Do the Protocols of the Elders of Zion show that there is a Jewish conspiracy aimed at world domination? Did Franklin D. Roosevelt conspire in the Pearl Harbor attack so that he could take the US into the Second World War? Was Hollywood a nest of communists in the 1940s? Was Marilyn Monroe murdered on the orders of Robert Kennedy? Was John F. Kennedy assassinated by the CIA? Are the Merovingians the descendants of Jesus? Was Robert Kennedy assassinated by the CIA? Was Hilda Murrell murdered by the British Government? Was there a cover-up about Ted Kennedy's car going into the water at Chappaquiddick Island?

Was Diana, Princess of Wasles, murdered on the orders of the Duke of Edinburgh? Was Dr David Kelly murdered on the orders of Tony Blair? Did Bush and Cheney arrange for the 9/11 twin towers attack so that they could start a war in the Middle East?

Yes, say the conspiracy theorists to all these questions, in hundreds of lucrative books weaving tangled webs of coincidence, speculation and fantasy.

Indeed, conspiracy theories are commonplace, to the extent that almost any occurrence of significance in the world rapidly comes to be viewed as the work of hidden agencies machinating away behind the scenes to make things appear other than they really and sinisterly are.

David Aaronovitch sets himself the task in this sensible and absorbing book of examining some of the main conspiracy theories of the past 100 years, and offering an analysis of how they work and why they exist. In the course of debunking them he recounts some of the major events of recent history, from the springs of anti-Semitism in the interwar years to the 9/11, Madrid and 7/7 terrorist atrocities. The result is a rich and fascinating account, and the unpicking of the conspiracy theories themselves is especially instructive - and entertaining, for even though Aaronovitch manages to restrain his exasperation with the confectors of the theories, his wry sense of humour gets the better of him at times, further leavening an already tasty loaf.

Most of the conspiracy theories discussed are familiar in outline, as are the official versions of the same events. His unravelling of the theories is a model of common sense and responsible reasoning.

A big part of the book's value is its reflection on the reason why such “theories” are concocted and believed. Trying to understand this matters not least because most of their originators are, as Aaronovitch points out, educated people in other respects perfectly capable of being sensible. They include writers, professors, MPs, senators, television producers and millionaire businessmen. What is it, he asks, that makes such people believe “that the British Royal Family executes its more awkward members, that Robert Kennedy had a poisoned suppository inserted into Marilyn Monroe before [himself] being assassinated by a Manchurian Candidate, or that the Roman Catholic Church has for two millennia been suppressing the truth about the secret bloodline of Christ?” And what is it that explains how such otherwise intelligent and informed people could believe that Bush, Cheney, the CIA, the FBI, and the oil and arms industries, all together and jointly, in a massive conspiracy, arranged for the 9/11 attacks on their own fellow citizens, risking even greater murder and mayhem in Manhattan and Washington than was achieved - and that was murder and mayhem enough?

Aaronovitch's answer is that it is not enough to cite a sense of powerlessness and disenfranchisement on the part of conspiracists, trying to wrest a little control by blaming dark agents for what is going wrong in their lives and societies. It is not enough to blame alleged scarcity or contradictoriness of information about these events, leaving “unexplained holes” in the accounts that trouble onlookers and make them suspect that something more is implicated.

Such considerations play their part. But Aaronovitch finds persuasive the further possibility that conspiracy theories are the outward expression of inner problems, as suggested by Elaine Showalter's study of mass hysterias in modern culture with its rapid communication capacity, allowing people in search of explanations for their difficulties to find them in a fatigue-syndrome virus, a Gulf War toxin, a repressed memory of childhood sexual molestation, a plausible-seeming account of criminality and manipulation by dark organisations.

The power of these alternative “explanations” is that they offer narrative accounts that people can share; the discovery that others in the blogosphere have the same doubts or experiences as oneself is a powerful propagator of beliefs.

Moreover, the accounts differ across gender and time: 80 per cent of chronic fatigue syndrome sufferers are women, as are 90 per cent of repressed-memory recoverers. Aaronovitch hypothesises that conspiracy theories are “hysterias for men”. Add to this that the paranoia involved in the belief that bad people are conspiring to dupe, manipulate and harm us may paradoxically be a salve to a deeper wound, namely that we do not matter and that no one cares whether we exist or not, and the psychological mixture at work is even richer. This is a suggestion that Aaronovitch finds in the work of the psychoanalyst Stephen Grosz: paranoia as a defence against feelings of worthlessness.

Doubtless there is a number of co-operating reasons for the origination and acceptance of conspiracy theories. Whatever they are, Aaronovitch's concluding point is an important one: that conspiracy theories do harm, and can have dangerous effects on policy and international affairs, as illustrated by the Nazi determination to re-arm because of beliefs about alleged JewishBolshevik plans to take over the planet. For that reason it matters that “conspiracism”, as Aaronovitch calls it, should be combated with good sense and sound reasoning. He provides an excellent example of both here.

Voodoo Histories: The Role of the Conspiracy Theory in Shaping Modern History by David Aaronovitch
Jonathan Cape, £17.99; 368pp Buy the book

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