John Curtice: The 'miserable compromise' with modest gains for Clegg

Only in 2010 – when first-past-the-post also failed to deliver a majority – would a hung parliament have occurred under AV

So we all know why Nick Clegg wants a "Yes" vote in the AV referendum, don't we. AV would result in more Liberal Democrat MPs. Consequently, future elections would always produce a hung parliament – and thus Britain's third party would become perpetual kingmaker.

Well, yes and no. At every election since 1983 at least one poll has asked voters which party was their second preference. So we have a good idea of what would have happened if AV had been used before. The findings are not quite so encouraging for the Liberal Democrats as is often imagined.

True, the party would always have won more seats under AV. The Liberal Democrats have consistently been the most popular second preference of both Labour and Conservative voters. So in seats where the party came a close second under the current system, AV would have enabled it to come first on the back of second preferences of voters whose party was third.

But close Liberal Democrat second places have been rare. So the benefit the party would have derived from AV at recent elections would typically have been not much more than 20 seats or so. Only in 1997, when the Conservatives fell back so badly and Labour voters were particularly keen on the Liberal Democrats as their second preference, would the party have secured a much bigger bonus.

Moreover, at none of the elections between 1983 and 2005 would the increased number of Liberal Democrat MPs have resulted in a hung parliament. In part, this is because most of the elections during that period resulted in landslide victories. But it is also because of the way Liberal Democrat voters would have used their second preferences.

This has changed over time. Before 1997, voters for Britain's main third party preferred the Tories to Labour – and especially in 1987. As a result, AV would have helped the Conservatives leapfrog past Labour in some seats, thereby compensating the party for the seats where the Liberal Democrats would have edged past themselves. Mrs Thatcher would still have enjoyed big majorities in 1983 and 1987. But in 1997, just as the nation swung behind Labour, so did Liberal Democrat voters. Now they preferred Labour to the Conservatives. As a result, Mr Blair would have enjoyed an even bigger landslide, and indeed again in 2001.

Only in 2010 – when first-past-the-post also failed to deliver a majority – would a hung parliament have occurred under AV. It is, perhaps, little wonder Mr Clegg once described AV as a "miserable little compromise". For its introduction is likely to bring his party only a modest benefit – and still leave open the prospect of the occasional Conservative or Labour landslide.

The writer is professor of politics at Strathclyde University