Ruefully, and rightly as it turned out, one lifelong investigator of the Kennedy assassination predicted that there “won’t be any smoking gun” in the cache of nearly 3,000 JFK-related documents released late on Thursday night. It was a suitably ironic choice of phrase by Jefferson Morley, the editor of the JFKfacts website. Because this, of course, is a rare case where there was very much both smoke and a gun, in the form of the 6.5mm Carcano rifle used by Lee Harvey Oswald to shoot John F Kennedy on 22 November 1963. It’s just not the smoking gun Morley’s readers were looking for, the one that would prove a vast, hidden conspiracy to murder the 35th president of the United States.

There were plenty of juicy titbits in the papers all the same. Conspiracy theorists will seize on the CIA memo that reports that Oswald, while in Mexico in September 1963, spoke to a Russian diplomat identified as a KGB officer and member of Department 13, a unit “responsible for sabotage and assassination”. Others will delight in the ambiguous words of the FBI director, J Edgar Hoover, who two days after the killing wrote of the urgent need to “convince the public that Oswald is the real assassin”. My personal favourite is the mysterious phone call to a British local paper – the Cambridge Evening News – 25 minutes before the shot rang out in Dallas, instructing a reporter to call the US embassy in London to hear “some big news”.

All of which is intriguing, but not what the conspiracy-mongers wanted to hear. They hoped this windfall of papers might include the document that would prove, once and for all, what they’ve said all along: that JFK was killed by the Russians, the mafia, the Cubans, the other Cubans, the FBI, the CIA, or some combination of all of them. What they never feared, by the way, was a document that might point in the other direction, a text that would settle beyond all doubt that Oswald alone was the murderer. Such a document could not exist, for it is in the nature of conspiracy theory that any contrary evidence can be brushed aside. An incongruent letter or photograph can and will be dismissed as a fake, forged by the hidden conspirators and therefore providing further proof of their wickedness, ingenuity and willingness to stop at nothing.

The temptation is great to laugh off such thinking as the province of harmless potting-shed eccentrics, green-ink cranks whose tightly spaced letters could once safely be filed in the dustbin. But conspiracism – of which the JFK industry was the early exemplar – matters. It has become a defining, and dangerous, feature of the world we live in now.

Of course, there were conspiracy theories before the murder in Dealey Plaza. But JFK provided the template for the modern-day version of the form. Whether it’s the microscopic analysis of photographs and film footage, or the casual assumption that governments could enlist many hundreds, if not thousands, of officials to collaborate in a murderous lie, all of them taking their secret to the grave, the pattern was set with the Kennedy assassination – redeployed to cast doubt on everything from 9/11 to the death of Princess Diana, from the shooting of very young children at Sandy Hook to this month’s mass slaughter in Las Vegas.

Lee Harvey Oswald talks to the media as he is led down a corridor of the Dallas police station for another round of questioning. Photograph: AP

Again, there was a time when you might have dismissed such talk as the derangement of the bug-eyed, irrelevant fringe, but that’s no longer so easy. Not now that we live in the era of fake news and post-truth. Those terms are used to describe a trend that is pervasive and indeed mainstream, that in the US has captured the highest office in the land. The man who sits in Kennedy’s chair today is himself a conspiracy theorist, responsible for spreading one of the most malicious inventions: the wholly debunked claim that Barack Obama was not born in the US.

Superficially, conspiracy theory and post-truth might look different. The conspiracists insist that they are bent on uncovering the real truth, while post-truthers shrug their shoulders, suggesting such truth doesn’t really exist, can never be known and doesn’t really matter anyway. But what both have in common is an indifference to facts and evidence. The conspiracist brushes off inconvenient facts as bogus, while the post-truther says they have “alternative facts” of their own. But both end up in the same place, dismissing the hard, established evidence that is the basis of reason.

They share too a supposedly defiant attitude to the establishment and the powers that be, casting themselves as heroic truth-tellers who have broken free of the credulous herd – lions among sheeple. They have favoured targets in common too. Both the JFK obsessives and a post-truther such as Donald Trump – who, let us not forget, offered his own addition to the Kennedy conspiracy canon with an evidence-free claim that the father of his Republican primary opponent, Ted Cruz, was involved in the assassination – perennially cast the FBI and the CIA as the key tools of dark, unseen forces.

This soon descends into a cynicism about democracy itself. In this view, elections are a sham; politicians are mere puppets; the real masters are hidden and lurk in the shadows, pulling the strings. Which is why, incidentally, so many conspiracy theorists, like so many post-truth merchants of the populist hard right, end up reaching the terminus of antisemitism. For antisemitism is itself often rooted in conspiracy theory: the belief that the secret hand behind world events, manipulating each and every development, belongs to the Rothschilds or George Soros or, when no euphemism is required, the Jews.

Some of the best analysis of conspiracy theory and its enduring, apparently increasing, appeal strikes an unexpectedly compassionate tone. It argues that the imagined conspiracy is a necessary refuge for those unable to face the world as it truly is: messy, arbitrary and full of pain. David Aaronovitch, whose Voodoo Histories debunks the key conspiracy myths of our time, writes that many Americans simply could not accept that a giant like Kennedy had been taken from them by “a little man” like Oswald: “Oswald represents chaos, the rule of nothing, the indifference of fate. Real Oswald is more intolerable than the notion of the establishment plot.”

But conspiracy thinking is no longer harmless idiosyncrasy, if it ever was. Not when it leads to the bereaved parents of Sandy Hook or the wounded of Las Vegas being bombarded with death threats and online abuse, branding them “crisis actors” paid by the government to help stage a hoax. But there is a deeper danger too. All this energy spent trying to find the hidden hands that secretly plot our destruction is energy not spent looking for the truly hidden hand – which does not belong to one shadowy individual, or even a group, but rather to the much more complicated forces of politics, economics and history that are shaping us every day.

On Thursday we learned that 1,500 billionaires have now amassed $6tn of wealth, a level of inequality not seen since the Gilded Age. That’s not come about because of a secret meeting in an underground boardroom, but because of a system that is fatally flawed. Tackling that system may lack the satisfying clarity of exposing an evil government agency or clandestine cabal, ready to murder its own fellow citizens or assassinate a youthful, golden president. But it would get us closer to the truth.