Pie supporters must now vote for one of the two remaining candidates. The Apple Pie supporters split into two even groups: 8 vote for Ice Cream and 8 for Fruit. Of those who supported Ice Cream and Fruit in the first round, no one decides to change their vote. The results of the second round are, therefore, as follows:

Result: Ice Cream now has an absolute majority so is declared the winner.

Runoff voting attempts to reduce the potential for tactical voting by eliminating wasted votes. Under the first past the post (plurality) system, voters are indirectly encouraged to vote tactically by voting only for one of the two leading candidates, as a vote for any other candidate will not affect the result. Under runoff voting this tactic, known as compromising, is sometimes unnecessary because, even if a voter's favourite candidate is eliminated in the first round, she will still have an opportunity to influence the result of the election by voting for a more popular candidate in the second round. However, the tactic of compromising can still be used in runoff voting because it is sometimes necessary to compromise as a way of influencing which two candidates will survive to the second round.

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