Crucially, Slender Man seems to titillate the brain’s sense of surprise in exactly the same way. “Slender Man is minimally counter-intuitive because, on the one hand, we can attribute psychological motivations to him just as we would any other person,” says Tehrani. “But on the other, he appears to be able to violate the laws of physics, by appearing out of thin air, and the laws of biology – he can stretch and shrink his body and grow tentacles.” In other words, the tale offers just enough hints of the eerie to pique our curiosity, without leaving us feeling too alienated.

Delighting in disgust

In terms of their wider themes, psychologists have found that, perhaps unsurprisingly, the most popular tales also tend to evoke strong emotions – and the feeling of disgust seems to make a story particularly potent. Julie Coultas at the University of Sussex recently asked subjects to read and share different versions of common urban legends, some more disgusting than others. One, in particular, seemed to stay in her students’ mind, about a woman who takes her poodle to Vietnam. As the woman fumbles with her order for a delicious steak, the dog trots into the kitchen. It is only when the bill comes, minus the cost of the meat, that she realises she has eaten her beloved pet. Even a year later, the students were still struck by the tale, she says. “It was amazing to see the difference in recall between the high and low disgust content stories,” says Coultas. Perhaps that can explain why urban legends are so often in very bad taste.

We are also drawn to themes of survival – which is why many stories deal with life and death. That makes sense, given our evolution – stories would have been an important way of transmitting valuable information that could save our skin at a later point.

But the most memorable tales, according to Tehrani’s recent lab experiments, involve some kind of social connection; we just can’t forget a piece of lurid gossip. His participants were given a choice of tales and asked to choose one to read, remember and pass on to another person. Each tale reflected the above biases in a different way, and it seemed to have a big effect on their popularity. One told the story of a woman who died after a poisonous spider made a nest in her unwashed beehive haircut. Dealing with death, it was a classic survival tale – but although it piqued people’s interest, it proved to be less easily remembered than some of the others. In contrast, a more memorable story concerned a woman who had cybersex with an unknown man, only to find out months later that it was her father. It’s hardly Jane Austen, but the story requires you to consider others’ motives and decisions, tapping into our social bias. (Others, along a similar vein, might include the story of the inadvertent biscuit thief that ends in excruciating embarrassment.)