Tehrani’s analysis focused on Little Red Riding Hood in its many forms, which include another Western fairy tale known as The Wolf and the Kids. Checking for variants of these two tales and similar stories from Africa, East Asia and other regions, he ended up with 58 stories recorded from oral traditions. Once his phylogenetic analysis had established that they were indeed related, he used the same methods to explore how they have developed and altered over time.

First he tested some assumptions about which aspects of the story alter least as it evolves, indicating their importance. Folklorists believe that what happens in a story is more central to the story than the characters in it – that visiting a relative, only to be met by a scary animal in disguise, is more fundamental than whether the visitor is a little girl or three siblings, or the animal is a tiger instead of a wolf.

However, Tehrani found no significant difference in the rate of evolution of incidents compared with that of characters. ‘Certain episodes are very stable because they are crucial to the story, but there are lots of other details that can evolve quite freely,’ he says. Neither did his analysis support the theory that the central section of a story is the most conserved part. He found no significant difference in the flexibility of events there compared with the beginning or the end.

But the really big surprise came when he looked at the cautionary elements of the story. ‘Studies on hunter-gatherer folk tales suggest that these narratives include really important information about the environment and the possible dangers that may be faced there – stuff that’s relevant to survival,’ he says. Yet in his analysis such elements were just as flexible as seemingly trivial details. What, then, is important enough to be reproduced from generation to generation?

The answer, it would appear, is fear – blood-thirsty and gruesome aspects of the story, such as the eating of the grandmother by the wolf, turned out to be the best preserved of all. Why are these details retained by generations of storytellers, when other features are not? Tehrani has an idea: ‘In an oral context, a story won’t survive because of one great teller. It also needs to be interesting when it’s told by someone who’s not necessarily a great storyteller.’ Maybe being swallowed whole by a wolf, then cut out of its stomach alive is so gripping that it helps the story remain popular, no matter how badly it’s told.

Jack Zipes at the University of Minnesota, Minneapolis, is unconvinced by Tehrani’s views on fairy tales. ‘Even if they’re gruesome, they won’t stick unless they matter,’ he says. He believes the perennial theme of women as victims in stories like Little Red Riding Hood explains why they continue to feel relevant. But Tehrani points out that although this is often the case in Western versions, it is not always true elsewhere. In Chinese and Japanese versions, often known as The Tiger Grandmother, the villain is a woman, and in both Iran and Nigeria, the victim is a boy.

Mathias Clasen at Aarhus University in Denmark isn’t surprised by Tehrani’s findings. ‘Habits and morals change, but the things that scare us, and the fact that we seek out entertainment that’s designed to scare us – those are constant,’ he says. Clasen believes that scary stories teach us what it feels like to be afraid without having to experience real danger, and so build up resistance to negative emotions.

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*Folklorists: those who study traditional stories