Last week this tweet went viral.
Which led to this headline on The Independent's indy100 site, which repeated the central claim as fact – and quickly became the most viral election story of the whole week.
And it's not the only one! Many of the most widely shared election stories of the past week have been about the same idea – that an increase in the number of people between 18 and 24 voting could be enough to swing the election. "It's official: The more young people that vote the more likely the Tories are to LOSE", wrote The London Economic blog, while The Canary had "Students just blew the general election wide open with one hell of a shock for Theresa May".
Because we had nothing better to do, we decided to see if it was actually true. How many more 18- to 24-year-olds would need to vote to swing the election? Let's DO SOME MATHS!
Let's start with some basic numbers.
To keep this simple, we'll use the results from the 2015 election as our guide. (That's actually a bit generous to Labour's chances, as the Conservatives are currently polling much better than they did in 2015, but let's run with it.) How different would that election have looked if 18–24 turnout had been higher?
Overall, the Conservatives got a total of 11.3 million votes to Labour's 9.3 million – a deficit for Labour of slightly under 2 million votes. Now obviously, UK elections aren't decided by who gets most votes nationally. But it's a good place to start.
So, how many people under 25 didn't vote?
In mid-2015, when the last election was held, there were 5,878,472 people aged 18 to 24 in the UK, according to estimates from the Office for National Statistics. Let's remove Northern Ireland from that (where Labour don't have candidates, so you can't vote for them). That gives us slightly over 5.7 million potential voters aged under 25.
Polling firm Ipsos estimated in its large post-2015 election study that the turnout rate of people aged 18–24 was 43% in 2015. This means that roughly 2.45 million people of that age voted, while 3.25 million didn't. That's a lot of non-voters.
It's also the lowest turnout of any age group, by quite a long way.
Old people are super into voting!
But those who did vote in 2015 went strongly for Labour.
And of course, they wouldn't all vote Labour.
In fact, to overturn the Tory lead in 2015, you'd have needed a turnout among under-25s of, er...261%.
Okay, you're thinking, but what if those non-voters don't split the same way as voters did? What if they're much more likely to prefer Labour?
A survey for the Higher Education Policy Institute showed students backing Labour by 55% to just 18% for the Tories, with 93% of them registered to vote – both big increases on previous elections. That's the basis for quite a lot of the stories about how students could have a big shock in store for Theresa May.
Okay, setting aside the point that undergraduate students aren't necessarily representative of all young people, let's chuck those numbers in and see what pops out. If 55% of young people voted Labour, with 18% going to the Conservatives, and turnout was an unprecedented-in-literally-any-general-election-ever 93%, Labour...would still come up 931,000 votes short.
The better news for Labour is that on these figures, you'd only need 138% of all the young people in the country to vote in order to win the popular vote.
Or to put it another way, even at that wildly unrealistic 93% turnout, under-25s would need to split for Labour over the Tories by 85% to 15% for Labour to change the result. Whichever way you cut it, bluntly: It's not happening.