" Post-truth politics," "alternative facts," "fake news": how disdainfully the phrases drop from leftist lips. As the liberal writer Wes Williams puts it, "Right-wing media use 'alternative facts' on an almost daily basis."

In fact, all human beings subconsciously seek out data that sustain their prejudices. We see what we want to see, even in the most literal sense: In tests where words are slowly illuminated on a screen, we see the nice words before the ugly ones.

Think of any contentious situation — for example, a white police officer shooting an unarmed black man. Most people's responses will be subliminally influenced by what they expect. If they start from the proposition that cops are good people doing a hard job, they will likely conclude that the case is one of justifiable self-defense, or at the very least of excusable error. If their assumption is that the police prop up a racist system, they will probably think the cop was guilty.

Either way, they won't be aware of their partiality. In their own minds, they will have drawn the only reasonable inference from the facts. They will therefore believe that those who don't see what they do, despite being presented with identical evidence, are either fools or liars.

Both sides think they are deploying cold logic; neither is aware that it is being selective. The people who fume about the way their opponents deny "the scientific consensus" on global warming are themselves often prepared to defy "the scientific consensus" when it comes to, say, the heritability of IQ.

Consider the example of wealth distribution. Global poverty has fallen by two-thirds over the past three decades, and global inequality is also dropping. Most leftists, though, refuse to believe it — and, in fairness, not only leftists. Two-thirds of Americans believe that global poverty has risen over the past 30 years. This is partly a response to seeing charity appeals, reports of disasters and so on. But it is also a classic case of what liberals call "post-truth politics."

When confronted by hard data showing that incomes in developing countries are rising, that malnutrition is being eliminated, that disease is becoming rarer, that infant mortality is at an all-time low, people will often respond by saying "Tell that to X" — X being, say, "a Syrian refugee" or someone else currently in the news.

The belief that poverty is worsening is so intrinsic to leftists' worldview that they will cast around for any data, however flimsy, that prop up their prejudices. In the U.K., every measure shows a decline in poverty — both in absolute terms and in the relative terms that leftists generally favor. (The poverty lobby in Britain succeeded in getting the government officially to define a household as "poor" if its income is less than 60 percent of median household income. With pleasing karma, this the definition of poverty is now registering the steepest drop of all.)

Yet, as I type these words, I am looking at a front page headline in The Guardian, our main Left-wing newspaper, proclaiming that "Poverty Kills Children." Well, OK, it's still true that a child born to wealthy parents has better health prospects; but you'd never guess from the article that the gap is closing.

With every official metric showing poverty in decline, leftists point desperately to the increasing number of food-banks. Food banks are becoming more common for the same reason that smart-phones are: they didn't exist 20 years ago. They are not a measure of anything.

To repeat, we are all subject to self-serving biases, conservatives as well as liberals. What is peculiar to the Left is the self-righteousness, the determination to sacralize every argument: "Tell that to the people lining up at a food bank!"

There are plenty of instances of "fake news" that leftists eagerly promulgate. After the Brexit referendum, for example, all sorts of bogus stories about attacks on foreigners whizzed around social media, and a ludicrous figure about a 58 percent "rise in hate crimes" assumed almost canonical force. Something similar happened following the election of Donald Trump, with at least one Muslim "victim" later admitting she had made up a story about being harassed. But try questioning these stories at the time and see what rage you bring down on yourself.

No one has a monopoly on objectivity; all human beings are subject to what psychologists call "self-serving bias." So please, liberals, take a look in the mirror before you lecture the rest of us.

Dan Hannan is a British Conservative MEP.