One stock explanation for why opinion polls wildly underestimated Conservative supporters in last week’s election is the “shy Tory”. But “shy” isn’t quite the right word, is it? “Shamefaced” comes closer.

In this version of events, Conservatives lie to pollsters, perhaps pretending to be “undecided” when they know full well beside which unsavoury bedfellow they plan to pencil their filthy X. They’re like teenage boys who tell parents blandly they’ll just “hang” tonight, when they’ve been organising a circle jerk to internet porn for weeks. In this mythology, voting Conservative is one of those disgusting habits that you only indulge in the shady, permissive privacy of the polling booth.

To verify the “shy Tory” theory would require statistical analysis beyond my ken. Yet socially, the characterisation rings true.

Broadly speaking – and forgive the generalisations, but they’re irresistibly entertaining – across the western world, leftists tend to be political extroverts, who often literally wear the T-shirt. Anything but ashamed, they’re apt to broadcast their opinions to anyone who will listen (just look at the Guardian letters page). For the left-leaning, political identity is liable to be closely intertwined with personal identity. The left is collusive, if not presumptuous: should you get on well with leftists at a party, they will blithely assume that you share the same views on the invasion of Iraq, even if all you’ve talked about is the canapés.

Mainstream conservatives are more circumspect. They are aware that everyone disagrees with them, and so are socially cautious, if not wary or, outside their territory, paranoid. Thus Tories are more likely to feel out their company before advocating a political position in public. In this sense they are indeed “shy”, and on discovering that they are speaking to an opposition supporter more inclined to flight than fight. At a dinner party, they will often opt for keeping their mouths shut. To defend David Cameron risks not only seeming hopelessly unhip, but at the wrong table could court profound unpleasantness, and possibly ostracism.

I can only testify to experience of this country for the last 28 years, but surely the emergence of the “shy Tory” stereotype is historically recent. In an inversion of the old order, the privilege with which Tories are associated has become a cultural millstone. These days, if you’re not one of the “vulnerable”, no one gives a shit about your problems, and “posh” is now an insult. Witness the virtual evaporation of received pronunciation, and the Eton mess of glottal stops among wealthy independent-school students.

Tory supporters are not spontaneously ashamed; they have been made to feel ashamed. British leftists fiercely believe they are right, in the sense of correct, but also in the sense of just. Conservatives likewise believe they are right-as-in-correct. Yet Tories are less confident about whether their politics are right-as-in-just.

The issue is self-interest. You have permission in this country to defend your own interests, and to vote accordingly, only if you’re poor, or otherwise disadvantaged. For the prosperous – according to the UK tax code, anyone who makes more than about £42k per year – voting in accordance with self-interest is unseemly, ungenerous, greedy, and mean. Hence Tory-shaming, and Conservative constituents’ leery playing of political cards close to the chest.

This construct is problematic. Democracy functions when all voters back what’s good for them – good for the voters privately, and good (in their view) for the country. So landlords may vote against a party that would bring in rent controls, while tenants may support that party: the voices of all affected are heard. If the issue is tax – and isn’t it always – why should it be shameful to vote to keep more of the money you’ve earned, but noble to vote to appropriate other people’s money (how many Labour voters in last week’s election themselves owned houses worth over £2m?) and award it to yourself, or to the groups with which you personally sympathise? The casting of both ballots is selfish. But the healthy pursuit of self-interest is what makes the democratic world go round, and to expect any sector of the electorate to vote contrary to their own advantage defies human nature.

Public discourse in this country would be more civilised, productive, and robust if the left were less sanctimonious, less smugly certain of the righteousness of their cause, and more sensitive to the fact that everyone doesn’t see things their way – in which case results like last Friday’s might seem a measure less surprising. Conservative supporters might either have the courage of their convictions or, if truly ashamed, revise them, but they should at least refute the proposition that defending your own interests is only acceptable if you’re broke. This election was largely swung by the middle class of middle England, who determined that the economy was in safer hands with the Tories. Others may not agree, but that’s still a reputable position; it shouldn’t be a sordid little secret.

Ed Miliband’s vaunted “hard-working families” was code for “anyone in the basic-rate tax bracket or below”– casting higher-rate taxpayers as undeserving and shiftless. If this political culture would stop demonising doing all right for yourself, we might get more accurate polls.