Whydunnit case is closed but a real mystery remains

The most intriguing thing about the great mystery writer Agatha Christie is the great mystery that lies at the heart of her own life - the 11 days in 1926 when she disappeared, sparking a real-life whodunnit played out in the full glare of the sensation-hungry press. It's an incident that's almost too good to be true; the moment when the writer's life collided with one of her own novels, when fiction became reality. But 80 years on, the disappearance remains a mystery. Not so much a whodunnit but a whydunnit? Theories have ranged from amnesia, through revenge for her husband's adultery, to an elaborate publicity stunt. Christie herself admitted to some kind of breakdown, but of what kind nobody really knows. Andrew Norman thinks he has found the answer.

The details of the disappearance could have been lifted straight from one of Christie's own novels. On the evening of December 3, 1926, Christie, by then an established thriller writer, vanished from Styles, her Berkshire home. The next day, her abandoned Morris Cowley was found, crashed, at the bottom of a country lane near Guildford. But there was no sign of her. For 11 days the nation was gripped by the mystery. Had she been murdered - perhaps at the hands of her unfaithful former First World War pilot husband? Had the car crash robbed her of her memory? Had she drowned herself in the Silent Pool, a natural spring near the accident scene that was said to be the site of the death of a young girl and her brother? But then, on December 14, Christie was discovered safe in a spa hotel in Harrogate. She was under the delusion that she was a South African woman and had taken the name of her husband's mistress. She had been fully conversant with her fellow guests, reading about Christie's disappearance and commenting on how closely she resembled the vanished author. She had even placed an advert in The Times under her assumed identity asking for family and friends to contact her. Her husband, who had been brought by the police to Harrogate to identify her, was understandably taken aback when she proclaimed not to know him.

Norman, a former doctor, believes that Christie was suffering from a rare psychological condition called fugue state, or psychogenic trance. This illness is brought on by severe trauma or depression, and Christie was deeply depressed by her husband's infidelity and his request for a divorce.

Norman describes how, as a child, Christie had suffered from night terrors - recurring nightmares in which a terrifying figure called "The Gunman" threatened to separate her from her family. Christie was an overprotected and lonely child. She grew up afraid of abandonment, and her husband's betrayal pushed her over the edge - hence her disappearance and new identity.

It is as convincing a theory as any, and to back it up Norman cites a barely concealed piece of autobiography that Christie published under a pseudonym in 1934, Unfinished Portrait . In the novel, Celia (Agatha), distraught at her husband's affair, leaves home unexpectedly and assumes a new identity. It's as close as Christie came to explaining what really happened during those 11 days, and Norman cleverly shows how Celia's condition closely resembles a fugue state.

Norman is concerned only with the reasons for Christie's disappearance. For those looking for a full biography, this is not the place to start; the 20 years of her life from 1928 to 1948 are dealt with briskly in just four pages. So, somewhat ironically, although the mystery of those 11 days is satisfactorily resolved, Christie herself remains something of an enigma.

Robin Dashwood is a television and documentary director at the BBC.