AV Referendum: could third placed candidates end up winning?

"Under AV, the candidate who finishes third can be declared the winner."
David Cameron, 17 April 2011

With party leaders stepping up the campaign for and against AV ahead of next month's referendum, politicians of all hues have been finding themselves in some unlikely coalitions.

The Prime Minister was in just such a scenario this morning, as he shared a platform with Labour stalwart and former Home Secretary John Reid in campaigning against a change to the voting system.

But does Mr Cameron's claim that a candidate placing a lowly third at an election could end up eventually triumphing hold water?

"In theory yes, in practice no" is the response of the Australian Broadcasting Corporation's election expert Antony Green. He has analysed election results held under AV in Australia and found that "no candidate has ever been elected from third place in an Australian election conducted under the AV rules proposed for the United Kingdom."

This contradicts a claim made by Lord Ashcorft on Conservative Home, which gives the example of Henry Arthur Hewson, who, it is claimed, was elected to the Australian House of Representatives in 1972 having placed third in the first round.

Whilst Lord Ashcroft has not made any mistake in recalling the election result, it should be noted that that particular contest was held under the "full preferential voting" system — which obliges voters to use all their preferences on the ballot paper — and not the particular variant of AV being proposed for the UK.

Even with this full preferential system of voting, it is extremely rare for candidates placed third after the first round of counting to end up the victor. According to Mr Green: "Since 1949 there have been 3,354 House of Representatives electorates contests, and in only seven has a candidate placed third managed to win."

Looking further afield, San Francisco did bear witness to the election of a candidate who ranked third after the first count in the 2010 district elections. Furthermore, this result did occur under an optional preferential system (in which voters are not required to use their full slate of votes) known in the US as ranked choice voting (RCV). Whilst this bears a greater resemblance to the system of AV voters will be asked to decide upon on May 5, the San Francisco example does differ in only allowing the electorate to express up to three choices.

It is also worth noting that the San Francisco district election was unusual in that it featured 21 candidates with no incumbent and no obvious front-runner, meaning that after the first round, the leading four candidates were all within about one third of a per cent of each other.

So whilst it is difficult to conclude with certainty that no third placed candidate has ever gone on to triumph under AV, as much depends upon how narrowly that system is defined, it does appear to be a rare occurrence in the elections where it has been used.

Furthermore it is even more difficult, given the relative scarcity of such examples, to draw firm conclusions for the UK's political system, and David Cameron cannot be said to be wrong to claim that it is a possibility that a candidate placing third after the first round of counting could eventually win.