A number of people on social media today raised an eyebrow or two at YouGov findings that show people who voted SNP in 2015 were significantly more likely to simply stay at home in the 2017 election than voters for any other party (with the exception of UKIP). This has raised the possibility that much of the swing from the SNP to the Tories and Labour was caused by SNP abstentions, rather than net movement from one side to the other.
Unfortunately, there isn't enough information in the datasets to draw such a strong conclusion. This is a GB-wide poll, and the SNP's abstention rate is not being compared with that of the Scottish Tories or Scottish Labour, but with the Tories and Labour across Britain as a whole. That's bound to give a misleading impression, because turnout in Scotland dropped by several points this year, whereas it rose south of the border.
The most that can be said, therefore, is that this poll is consistent with the theory that the SNP suffered from differential turnout, but it doesn't provide proof. If that is what happened, presumably there were independence supporters who were fired up in the immediate aftermath of the 2014 referendum, but who this time weren't sufficiently inspired by the rather vague (and bland?) "Stronger for Scotland" message. I suspect the SNP missed a trick by downplaying independence during the campaign - they were probably worried about losing No voters, but the pre-election polls suggested most of those people had already drifted off anyway.
The poll's oddest finding is that, even after abstainers are excluded, only 33% of people who voted Plaid Cymru in 2015 stuck with the party this year. The equivalent figure for the SNP is 71%. It's hard not to be sceptical about that finding, because Plaid's vote share only slipped 1.7% (and they made a net gain of one seat!).
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