“Youthquake” was the Oxford English Dictionary’s word of 2017. It describes the apparent surge in turnout among young people at last year’s election, and has been used to explain Labour’s unexpectedly good performance.

But data published today by the British Election Study – considered the “gold-standard” of post-election analysis – shows there was no youthquake.

How did so many analysts get it wrong? And if it wasn’t a youthquake what dunnit, how can we explain Labour’s unexpected gains at the election?

What does the data show?

We already know that young people tend to prefer Labour to the Conservatives, but they’re also much less likely to vote than older people.

The youthquake theory was based on the idea that Labour – and particularly Jeremy Corbyn – had somehow managed to convert Labour-supporting young people into Labour voters.

Today’s report confirms that Labour was popular among young people. But it also finds that Labour failed to get those young people into the voting booth.

The BES says there is “no evidence of a surge in voter turnout amongst the youngest eligible voters (indeed turnout in the youngest age group is actually slightly lower in our 2017 survey).”

It goes on: “There was likely a small increase in turnout across a large age range, with a slightly larger rise for those aged 30-40. The margin of error means that we cannot rule out a small increase (or decrease) in youth turnout in 2017.

“We can be confident, though, that there was no dramatic surge in youth turnout of the sort suggested by some other surveys. In short, there was no ‘youthquake’.”

But why did it become received wisdom?

In the immediate aftermath of the election, there was some data to support the idea of a youthquake. One post-election poll suggested that turnout among 18 to 24-year-olds was up by 16 per cent compared to the 2015 election.

And it seemed to make sense. As today’s report points out, “political commentary is prone to believing that what politicians set out to do is effective, even if there is no concrete evidence that it is.

“Increasing youth turnout was part of Corbyn’s political strategy. As Labour did unexpectedly well, it is not unreasonable to think that strategy might have paid off, even though this turns out not to be the case.”

How do we know this research is more reliable than the earlier polls?

On 20 June, Ipsos MORI estimated that turnout among 18 to 24-year-olds had risen by 16 percentage points since 2015. This was based on telephone and online interviews of over 7,000 adults.

This type of survey is common as it can be conducted over a few days and turned around quickly. But even pollsters themselves accept that it’s not a foolproof method.

Another leading pollster, Opinium, points out that: “Both [online and telephone polls] have fast turnaround times of a few days … This means that online and telephone polls have to take whoever they can get.” This can mean that certain types of people are over-represented in online and phone polls. Pollsters do take steps to address this, but it’s hard work.

The British Election Study, on the other hand, “can select precise respondents and make multiple attempts to contact them, resulting in a more representative overall sample at the cost of a significantly longer timespan.”

The BES goes further still: “By comparing the face-to-face survey with our internet panel (which shows very similar results to the pre-election polls) we can assess where the polls failed to achieve representativeness and how this affected their results.”

In other words, the BES can get closer to the truth and help other pollsters understand the limits in their own data.

So where did Labour get their extra votes from?

Both Labour and the Tories increased their vote share at the election, and Labour exceeded expectations. But if it wasn’t a youthquake, where did they get their extra votes from?

Earlier research by the British Election Study has shown that Labour picked up votes from other parties. For example, nearly two thirds of people who voted Green in 2015 switched to Labour in 2017. Labour also won over more than half of all Remain voters.

Labour also managed to attract “switching” voters – in other words, people who changed their mind about who to vote for during the campaign itself. Labour picked up 54 per cent of switchers, while the Tories attracted only 19 per cent of people who changed their minds. This may be to do with Jeremy Corbyn’s surge in personal ratings over the same time.

Marking our own homework

We very nearly fell for the youthquake myth ourselves.

The day after the election, FactCheck concluded, based on the available evidence, “the most likely scenario is that there was a much larger-than-expected turnout among a demographic already inclined to vote Labour, but we await better research on youth turnout.”

We were also sceptical about the very early claims that there had been a turnout of 72 per cent among young people.

And by the end of June, we were already getting evidence that more young people didn’t vote at all than voted Labour.