Voodoo Histories, By David Aaronovitch

Why do conspiracy theories gain currency, and just how pernicious can they be?

Most students of history know that there is no such thing as the future; there are futures, plural, and the historian's task is to examine yesterday, compare it with today, and thence arrive at a plausible description of tomorrow. Concealed within this approach is the recognition that, contrary to what much of Western society believes, there is no purpose to history. Events aren't always determined, in the broader sense of that term. Stuff just happens. Life is random and unfair and nobody controls their destiny entirely. Not only might you get hit by a bus tomorrow; it could actually be that unfashionable thing: an accident.

This prospect is too much, not just for some people, but most of us, which goes some way to explaining the enduring force of religious belief in societies increasingly dependent on scientific knowledge. The yearning for meaning overwhelms us, and so we invent relations between events that are not, in fact, connected.

This, at least, is the approach of the conspiracy theorist. Conspiracy theories have been a staple of Western culture for centuries – the circumstances of Christ's death, never mind Marlowe's, come to mind – but it's true that in the 20th century they flourished as never before. Now, in the age of dispersed information and the internet, their prominence is swelling yet further.

Chief among the several merits of David Aaronovitch's forensically argued book, subtitled "The Role of the Conspiracy Theory in Shaping Modern History", is the fact that his source material is so riveting. Conspiracy theorists don't go in for the mundane. The idea of alternate histories, of plausible counterweights to the official version of events, is intrinsically gripping.

The trouble is, the truth remains true even when boring. Aaronovitch goes through a catalogue of conspiracy theories and – conscious of being a killjoy – debunks them at premise, midpoint and conclusion.

Take the murder of President Kennedy on 22 November 1963. Conspiracy theorists ask how Lee Harvey Oswald could have fired two out of three shots on target in 5.6 seconds. They look at Kennedy's controversial stances on Cuba and Vietnam, and the alleged discord between the CIA and Oval Office. They note that Robert Kennedy, another handsome young Democrat, was assassinated too. They consider it peculiar that Oswald died before making it to court. It must stack up.

Except for the boring truth. In fact, Oswald had 7.1 seconds (not 5.6) to strike a target moving along his telescopic line of vision, not across it. And he was a practised shot, having tried seven months earlier to assassinate Major General Edwin A Walker, a prominent public figure, with the very same rifle. These facts alone destroy two of the central tenets of the industry that blossomed after JFK's death.

Other conspiracy theories similarly rub up against tedious practical objections. The moon landings can't have been faked, because not a single person out of the thousands necessarily conscripted into life-long secrecy has broken their code of omertà. Diana wasn't carrying a Muslim foetus, and her driver was drunk. The second President Bush really wouldn't have formed a pact with the Taliban to provide a pretext for the capture of Iraqi oil, via terrorist attacks on 11 September. The second President Roosevelt similarly didn't orchestrate Pearl Harbor, and Robert Kennedy didn't have "a poisoned suppository inserted into Marilyn Monroe before being assassinated by a Manchurian candidate".

That last example is a fairly harmless one. But when the terrorists of Hamas collude with Nazism to insert passages from the (fabricated) Protocols of the Elders of Zion into their covenant, verbatim, or Stalinists justify the torture and execution of Trotskyites during the Moscow Trials of the 1930s by reference to socialist utopia, conspiracy theories become very far from innocuous.

Aaronovitch ought to have had the courage to admit that, as a champion of the most recent Iraq War, he too fell for a conspiracy, in relation to weapons of mass destruction. And though, in his marvellous introduction, he shows how often conspiracy theorists pose as champions of the oppressed – history written by the silent losers, if you like – he doesn't give enough credit to the uncovering of pernicious conspiracies that in fact took place, such as the Vatican's concealment of the rape of children.

Nevertheless, he has performed an admirable public service. It turns out that history doesn't happen for a reason. Destiny is fantasy. Meaningless events might make us sad, but c'est la vie. We cannot prepare for tomorrow if, by dint of paranoia, we remake yesterday as an illusion.