Last week, YouGov released a new poll of Labour leadership selectors that suggested that Jeremy Corbyn may very well win in the first round. Corbyn’s meteoric rise from charity case to front runner has been all the more remarkable because, in the words of Alastair Campbell, “Jeremy Corbyn as … every piece of political intelligence, experience and analysis tells you will never be elected Prime Minister.” So what’s going on here? Is conventional Westminster wisdom wrong? Is the favourite of the unions capable of repeating the success of Syriza? Or has Labour rediscovered its “desire never to win again”? Perhaps neither.

A useful insight might come from public choice theory, and in particular from a highly-regarded and heavily-cited book by Geoffrey Brennan and Loren Lomasky. In Democracy and Decision: the pure theory of electoral preference, Brennan & Lomasky offer a new explanation for the “paradox of voting”, the rationality-defying fact that people vote despite the fact that the probability of one’s vote mattering is almost zero. As Sam observed on these pages:

No individual can reasonably expect her vote to determine or even influence the outcome of an election. In America, the chance of a one-vote victory margin that would determine the 2008 presidential election was about 1 in 10 million in some swing states, and 1 in a billion in places like California or Texas.

Thus a rational voter would be a non-voter, avoiding the cost of registering for and participating in something that they cannot possibly hope to affect. And yet people vote.

Brennan & Lomasky offer a different explanation. Individuals do not vote primarily to affect the outcome (which they know they cannot) but to express an preference; indeed, to express themselves. Much as we might shout at a football match on television or curse out loud when on our own, there is something inherent in the human psyche that wishes to express its opinion. What is more, the way in which we express ourselves helps define who we are, and enables us to feel good about ourselves.

The crucial point here is that there is absolutely zero cost to expressing oneself any way one pleases at the ballot box, because one’s vote is hardly likely to matter. For the same reason, the only tangible benefit one is likely to reap from voting is that feeling one gets for choosing “the right” candidate. Vote Labour and you are a caring person; vote Conservative and you are a responsible person; vote UKIP and you are a proud patriot; vote Green and you want to save our planet...

Which brings us back to Mr Corbyn. John Mann, Labour MP for Bassetlaw, tweeted that “Quite a number of Corbyn supporters [said] to me that principled opposition is better than seeking an electoral majority.” He dismissed this as “The elite speak[ing]”.

But maybe what is going on here is that Labour supporters, bruised by a crushing defeat and frustrated by the thought of another five years of Conservative rule, are voting not rationally but expressively. Remember, the chance that any single vote matters is going to be tiny: Edlin, Gelman, and Kaplan suggest that “The probability of a pivotal vote is inversely proportional to the number of voters…”; that means that a Labour leadership selector has a 1-in-400,000 chance of being decisive.

But they have a precisely 1-in-1 chance of defining who they are by how they vote. With a probability of one they can make a statement that they are caring and principled, that they believe in social justice, that they reject the Conservative dogma that has dominated electoral politics for a third of a century. Conventional Westminster wisdom may be right that Corbyn isn’t electable, but whether Corbyn becomes leader is not a function of their single vote, whereas that single vote says everything about the voter and their values.

What results, as public choice theorists know only too well, is a collective action problem. No individual can affect the outcome and therefore the worst outcome results. It is important to acknowledge, also, that it is highly unlikely that voters are fully conscious of how the incentives affect their behaviour. But it does explain why supporters whose party has only ever won when it has tacked to the centre are nonetheless willing to vote for the most extreme candidate on the ticket despite the fact that their last, somewhat off-centre leader, lost them the general election.

It may not be the “right” thing to do, but God! It feels good!