IN the run up to the General Election, Labour and SNP plans to cut the deficit were almost identical. Indeed, though the think tank's figures were disputed by the Scottish Government, the Institute for Fiscal Studies concluded the SNP was taking the slightly more austere approach of the two parties.

The tone and rhetoric of the campaigns could not have been more different, however. The SNP vowed boldly to "end austerity". Nicola Sturgeon pulled off the seemingly impossible trick of simultaneously deriding Labour's plan as no different from the Tories' while offering to work with Ed Miliband but never with David Cameron.

Labour, fighting desperately to convince voters it would be a responsible custodian of the economy, took a different tack. Eds Miliband and Balls sent a message they would be tough on the deficit. Cuts had to be made and would be made, they stressed.

When Labour was all but wiped out in Scotland, those on the left of the party drew a clear lesson: Scottish Labour must position itself to the left of the SNP to have any hope of staging a recovery.

Centrists instinctively rejected this analysis, noting that, for all their radical soundbites, the Nationalists have governed firmly from the centre at Holyrood. Their case appears to have been strengthened by the latest findings from the British Election Study, the long-running and highly respected academic programme which has unpicked every UK poll for the past half a century in an attempt to understand why people voted the way they did.

In a recent seminar, Professor Ed Fieldhouse of Manchester University crunched the numbers on three features of last May's contest: the collapse of the Lib Dems; the prospect of a hung parliament and the SNP's rout of Labour in Scotland.

On the last of these he concluded that the SNP's anti-austerity stance was "not critical" to the party's success. Rather, the biggest factor pushing people to vote SNP was the independence referendum. If you voted Yes last September, it is more than 90 per cent likely you voted for Nicola Sturgeon's party in May.

Strikingly, the study revealed more than three quarters of previous Labour supporters who backed independence switched allegiance to the Nationalists eight months later, a figure which lays bare the abject failure of Jim Murphy's original election strategy of targeting and winning back so-called "Glasgow Man".

"The referendum itself cemented people's support for the SNP. People who previously voted Labour became more inclined to support the SNP because they voted Yes," argued Professor Fieldhouse. "It was actually something about the referendum itself," he added, noting that Labour's image had become tarnished by the referendum, in particular by the party's decision to join forces with the Tories' in the Better Together campaign.

His conclusion that the SNP's anti-austerity stance did not play a decisive role was based on the surprising finding that people backed the party whether or not they agreed with Ms Sturgeon on the issue. As you'd expect, 90 per cent of people who said they were anti-austerity and who voted Yes ended up giving the Nationalists their vote. But so did 90 per cent of Yes voters who were not anti-austerity, who believed it was important to eliminate the deficit. In other words, Yes voters who did not believe in the SNP's key, left-wing election message still voted overwhelmingly for the party.

Others factors aided the SNP. Professor Fieldhouse found the change in leadership from Alex Salmond to Ms Sturgeon "helped considerably" to win over former Labour voters. People's views on devolution and the performance of the SNP government played a part.

In the end, though, it was the referendum - and backing Yes - that emerged as "the most important variable" in discovering how people voted in the General Election.

These British Election Study conclusions pose all kinds of difficult questions for Scottish Labour which, faces a Holyrood election in just nine months' time.

But if they are correct, the party should be wary of trying to position itself miles to the left of the SNP when it draws up its manifesto for May 5 next year. This hadn't been much of an issue in a Scottish leadership race between two centrist candidates - until Corbynmania broke out. Now the possibility of the unflinchingly left-wing MP for Islington North leading the UK party has made the debate unavoidable.