Britain goes to the polls next week and the Labour leader, Ed Miliband, is clear about what the choice is.

"For too long, you have been told something that simply isn't true," Miliband assured supporters as he announced his program. "That's what's good for the richest and most powerful is always good for the whole of our country. That as long as a few individuals and companies are OK, we can just wait for the wealth to trickle down to everyone else."

Really? We've been told that? By whom? Who has spouted such bilge? Type "trickle down economics" into Google and it'll prompt you with "myth", "criticism", "debunked" and "doesn't work." But you'll search in vain for anyone actually proposing the idea.

Not that this deters leftist politicians, election after election, from tearing into it. Here, for example, is Barack Obama in 2008: "We can't afford four more years of the theory that says we should give more and more to those with the most and hope that prosperity trickles down to everyone else."

It's tempting to dismiss this as political knockabout, the kind of overheated propaganda that elections seem to bring out in even the calmest politicians. But lefties really do believe this stuff. Ask your liberal friends — or, if you're really brave, ask Twitter — to name a single economist or politician who has asserted that the best way to stimulate an economy is to give rich people more to spend on Lamborghinis. You'll simply be told, as though it were self-evident, that such a belief underpinned the Thatcher and Reagan governments.

A couple of years ago, Thomas Sowell tracked down the origin of the phrase "trickle down". It turns out to have been coined by FDR's speech writer, Samuel Rosenman, who attacked "the philosophy that had prevailed in Washington since 1921, that the object of government was to provide prosperity for those who lived and worked at the top of the economic pyramid, in the belief that prosperity would trickle down to the bottom of the heap and benefit all."

Then, as now, the notion was a left-wing fantasy. What actually happened in the Twenties under Calvin Coolidge, as Amity Schlaes showed in her biography, was that tax rates were cut in order to boost revenue. The taciturn old congregationalist had grasped the logic of the Laffer Curve avant la lettre. As a result of his rate reductions, the wealthiest Americans ended up contributing far more in both absolute and proportionate terms.

We free-marketeers believe, not in trickle-down, but in trickle-up. Capitalism, uniquely, rewards people who sell to the mass market. I am typing these words on a program that I bought from Bill Gates. My purchase enriched him, adding fractionally to his net wealth; and it also enriched me, making my life more convenient. Like most successful people, Bill Gates became rich by persuading lots of poorer people to buy something from him; in consequence, we are all better off.

Try running that argument past your liberal friends, though. The chances are that even the most open-minded among them will give you a slightly glazed look. Even if they grasp the idea, they will soon forget it and go back to telling you that "everyone knows" that conservatism is all about the defense of privilege and oligarchy. This is because, to a greater degree than most of us care to admit, our political leanings are expressions of our character traits, and are not dependent on empirical data.

If you start from the conviction that you're standing up for the underdog, you will naturally assume that your political opponents are for the powerful. You will subliminally screen out evidence that challenges that view. As Danusha Goska put it in American Thinker not long ago, "Never, in all my years of leftist activism, did I ever hear anyone articulate accurately the position of anyone to our right. In fact, I did not even know those positions when I was a leftist."

Do rightists also caricature their opponents? Yes, but not to anything like the same extent. A 2012 study by Jesse Graham, Brian A. Nosek and Jonathan Haidt asked conservatives and liberals to answer a series of questions as themselves, and then to answer them in the imagined personae of a typical conservative and a typical liberal. It found that the liberals were the least able to accurately to guess their opponents' views, seeing conservatism as a kind of moral failure.

They're not playing to the gallery. They're not sloganizing. They genuinely believe that we conservatives went into politics because we hate the poor. Their conviction is so solid that facts crash against it as waves against a rock. Again and again, leftists will line up behind policies that hurt the general population in order to reward vested interests: the bailouts and the TARP boondoggle, the alternative energy scams, the trade barriers that favor privileged industries, the regulations that drive up prices. The practical consequences of these policies matter far less to their advocates than the opportunity to signal their good intentions. And if their intentions are good, ours must be bad. Psychologists call it "self-serving bias".

In all the years I've been in politics, I'm not sure I've shaken a single socialist out of the "you conservatives hate poor people" shtick. The only way to answer, I've found, is to say: "Yup, you're right: we want to turn them into rich people."

Dan Hannan is a British Conservative MEP.