Despite her worldwide sales of two billion, critics such as the crime writer P D James pan her writing style and “cardboard cut-out” characters. But the study by neuro-linguists at the universities of London, Birmingham and Warwick shows that she peppered her prose with phrases that act as a trigger to raise levels of serotonin and endorphins, the chemical messengers in the brain that induce pleasure and satisfaction.

“Christie’s language patterns stimulate higher than usual activity in the brain,” said Dr Roland Kapferer, who co- ordinated the research. “The release of these neurological opiates makes Christie’s writing literally unputdownable.”

The study — The Agatha Project — involved loading Christie’s novels onto a computer and analysing her words, phrases and sentences.

One finding was that she used a very limited vocabulary. “It means that readers aren’t distracted and so they concentrate more on the clues and the plots,” said Dr Pernilla Danielsson from the school of humanities at Birmingham University.

Despite the grisly theme of her novels, researchers say that much of her vocabulary is pleasing and gentle.

Favourite words or phrases, repeatedly used in a “mesmerising” way, help to stimulate the pleasure-inducing side of the brain. They include she, yes, girl, kind, smiled and suddenly. Common phrases include “can you keep an eye on this”, “more or less”, “a day or two” and “something like that”.

“Christie does not give her readers too much detail,” said Kapferer. “Her narrative lays down what can be described as a general mist of ideas in which the reader is shrouded until the plot has unfolded. It creates what we call ‘minimum cognitive distraction’.”

Another trick deployed by Christie, who died in 1976, is the frequent use of the dash to create “a faster-paced, unreflective narrative”. Each phrase followed by a dash entices readers to carry on and they are seldom distracted by the adjectival or adverbial phrases used by other crime writers.

However, Mathew Prichard, Christie’s grandson, was sceptical of the research. “It’s not really a mystery. She was simply a writer of great plots,” he said.

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