“In fact, in each election in the past 30 years, the only party who would have gained more seats every time if AV was used is – that’s right – the Lib Dems.” Caroline Flint, The Mirror 4 May 2011. 

Labour's Shadow Communities and Local Government Secretary is among the Opposition front-benchers to back the current electoral system, putting her case today to readers of the Mirror.

So what reasons did she give? As well as repeating the line used by David Cameron that AV is 'complicated, unfair and expensive', Ms Flint argued that the Liberal Democrats were the only party that would have benefited from AV in every election over the last 30 years. 

The question of how AV would have altered past British elections has been the subject of frequent debate, and so Full Fact took a look at the modelling work that has been done on past elections. 

In their report on AV, the Political Studies Association have reproduced an overview of the projections made for elections between 1983 to 2010. 

The modelling from 1983 to 2005 has been done by Prof. John Curtice, while the British Election Study looked at the 2010 election. All of the modelling is based on surveys looking at voters' second preferences. 

It shows that the Liberal Democrats are the only party that would have been likely to benefit under AV in each and every election over the last 30 years, as stated by Caroline Flint. 

However it is not necessarily true that AV will thus hurt the two biggest parties. Under AV Labour and the Conservatives are both likely to win fewer seats in close elections; but both win more seats when they have strong electoral leads. 

The tendency for AV to reinforce large majorities is best demonstrated from the 1997 projections, where the unpopularity of the outgoing Conservative party would have been reinforced through second preference votes going against the party. The information shows this could have reduced the party's representation to just 70 seats, making them the 3rd party in Parliament behind the Liberal Democrats. 

Under AV minority parties are unlikely to have gained additional seats. 

In looking at past elections it is important to keep in mind that modelling cannot predict with absolute certainty how voter behaviour would have altered the results had past elections in fact been held under AV. The way that voters respond in practice to the opportunity to allocate their votes to small parties would be important in shaping the national outcome of elections under AV. 

Conclusion

Caroline Flint was right to argue that the Liberal Democrats would likely have benefited in each election over the last 30 years under a system of AV. They would also have been the only party to do so; although Labour and the Conservatives would each have benefited under AV in years where they had strong electoral majorities.