The "marketplace of ideas" is failing. That is the message of Stefan Stern's complaint that, in this election even more than others, lies seem to be winning.

Of course, Stefan is right to say that politicians have always been economical with the actualite. But there is, surely, a big difference between Johnson and, say, Thatcher. Whilst we lefties hated much of what Thatcher did, we only rarely thought she was lying. Being a liar was the least of Thatcher's sins. But it is the essence of Johnson's identity.

This was not supposed to happen. Liberals used to think that the marketplace of ideas was like other markets - that good products would drive out bad, that, as Mill said, "wrong opinions and practices gradually yield to fact and argument.

But they don't. A big reason for this is that, just as ordinary markets can fail, so too does the market in ideas. And it does so for reasons we can also see sometimes when other markets fail. Here are five such reasons.

1. Monopoly. The problem here is not merely that the press is owned disproportionately by billionaires. It is also that almost all prominent journalists (though definitely not junior ones) will be hit by Labour's tax rises, which surely imparts a bias, or at least reinforces one. And then there's the fact that incentives to be first with a story leads to an over-reliance upon misleading (often Tory) sources: Laura Kuenssberg's libelling of a Labour activist yesterday fitted the pattern described by Peter Oborne. On top of this, there's what Musa Okwonga calls "the innate bias towards parties in power."

Perhaps a bigger problem is that journalists, like all professionals, have their perspective distorted by their training, and indeed are selected to have such distortions. What's especially damaging here is that they like the "theatre of politics" - the "knockabout", the "who's up and who's down."

This doesn't just cause them to regard the Tories utter mess of Brexit as being drama rather than chaos. It also causes them to underweight two other perspectives.

One is these is that politics isn't a game, but a matter of life and death - austerity, poverty and benefit sanctions kill. The other is that regarding politics as theatre leads journalists to think of even moderately tricky issues as - in John Humphrys' words - as "all getting a wee bit technical and I'm sure people are fed up to the back teeth of all this talk of stuff most of us don't clearly understand." The apotheosis of this is the oft-repeated claim that Labour Brexit policy is unclear because it cannot be explained in a single word.

Of course, all of us have partial points of view. The market is ideas is healthy when these points of view collide. It becomes imperfect when one such partial point of view acquires a near-monopoly position. And this has become the case with political journalists.

2. Inadequate incentives. In theory, people should have good incentives to find best value in the market, because they'll waste money if they don't. Even in ordinary everyday markets, such incentives often aren't sufficient. People waste money even on food shopping - and don't get me started on fund management. This is especially true in the market for ideas. Because our individual vote doesn't affect the outcome of elections, people have no incentive to get smart about politics. They are rationally ignorant. This is an environment in which lies can thrive.

3. Unpriced externalities. One reason why we lack incentives to know about politics is that the cost of bad decisions are borne not just by ourselves but by everybody. As Jason Brennan has said:

When a democratic majority picks a policy, this is not akin to you picking a sandwich from a menu. When the majority chooses, it chooses not only for itself, but for dissenting voters, children, foreigners, nonvoters and others who have no choice but to bear the consequence.

In this sense, a bad vote is like pollution. It imposes a cost upon everybody. And if people don't have to pay the cost for such externalities, we'll get too many of them.

4. Positive feedback. Conventional markets work best when there is negative feedback - when an unwarranted price increase leads to lower demand, thereby discouraging such increases. In the marketplace for ideas, however, we sometimes see the opposite - positive feedback. Barefaced lies do not lead to their teller being disgraced. Instead, they can successfully manipulate the agenda - for example the infamous £350m on the side of the bus encouraged people to think about the cost of EU membership. And they can sometimes stick in the memory. As Tim Harford has written:

Repeating a false claim, even in the context of debunking that claim, can make it stick. The myth-busting seems to work but then our memories fade and we remember only the myth. The myth, after all, was the thing that kept being repeated. In trying to dispel the falsehood, the endless rebuttals simply make the enchantment stronger...Facts, it seems, are toothless. Trying to refute a bold, memorable lie with a fiddly set of facts can often serve to reinforce the myth.

In this way, the market does not drive out lies, but actually encourages them.

5. Arms races. This is true in another way. When one party gets away with lying, its opponents also resort to lies, in the same way that an athlete competing against a drugs cheat might take drugs himself simply to avoid being disadvantaged. This leads to an arms race in which both sides lie more and more. Labour is sometimes accused of lying, for example by claiming the Tories will sell off the NHS or that they will save families £6700 a year. But its accusers fail to ask: might they be lying because they've learned that it pays to do so?

My point here is a simple one. There are big market failures in the market for ideas - bigger, perhaps, than in most markets. Ordinarily, such market failures would constitute a strong case for government intervention to mitigate them. Except that in this case, the government actually benefits from these failures. It is, therefore, difficult to see how political discourse can ever become worthwhile again.