Tactical voting

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In voting systems, tactical voting (or strategic voting or sophisticated voting) occurs, in elections with more than two viable candidates, when a voter supports a candidate other than his or her sincere preference in order to prevent an undesirable outcome.

It has been shown by the Gibbard-Satterthwaite theorem that any voting method which is completely strategy-free must be dictatorial.

For example under "First past the post" a voter might sometimes gain a "better" outcome by voting for a less preferred but more generally popular candidate while under AV a voter might sometimes get a better outcome by voting their first choice for their least favourite candidate.

A false counter example to this that is often given is the random ballot voting method, which randomly selects the ballot of a single voter and uses this to determine the outcome. This is strategy-free but it is dictatorial (albeit with the dictator being picked at random). This counter-example is often used to imply that the G-S Theorem does not apply to non-deterministic voting systems. That is not true, it does apply to probabilistic voting systems if they are non-dictatorial.

However, the type of tactical voting and the extent to which it affects the character of the campaign and the results of the election vary dramatically from one voting system to another.


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