Regular readers will know that I have often complained about surveys claiming to show that an issue or policy stance will make people more or less likely to vote for a party. I’ve written about it previously here, here and here.

Polls like this are very popular with pressure groups and political campaigns because they inevitably end up suggesting that the issue they are campaigning on is really important and will swing votes, and MPs better listen to them and do what they say or they’ll lose their jobs. The problem is people don’t actually behave like that – we know from academic studies that people don’t vote on single issues, they vote on broad things like party identification, perceptions of party leaders, perceptions of competence, economic trust, party image and so on. Individual policy stances play into these things, but to reduce the complex drivers of voting behaviour to polling questions purporting to show that “If party X follows policy Y then 14% of people won’t vote for them” is simplistic and naive.

I won’t recite at length why these questions give such misleading data, but there are a number of reasons. Firstly respondents use them to indicate support or opposition to a policy or a party regardless of whether it would actually change their vote. Secondly it gives false prominence to a single issue, when in a real election campaign that issue would be considered alongside other important issues like the economy, leadership perceptions, party competence and so on. Thirdly people simply aren’t very good judges of what drives their own decision making. Working together these produce results that overestimate the impact of single issues on voting intention.

Yet, it is a question that people eternally want asking – and keeps provoking rather silly news stories, full of hyperbole about gay marriage dooming the Conservative party. Hence it is worth trying to do it sensibly.

In the Sunday Times/YouGov poll this week they first they asked what three or four issues would be most important to people in deciding their vote at the next general election. It’s still simplistic of course, and still depends on people understanding the drivers of their own voting intentions when they don’t, but at least it doesn’t give an issue false prominence.

Asked this way, 56% say the economy will be an important issue in how they vote, 42% immigration, 36% health, 28% unemployment and so on down to gay marriage, of which 7% of people say it will be an important issue in deciding their vote. This includes 5% of current Tory voters and 5% of 2010 Tory voters.

Of course, not all these people who care about the issue are necessarily opposed to it. YouGov asked those 7% who said gay marriage was likely to be an important factor in deciding how they voted whether it would make them more or less likely to vote for a party – 54% said more likely, 44% less likely. In other words three and a bit percent of voters claim they are more likely to vote for a party that supports gay marriage, three and a bit percent claim they are less likely.

Looking just at Tory voters, that 5% of Tory voters who say it will be an important issue in deciding their vote are mostly people who would be less likely to vote for a party that supported it… but this still equates to just 4% of Tory voters – that is, about 1 percentage point of their current 34 percentage points of support. Hardly a huge election winning or losing issue.

Even this is likely to be an overestimate of course, because as I said at the start of the post, people aren’t very good judges of what drives their voting intention and individual issues aren’t actually much of a driver of voting intention; party identification, competence, the economy, leadership perceptions are the sorts of things that actually drive votes. On top of that, gay marriage is very high up the news agenda right now – I expect it will be a far less high profile issue in 2 years time. It also ignores whatever positive impact the issue might have.

In terms of a direct effect, my guess is that, by the time of the next election, gay marriage will have negligible impact on Tory support. Potentially more important is the indirect effect – whether, on one hand, that it adds to perceptions amongst traditionalist voters that David Cameron does not understand or reflect their views or, on the other hand, helps build perceptions that the Conservatives are a more modern and tolerant party that is at ease with the modern world.

Things like this are almost impossible to measure in polls, but are probably far more important. My own view is that Michael Ashcroft is right on this – in terms of impact on party perception Cameron may or may not have been right to come down the route of pushing gay marriage, but now he is here he must continue. Changing his mind won’t convince his detractors that he actually agrees with them, it will just make him look weak, while fatally undermining whatever positive impact it has on making the Conservative party look more tolerant and in tune with modern Britain (that said, I’m far from convinced that it is having a positive impact on that front, because the impact of David Cameron supporting gay marriage risks being cancelled out by the impact of right-wing Conservative backbenchers opposing it).

Meanwhile YouGov’s weekly poll for the Sunday Times has topline figures of CON 34%, LAB 41%, LD 12%, UKIP 8%. The mid-week Sun polls clearly suggested the referendum boost was on the wane, but these figures suggest it hasn’t completely gone yet.