In voting systems, tactical voting (or strategic voting) occurs when a voter misrepresents his or her sincere preferences in order to gain a more favorable outcome. Any minimally useful voting system has some form of tactical voting. However, the type of tactical voting and the extent to which it affects the timbre of the campaign and the results of the election vary dramatically from one voting system to another.
Types of tactical voting
There are different types of tactical voting:
Compromising (sometimes favorite-burying or useful vote) is a type of tactical voting in which a voter insincerely ranks an alternative higher in the hope of getting it elected. For example, in the first-past-the-post election, a voter may vote for an option they perceive as having a greater chance of winning over an option they prefer (e.g., a left-wing voter voting for a popular moderate candidate over an unpopular leftist candidate). Duverger's law suggests that, for this reason, first-past-the-post election systems will lead to two party systems in most cases.
Compromising-compression is a compromising strategy that involves insincerely giving two candidates an equal ranking. Compromising-reversal is a compromising strategy that involves insincerely reversing the order of two candidates on the ballot.
Burying is a type of strategic voting in which a voter insincerely ranks an alternative lower in the hopes of defeating it. For example, in the Borda count, a voter may insincerely rank a perceived strong alternative last in order to help their preferred alternative beat it. A real-world analogy would be voters of one party crossing over to vote in the other party's primary against the candidate they think might beat the candidate of their party.
Burying-compression is a burying strategy that involves insincerely giving two candidates an equal ranking (or truncating, which generally amounts to the same thing). Burying-reversal is a burying strategy that involves insincerely reversing the order of two candidates on the ballot.
Push-over is a type of strategic voting that is only useful in methods that violate monotonicity. It may involve a voter ranking an alternative lower in the hope of getting it elected, or ranking an alternative higher in the hope of defeating it. Also known as a paradoxical strategy.
Strategy-free voting methods
It has been shown by the Gibbard-Satterthwaite theorem that it is impossible for a voting method to be both strategy-free and deterministic (that is, select the same outcome every time it is applied to the same set of ballots). The random ballot voting method, which selects the ballot of a random voter and uses this to determine the outcome, is strategy-free, but may result in different choices being selected if applied multiple times to the same set of ballots.
However, the extent to which tactical voting affects the timbre and results of the campaign varies dramatically from system to system: see below.
Examples in real elections
In United Kingdom elections, there are three main parties represented in the Parliament: the Labour party, the Conservative party and the Liberal Democrats. Of these three, Labour and the Liberal Democrats are most similar. Many people who prefer the Liberal Democrats vote for the Labour candidate where Labour is stronger and vice-versa where the Liberal Democrats are stronger, in order to prevent the Conservative candidate from winning.
In the 1997 UK General Election, the Democratic Left organised GROT - Get Rid Of Them - a tactical voter campaign. In 2001, the Democratic Left's successor organisation the New Politics Network organised a similar campaign tacticalvoter.net. Since then tactical voting has become a real consideration in British politics as is reflected in by-elections and by the growth in sites such as www.tacticalvoting.com who encourage tactical voting as a way of defusing the two party system and empowering the individual voter.
Rational voter model
Academic analysis of tactical voting is based on the rational voter model, derived from rational choice theory. In this model, voters are short-term instrumentally rational. That is, voters are only voting in order to make an impact on one election at a time (not, say, to build the political party for next election); voters have a set of sincere preferences, or utility rankings, by which to rate candidates; voters have some knowledge of each other's preferences; and voters understand how best to use tactical voting to their advantage. The extent to which this model resembles real-life elections is the subject of considerable academic debate.
Because tactical voting relies heavily on voter's perception of how other voters intend to vote, campaigns in electoral systems that promote compromise frequently focus on affecting voter's perception of campaign viability. Most campaigns craft refined media strategies to shape the way voters see their candidacy. During this phase, there can be an analogous effect where campaign donors and activists may decide whether or not to support candidates tactically with their money and labor.
In rolling elections, or runoff votes, where some voters have information about previous voters' preferences (e.g. presidential primaries in the United States, French presidential elections), candidates put disproportionate resources into competing strongly in the first few stages, because those stages affect the reaction of latter stages.
Views on tactical voting
Some people view tactical voting as providing misleading information. In this view, a ballot paper is asking the question "which of these candidates is the best?". This means that if one votes for a candidate who one does not believe is the best, then one is lying. Labour Party politician Anne Begg considers tactical voting dangerous: 
- "Tactical voting is fine in theory and as an intellectual discussion in the drawing room or living rooms around the country, but when you actually get to polling day and you have to vote against your principles, then it is much harder to do".
While most agree that tactical voting is generally a problem, there are some cases when a strictly limited amount of it may bring about an more democratic result. Since Arrow's impossibility theorem proves that any voting system is arguably undemocratic in at least some case, tactical voting may be used to correct such flaws. For instance, under purely honest voting, Condorcet method-like systems tend to settle on compromise candidates, while Instant-Runoff Voting favors those candidates which have strong core support - who may often be more extremist. An electorate using one of these two systems but which (in the general or the specific case) preferred the characteristics of the other system could consciously use strategy to achieve a result more characteristic of the other system. Under Condorcet, they may be able to win by "burying" the compromise candidate (although this risks throwing the election to the opposing extreme); while under IRV, they could always "compromise".
The problem is that such tactical voting would tend to overshoot and give undesired results. This greatly complicates the comparative analysis of voting systems. If tactical voting were to become significant, the perceived "advantages" of a given voting system could turn into disadvantages - and, more surprisingly, vice versa.
Tactical voting in particular systems
Steven Brams and Dudley R. Herschbach argued in a paper in Science magazine in 2000 that approval voting was the system least amenable to tactical perturbations. This may be related to the fact that approval voting does not permit preferences ('likes' or 'dislikes') to be stated at all, permitting only a statement of tolerances, that is, "which candidate could you stand to see win", as opposed to "which candidate would you like to see win".
Due to the especially deep impact of tactical voting in first past the post electoral systems, some argue that systems with three or more strong or persistent parties become in effect forms of disapproval voting, where the expression of disapproval, to keep an opponent out of office overwhelms the expression of approval, to approve a desirable candidate. Ralph Nader refers to this as the "least worst" choice, and argues that the similarity of parties and the candidates grows stronger due to the need to avoid this disapproval.
- Tactical Voting Can Be a Weak Strategy -- Article on tactical voting within larger strategic considerations
- tacticalvoter.net -- UK Tactical Voting
- VotePair.org VotePair is a banding together of the people who started tactical voting online in the 2000 elections..
- Making Votes Count, Gary Cox (1997)
- The Proof of the Gibbard-Satterthwaite Theorem Revisited, Lars-Gunnar Svensson (1999)
- Brams, Herschbach, "The Mathematics of Elections" (sic?), Science (2000)
- Extending the Rational Voter Theory of Tactical Voting, Stephen Fisher (2001)
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