Elementary school

Worldwide, the novels of Agatha Christie outsell everything apart from The Bible. Meanwhile, letters from Sherlock Holmes fans all over the globe still pour in to 221b Baker Street. A recent missive from the police force in Worth, Illinois, asked for the detective's help with an unsolved murder.

But why are we so in thrall to such strange figures as Poirot and Holmes with their ridiculous moustaches, deer-stalkers and tics? Why do we have bookshops such as Murder One, entirely devoted to the single genre of detective fiction? In The Great Detectives, a new four-part series for BBC2, the author Nigel Williams (right) whips out his magnifying-glass and attempts to get to the bottom of "The Mystery of The Popular Sleuth" by investigating, in turn, Sherlock Holmes, Hercule Poirot, Philip Marlowe and Jules Maigret.

In Williams's view, detective fiction provides a model of justice that is all too frequently missing in real life. "So often in reality, detectives don't actually catch the right people. There is a morality in detective fiction which the literary novel has forsaken. In detective novels, bad people are punished - that is satisfying. We want to see baddies get their comeuppance, and here they do. In real life, we can never actually find a policeman when we need one. It's like that sketch in Big Train where the policeman runs away when a member of the public approaches him."

To Williams, the unambi-guous resolutions in detective fiction also impose a sense of order on a chaotic world. "It's to do with fear," he reckons. "Years ago, I talked to a policeman. He said, `My job is all about calming people's nerves. People feel that everything must be all right if I'm there.' In a society which was famously declared not to exist, that's fantastically reassuring."

Holmes, the Ur-detective, brings us just such reassurance. One "Sherlockian" in The Great Detectives believes that we are comforted by Holmes's powers of deduction. The detective makes us think: "This is a terrifying place, but one where we can find a guide in whom we have confidence and belief."

"Holmes still seems to define what a sleuth, real or imaginary, is," says Williams. "He satisfies the schoolboy in all of us, hunting down facts and thinking, `Gosh, what a super wheeze'."

Detective fiction has become as British as afternoon tea. "In one of his travel books, Eric Newby stays with a family in the remotest Apennines in Italy. They welcome him by saying, `Ah, London. Fog and murder and Sherlock Holmes'," Williams laughs.

Why are we, more than other nationalities, attracted to the genre? "Martin Sherman, the American playwright, once said, `You English are so passionate', which is true," Williams continues. "Our emotion is bottled up behind a respectable facade. That's what powers British life. There is an intense morality behind that very British sense of reserve."

We are also drawn to the fact that detective fiction operates on several different levels. "It's a form that looks as if it's based on logical thought but is actually completely magical and mystical," Williams asserts. "For a scientific idiot like me, it flatters your sense of getting control over phenomena while playing the oldest tricks in the book on you."

Does our love affair with detective fiction show signs of waning? Not according to Williams. "The genre is just getting bigger. It's related to our desire for answers. We may be losing faith in religion, but we all still have to grapple with questions of morality. Detective fiction provides a simple, easily exportable form of morality. It's soon going to take over the planet."

It already has. The pipe-smoking, deer-stalker-wearing silhouette of Sherlock Holmes was recently used to advertise financial products in Japan. A most singular fate, Watson.

`The Great Detectives' starts at 8pm on BBC2 tomorrow

James Rampton