Joan Smith: Was Jesus an ice-skating champion? Read on ...

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Here's a confession: I've never read The Da Vinci Code, although I feel as though I have. It's one of those books that follows you around, screaming its existence in a way that really seems quite impolite. My theory is that the more books you read - two or three a week in my case - the less likely you are to have got to the end of Brown's thriller.

It involves Opus Dei and the Catholic Church supposedly trying to cover up the fact that Jesus married Mary Magdalene and had children. This seems to me to fall firmly into the "so what?" category. I'd like to think it reflects a lingering anti-Clericalism, traceable back to the French Revolution, which certainly seems to be a factor in the Catholic Church's hostile reaction to the movie version, which opens tomorrow.

I suspect, though, that it has more to do with a longing for novelty and a general suspicion that "they" are lying to us, whether they happen to be priests, government ministers or members of the royal family. And they are, naturally, still orchestrating a massive cover-up of the murder of Diana, Princess of Wales.

Whenever I hear anyone put forward this thesis, I question them closely as to how the chief suspects, usually identified as British intelligence acting in concert with the Duke of Edinburgh, could have been certain of securing the desired outcome through so dubious a method; if you really wanted to bump someone off, you'd hardly rely on her getting into a car with a drunk driver and failing to wear a seatbelt. But there is always an unexplained element, the white Fiat Uno on this occasion, to bolster the most outlandish conspiracy theory; according to opinion polls, astonishing numbers of people believe some version of this nonsense. Next year, as we approach the tenth anniversary of the Princess's death, there will no doubt be a flood of books and articles claiming she was assassinated to prevent her marrying a Muslim. If such stuff is to your taste, you can already read it weekly in that chief purveyor of Diana conspiracies, the Daily Express.

Curiously, this has had little effect on attitudes to the royal family, in much the same way as The Da Vinci Code - which has, after all, been on the bestseller lists for years - barely impinged on the global popularity of the late Pope John Paul II. In that sense, conspiracy theories are clearly recreational, a bit like the baroque mourning that followed Diana's death. They exist in a parallel universe where they do not connect with people's everyday lives, except, of course, in the case of those Americans unlucky enough to have been abducted by aliens for the purpose of bizarre sexual experiments.

I wish I could say the same about that other fertile subject for conspiracy theories: the terrorist attacks on the East Coast of the US on 11 September, 2001. Yesterday, the US government finally released the only film footage of the attack on the Pentagon, taken by a couple of security cameras, in the hope of rebutting claims that the building was really attacked by a cruise missile. It didn't work: conspiracy theorists rushed to appear on TV, insisting that the white object travelling towards the Pentagon before the conflagration could not possibly have been a hijacked plane.

Earlier this year, at the launch in London of a book about Osama bin Laden, a member of the audience demanded to know why the author assumed al-Qa'ida was behind the suicide-bombings. This would be a reasonable question if it were based on the premise that al-Qa'ida is not a conventional military organisation with a top-down command structure. But that's not what the questioner had in mind; he was referring to what he described as the "mass of evidence" that's emerged to suggest that the American government was responsible for 9/11.

I am hardly the greatest fan of the Bush administration but even I do not think that Bush, Cheney and Rumsfeld are wicked enough to conspire to murder nearly 3,000 Americans in New York and Washington. Nor do I think the real culprit was Israel, or believe the canard that several thousand Jews were warned in advance not to go to work that day.

These are pernicious fantasies, and the fact that they are born out of a distrust of the American government's aggressive unilateralism neither justifies them nor helps us resolve an exceptionally grim international situation.

The problem is that people have begun to apply the same standards of proof, which is to say none at all, to real events as they do to an inconsequential piece of writing such as The Da Vinci Code. Readers may enjoy fantasising that Jesus was secretly married or was indeed an early ice-skating champion. But we all need to be able to distinguish fact from fiction.