One of the purported benefits of the Alternative Vote is that it would end the need to vote ‘tactically’ in elections, but this claim has been challenged by some commentators. Andy White of the Electoral Reform Society explains why, in practical terms, the claim stands up to scrutiny.

What is tactical voting?

A tactical vote is an insincere vote: one in which a voter’s ranking of candidates on the ballot paper does not reflect his or her true intentions, and which produces a more desirable result than if the voter had voted sincerely. For example, under our present system of ‘first past the post’ elections, it is (or was) common for Labour supporters in unwinnable seats to plump instead for the Liberal Democrat candidate, in order to prevent the Conservative from winning.

This strategy was illustrated at the 2010 general election in the seat of Eastleigh, where Chris Huhne beat his Conservative opponent by appealing to Labour supporters. You can see how this choice was presented to voters in the chart in the bottom right of this election leaflet.

The Gibbard-Satterthwaite theorem

The Gibbard-Satterthwaite theorem to which both Rafael Hortala-Vallve and Roger Mortimore referred in recent LSE blog posts, proves that all voting systems are susceptible to tactical voting, “in the sense that there are conditions under which a voter with full knowledge of how the other voters are to vote and of the rule being used would have an incentive to vote in a manner that does not reflect his preferences.”

Nobody disputes the theorem, but a key phrase in its construction is ‘full knowledge’. Tactical voting is easy under first past the post, because only vague knowledge is required to make a correct decision. If I am a UKIP supporter in a seat that has only ever been held by Conservative or Labour MPs, then I can confidently predict that a tactical vote for the Conservative candidate will have the desired effect.

Indeed, tactical voting is so simple and effective under first past the post that the phenomenon has its own dedicated websites and guides in tabloid newspapers. Dr Stephen Fisher of Oxford University estimates that nearly a tenth of all votes in British general elections are tactical (see this paper from 2001).

The proportion of seats susceptible to tactical voting (i.e. so-called ‘marginal’ constituencies, where more than one party stands a realistic chance of winning) is up to half of all seats in Britain, and these are the seats which decide our general elections.

Tactical voting with the Alternative Vote

Roger Mortimore has posted an excellent description of how, in theory, one might be able to vote tactically in an Alternative Vote election. I suggest readers follow his worked example before returning to my argument.

You can see in Mortimore’s example that detailed prior assumptions are required to vote tactically in an Alternative Vote election. In the example Mortimore gives, the swing needed for one candidate to overtake another (about 4 per cent) – and thus change the character of the contest – is within the margin of error of a typical opinion poll. Moreover, even if voters are able to coordinate an effective tactical strategy, the number of constituencies susceptible to it is greatly reduced (there are about fifty ‘three-way marginals’ in Britain).

I contend that the level of prior knowledge and coordination required to vote tactically in an Alternative Vote election is so great – and the risks so much clearer than the rewards – that there is no incentive to attempt it. Tactical voting under first past the post requires only a certainty that one’s favoured candidate cannot win. With the Alternative Vote, conversely, one must accurately project the vote shares and transferred preferences of three different candidates (taking into account the fact that one’s co-conspirators will also be voting insincerely). For these reasons, the Gibbard-Satterthwaite theorem does not contradict claims that the Alternative Vote would end tactical voting in British general elections.

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