Firm adherents to the view that the CIA killed JFK or that the 9/11 terror attacks were carried out by Israeli secret agents will not have their minds changed by Voodoo Histories. It isn't the author's fault. David Aaronovitch's exhaustive inquiry kills those and other popular conspiracy theories stone dead. But Aaronovitch is a columnist for the Times. In the cosmography of conspiracy, that makes him a tainted source, sure to be complicit in the great deception. The fact that a review in another establishment newspaper might judge Aaronovitch to be right will only confirm the existence of a mass media stitch-up. In that sense, conspiracism is a kind of faith, a belief in a higher order of truth that transcends evidence. It absorbs rebuttal and reconfigures it as proof of more conspiracy.

While the resolute believers are a lost cause, Voodoo Histories should persuade the casual sceptic. Anyone who has toyed with suspicion over the accidental nature of Princess Diana's death, or the self-inflicted nature of David Kelly's, will have that flirtation brutally curtailed by Aaronovitch's caustic rationality. Scepticism, he reminds his reader, should cut both ways. If it seems implausible that a small band of terrorists should successfully conspire, undetected by the full might of US intelligence, to crash airliners into the World Trade Centre, how much more plausible is the alternative? An official conspiracy would have to recruit thousands of willing (and psychotically immoral) civil servants and intimidate thousands of witnesses into silence.

Then there is the cost. The effort and expense required to keep up the pretence of a fake Apollo mission over 40 years would far exceed the trouble of simply landing a man on the Moon. If the conspiracy theorists are right, there would be a secret clause in every annual US budget authorising hush money for retired Nasa engineers. Aaronovitch demolishes his chosen conspiracies twice over. First, he makes it clear how inconceivable they are compared to the relative likelihood of the official version of events being true. Second, he takes them on their own terms, examining the so-called evidence in web pages, pamphlets and pseudo-science articles and refuting it with better sourced, more reliable evidence. He salvages facts from the wreckage of history submerged in oceans of amateur, bogus revisionism.

But Aaronovitch aims to do more than expose popular nonsense. He wants to account for our collective appetite for conspiracy in historical, social and psychological terms. It is an approach that owes more to scholarship than journalism and, admirable though the diligence is, it sometimes makes for sticky reading.

When Aaronovitch does sprinkle the debunking with humour, it is a kind of acid drollery, barely disguising the author's scorn for mankind's propensity to be stupid. When rubbishing some preposterous Holy Grail story, for example, he cannot resist the atheist sneer that "with sections of the established church still professing to believe in saints, miracles and manifestations, in the liquefying of sacred blood and the transubstantiation of comestible items, such theories seemed no more far-fetched". It sometimes feels as if the working subtitle for Voodoo Histories was "Sometimes I Wonder Why I Even Bother".

But the point about an overlap between sacred doctrine and wacko paranoia is an important one. Aaronovitch tries to sidestep it early on with a taxonomy of conspiracy theory that excludes major religions. But he then quickly gets drawn into major historical controversies alongside pop cultural frippery. The Moscow show trials of the 1930s feel somehow of a different order to the question of whether or not Marilyn Monroe was killed by a secret enema.

Yet Aaronovitch scarcely clocks the difference in scale as he mercilessly purges wrongness. You could almost start to feel sorry for some of the small-time hustlers he exposes peddling their dodgy bootleg history off the back of an internet lorry. Almost, but not quite. Aaronovitch is right to argue that spreading lies about big events is not a victimless crime. At a personal level, most conspiracy theories include terrible defamations. You might not fancy being stuck in a lift with the Duke of Edinburgh, but that doesn't mean he killed his daughter-in-law in cold blood.

More toxic than a bit of tabloid libel is the theorists' implied contention that all politics is remorselessly deceitful; that the order to deceive comes from a hidden source; that nothing happens by accident; that someone is pulling the strings. It is a nihilistic view of public office that sees democracy as a naive delusion and tends towards political extremism. Aaronovitch's research reveals an overlap in patterns of argument and in personnel between the conspiracy-theory community and the far right. Rummage around in the intellectual baggage of many paranoids and, soon enough, you'll find an assertion that the Jews are behind it all.

But the journey from ufology to Mein Kampf isn't inevitable. Most of the conspiracy theorists in Aaronovitch's account are dysfunctional fantasists and hucksters, driven by "glory, money, stupidity". Their obsession with a hidden meaning in events reflects repressed fear that their lives have no meaning. They are made anxious by the untidiness of truth, preferring to believe in a brilliantly malevolent government than in a disorganised, neglectful one. David Aaronovitch, by contrast, is not afraid to stare the truth in the face and call it boring. And while that probably makes Voodoo Histories a necessary book, it doesn't always make it a compelling one.

We in the west, says David Aaronovitch, "are currently going through a period of fashionable conspiracism". But perhaps this is a condition of modernity in general rather than of our own period in particular. In the aftermath of the first world war, for example (to which it would be asinine to compare current frets), conspiracy theories blossomed, expressions of the facts of economic and social volatility.

