An Exeter University expert on Agatha Christie has taken part in a project to solve the mystery of how she constantly keeps her readers guessing.
The works of the world's most popular crime novels have been analysed by a team including Dr Jamie Berntal.
They found that the killers are always introduced early in the book - and that female murderers are often discovered through a domestic item.
Modes of transport that feature in the story can also provide a clue - land-based transport such as cars and trains point to a female killer, while ships and planes can indicate that it's a man who 'dunnit'.
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