John Rentoul, the political commentator, and I have a friendly competition to find ever more outrageous examples of a genre of newspaper commentary that he calls "Questions to Which the Answer is No". This is a headline that floats a bogus and unsupported theory by posing it as a question. My favourite is a (genuine) Daily Mail headline asking: "Has Marilyn Monroe been reincarnated as a shop assistant called Chris?"

But our joke has fallen flat owing to an example that violates all bounds of decency. Amid the harrowing coverage of the murder of 26 children at Newtown, I found an article entitled: "Did Mossad death squads slaughter American children at Sandy Hook?"

The argument is monstrous, calumnious, demented bilge and comes from no reputable news outlet. It was published by Press TV, the English-language arm of Iranian state propaganda, and a website called Veterans Today. Despite its name, Veterans Today has nothing to do with the welfare of retired servicemen: it promotes conspiracy theories, including Holocaust denial.

The author is Jim Fetzer, a leading figure in the 9/11 Truth Movement, which maintains that the destruction of the Twin Towers in 2001 was perpetrated by the US Government. I cite Fetzer not for any merits of his argument (there is none) but because he illustrates a thesis I now regard as proved. Conspiracy theories are inherently, and not merely incidentally, a threat to the Jews.

Conspiracies do happen. Watergate, the Suez invasion and (in Israel) the Lavon affair show that even democratic governments have unavailingly resorted to them. But a conspiracy theory argues more than that truism. It explains history with reference to an overarching plot by powerful interests, and disposes of every piece of countervailing data by positing one more twist in the supposed conspiracy.

They alight on small items of discrepant data

This is where the danger, and not only the intellectual disrepute, of conspiracy theories lies. If their exponents need to keep expanding the imagined conspiracy in order to explain inconvenient facts, they will sooner or later (and usually sooner) turn to a highly familiar staple of such notions. They will blame the Jews.

That depraved mode of reasoning characterises the 9/11 Truthers. Like all conspiracy theorists, they alight on small items of discrepant data that inevitably arise amid a complex set of events. They then draw the wholly unwarranted and implausible inference that the discrepancies point to an official conspiracy. That in turn prompts the question of the identity of the purported conspirators.

The baseless libel that the Jewish state shoots American schoolchildren belongs in a tradition that has done catastrophic harm. Norman Cohn, the historian, wrote a seminal work in 1967 on the Tsarist antisemitic fabrication The Protocols of the Elders of the Zion. He called it Warrant for Genocide, for it fuelled Nazi fantasies of global Jewish conspiracy.

It's obvious that a malevolent conspiracy theory such as Holocaust denial draws from the wellspring of antisemitism. It's not quite so obvious that ostensibly apolitical conspiracy theories do likewise. But they do.

Consider the proposition, dramatised in Roland Emmerich's film Anonymous, that the works of Shakespeare were in reality written by the Earl of Oxford. The originator of this hoary conspiracy theory of the 1920s decried democracy. Its most prominent recent advocate, Joseph Sobran, was a notorious antisemite who addressed a Holocaust denial conference in 2002.

In the fantastical world of the 9/11 conspiracy theorists, it is again the Jews who are behind momentous world events. It is nonsense, of course; but it is worse than that.

Oliver Kamm is a leader writer for The Times