For these facts it was natural to seek a cause. As Aaronovitch himself says of 1919 in Voodoo Histories: "Everywhere people stared out of the abyss, their certainties and traditions replaced by extreme anxiety and dangerous novelty." Mainly, the cause they fixed on was the Jews, who were suspected of somehow being behind the turbulent new times, plotting and controlling and string-pulling.

One gets a strong sense from this book that it is Aaronovitch's intellectual curiosity about the early phases of that tragic (but also, in his capable hands, sometimes comic) ascription of control that has driven the whole project. For all that, he makes a very good case for a general theory of conspiracy theories. It is his working out of this - a teasing out of argument through the particularities of individual narratives - that makes Voodoo Histories a success, despite some flaws.

The narratives themselves concern the Protocols of the Elders of Zion, Pearl Harbor and its relation to Roosevelt's keenness to enter the second world war, the assassination of JFK, Marilyn Monroe, the death of Princess Diana, and so on. To these long-time staples of the paranoid imagination, he adds the death of the CND activist Hilda Murrell in 1984, the death of Dr David Kelly in 2003, 9/11 and the bloodline-of-Christ myth associated with The Da Vinci Code

It is the fabrications associated with the tumbling of the World Trade Center in 2001 that are the most fascinating. These tend to centre on the idea that the terrorists were dupes of a "parallel network" or "secret team" of state-sponsored intelligence operatives. As Aaronovitch observes, what is interesting about these theories is their holders' "absolute contempt" for the idea that Arabs might have done this on their own. Occidentalism is thus paradoxically reinforced by those nominally opposed to it.

One aspect of conspiracy theories he is good on is how they propagate by relying on previous conspiracy theories as scaffolding. So the attack on Pearl Harbor as a deliberately engineered ploy by Roosevelt to get the US into war is linked with the Bush-Cheney faction's supposed provocation of 9/11 in order to enable war in Iraq. The next stage in this tale - too recent for Aaronovitch's book but all over the internet - could be the Federal Emergency Management Agency's supposed purchase of half a million plastic coffins, which is somehow tied up with concentration camps, an extermination programme and a coming police state in America, as well as now being linked with swine flu.

Aaronovitch rejects the definition that conspiracy theories are simply conspiracies that don't exist. Nor does he see them just as a motive force in historical events, taking over the minds of individuals and sometimes whole events. His own take is more robust - "I think a better definition of a conspiracy theory might be: the attribution of deliberate agency to something that is more likely to be accidental or unintended". But he remains open to the charge that his book is a multiple example of an opposing "contingency theory" - the idea that arguments against conspiracism actually constitute another questionable ideology, one that maintains the status quo by turning a blind eye to the large relations underlying a society. There is no conspiracy; everything is contingent. This inherently conservative idea is not something one could ever accuse Aaronovitch, a former communist, of holding. Even his shift to the right in recent years has involved rubbing a wire brush up against orthodoxy.

Part of contemporary conspiracism he ascribes to purely commercial motives: the proliferation of books, films and television shows alleging secret plots behind the patterns of both everyday life and large events such as 9/11 or the invasion of Iraq. Other reasons centre on certain social needs relating to historical knowledge, education and explanation; Aaronovitch's impressive skill in these areas distinguishes his book, which debunks conspiracy theories, from those that promulgate them.

However, even the most sceptical of readers will experience a strange sense of perverse enjoyment as they are drawn into a spider's web of tales that Aaronovitch narrates only to knock down. Between these dubious pleasures, many acute observations are made. One is that conspiracy theories appeal because they seem to fit reality, indeed to explain it: "belief in the Protocols was not just a prejudice; it was a fully worked out view of how, as the American author Stephen Bronner puts it, 'history operates behind our backs'."

This has been the premise of many popular dramas, such as Edge of Darkness, Defence of the Realm and A Very British Coup in the 80s, all of which Aaronovitch dissects. These narratives and their more recent followers tend to dramatise the secret state as the operator of history, against which a dissident must struggle, often with the media somewhere in the middle. The release of Kevin Macdonald's new film State of Play (itself a remake of a 2003 British TV series) shows how the pattern continues. The fact is, this kind of drama has been around for a long time in Hollywood - it probably goes back to such films as Three Days of the Condor (1975), Chinatown (1974) and, further back, The Manchurian Candidate (1962). We have a need for conspiracy theories, it seems, even when we know better.

At the end of Voodoo Histories (after taking a pop at Spike Lee), Aaronovitch sketches out some possible reasons for that need. They involve the impulse to dramatise and a biological human requirement for causality - and also the notion that we desire such stories to stave off the idea of chaos, even if their actual material is disturbing. And here is the most disturbing one of all: Aaronovitch is not actually a scary lizard who has all along been working on behalf of vested interests from the planet Zog; he is a human being who has written a useful book about an important subject. Amazing, isn't it?

 Giles Foden's new novel, Turbulence, is published by Faber in June