"Trump wears deception as a foreskin." Thus Nathan, a small business owner, talking to me in Chicago the other day. I couldn't have put it better myself. A recent analysis concluded that Donald Trump averages one lie or inaccuracy for every five minutes of speaking. There's a great debate raging here about how the American media should cover this narcissistic, bragging, mendacious, ignorant, dangerous demagogue. But what's happening to the media themselves is part of the problem.

The journalistic consensus seems to be that television anchors and news reporters should call him out on his serial whoppers, as NBC's Lester Holt did when moderating last month's debate, and not preserve a false balance between two candidates of such disparate quality and seriousness. To do that would be to fall prey to what the media commentator, Brooke Gladstone, slyly calls Fairness Bias: "Ah yes, Professor Smith, thank you for arguing that the earth is round, and now let me give equal time and ostensible respect to Mr Jones, who argues that the earth is flat." If you want a good recent example of fairness bias, just recall the coverage of the Brexit referendum campaign by our timid and intimidated BBC.

Interestingly, even that The New York Times - not for nothing known as the "grey lady" - has abandoned her usual strict evenhandedness and spinsterly circumspection. It's not just that hardly a day goes by without there being not just one but two or three comment pieces excoriating Trump. It's also that the news coverage, as well as offering excellent long investigative pieces revealing Trump's past as a businessman, charlatan and bigot, now slips in pejorative adjectives, adverbs and phrases that would previously have had the grey lady tut-tutting from behind her teacup.

Now I can totally understand why The New York Times has departed from its usual practice. As it argued in an editorial, Trump is "the worst nominee put forward by a major party in modern American history". He is a threat to civil peace at home and the country's standing abroad. An Italian friend compares it to La Repubblica's reaction when faced with the resistible rise of Silvio Berlusconi.

Unfortunately, this taking sides may reinforce a structural trend that is itself corrosive of American democracy. The most characteristic American argument for free speech and for what we still anachronistically call a free "press" - as explicitly mentioned in the First Amendment - is that this is necessary for democratic self-government. Only if citizens can hear all the relevant arguments and evidence, as ancient Athenians did when they gathered on the Pnyx at the foot of the Acropolis, will they be able to make an informed choice and therefore meaningfully be said to be governing themselves: first voice, then vote. So, you have to hear the arguments and evidence from both sides.

But in this respect, the television duel between the two candidates was the exception that proves the rule: a brief moment of shared experience in the public square. The rest of the time, American voters are off in their own echo chambers, hearing views that reinforce their own. This echo chamber effect was first noticed on the internet, where people identified the "information cocoon" and "filter bubble", but this is now a defining feature of the entire media landscape, not just online and not only in the United States of America. There is both a free-market profusion of sources of news and views, and a corresponding fragmentation. Trump voters get theirs from Fox News, right-wing radio talk shows, internet sources like Breitbart (whose supremo Trump now employs as a key campaign adviser) and their Facebook friends; Clinton voters get theirs from MSNBC, NPR, PBS, internet sources like Slate or HuffPost, like-minded people on social media - and that now clearly anti-Trump organ, The New York Times.

Since the internet has demolished the traditional business model of newspapers, while allowing a fantastic profusion of sources, everyone ( The Guardian included) is competing furiously for eyeballs and mouse clicks in this crowded, hectic, 24/7 arena - the virtual equivalent of a traditional stock exchange trading floor or an Indian street market. Shout, shout, SHOUT. If it bleeds it leads, if it roars, it scores. Nuanced, balanced, evidence-based reporting and analysis, such as your humble servant still attempts to purvey, has difficulty making itself heard amid the hubbub. Technological possibilities, commercial imperatives and perhaps also cultural changes combine to turn deliberative democracy into infotainment.

Reality TV trumps reality - and "trumps" is the operative word. Like Berlusconi, the showbusinessman and former reality TV star, Trump, is both a product and a creator of this brave new world. He is the Jerry Springer of American democracy. Besides the so-called alt-right we now have alt-reality.

In alt-reality, facts, evidence and expert opinion give way to myths, wild exaggerations, lies and powerful simplistic narratives: Trump's "make America great again", the Brexiteers' "take back control". Historians of propaganda know that lies prevail by sheer mind-numbing repetition, crowding out the truth. The 24/7 multi-platform echo chambers of partisan media and prejudice-reinforcing social media have a similar effect.

I once had the hilarious experience of defending a book of mine called Facts Are Subversive on the satirical Colbert Report. What do you mean? cried Stephen Colbert, I don't want facts to subvert me and make me feel uncomfortable, I want things to make me feel good! He famously invented the term "truthiness" to describe this comforting alternative to truth. Well, it's a good thing that Colbert has moved on to another comedy show, because in the meantime real life has overtaken his satire. Trump is the grand master of truthiness. Although he has now abandoned the birther lie, ludicrously blaming it on Hillary, one of his comments after Obama had made public his birth certificate perfectly exemplifies his truthiness. Trump often covers himself by saying "a lot of people think", but this comment in an interview went one further, from think to feel. It said, "A lot of people feel it wasn't a proper certificate." And you know what, I feel that the earth is flat.

Both candidates actually alluded to this separating out into rival echo chambers in Monday's televised debate. Hillary got off her well-rehearsed line "Donald, I know you live in your own reality." The Donald's line was less practised, unintentionally funnier, and more revealing: "I think the best person in her campaign is mainstream media." The sentence is characteristic of so much populist rhetoric across the world, from the US to France and Poland to India, suggesting at one and the same time that your supporters are an embattled group being oppressed by powerful liberal elites, and that they alone are the "real people" (a phrase often used by UKIP's Nigel Farage). The distortion is worse on the populist right, but we must recognise that tendentious media polarization, simplistic shouting and echo chambers are a problem on all sides. Although the US has free, uncensored, diverse media, these are less and less offering the shared public square we need for deliberative democracy. A noble American cliché invites us to believe in the "marketplace of ideas". What we are witnessing in this election is a market failure in the marketplace of ideas.

The author is professor of European Studies at Oxford University, and a senior fellow at the Hoover Institution, Stanford University. His new book, Free Speech: Ten Principles for a Connected World, was published recently