More prosaically, consider the “Bloody Mary” ritual carried on (usually) by preteenage girls, who recite “Bloody Mary” into a darkened bathroom mirror, half-expecting to see a monstrous image, or groups of high school students “legend tripping” to a local haunted house.

That said, we also have to recognize how Slenderman, as an Internet phenomenon, differs from past horror folklore. The web takes oral traditions — jokes, rumors, legends — and transmits them with a speed that far surpasses the word-of-mouth circulation of most pre-Internet folklore. Dead-celebrity jokes and 9/11 conspiracy theories go viral within hours.

In the past, Slenderman might have remained localized, a regional legend about a haunted neighborhood in Northern California or suburban Virginia. Instead, he is everywhere, at the same time.

And it makes sense that Slenderman appears predominantly in photos and film, rather than text. As digital cameras and home-editing programs proliferate, Internet memes are becoming increasingly visual — think LOLcats and “Thanks Obama,” among countless others. As a visual meme, Slenderman is at once more accessible and more frightening, in ways that oral memes can’t replicate.

Moreover, the Internet has become a melting pot where long-held distinctions among folklore, mass culture and literary or high culture break down. The stories of H. P. Lovecraft, a once obscure early 20th-century horror writer, have gained immense mainstream currency thanks to the web, where people who may never have read the books still revel in breaking apart the complex and immense world of Lovecraft’s extraterrestrial monster Cthulhu, for visual fodder for everything from toys, games and fan fiction to YouTube films and even erotica.

In that regard Slenderman, who is both a very specific character and an infinitely pliable meme, is like Cthulhu, without the literary origins.

As a folklorist, I can’t comment on what may have motivated those two girls in Wisconsin. I’ll leave that to the psychologists. But I can say that we’ve seen this pattern before, especially when it comes to technology, pop culture and violence: killings said to be inspired by Dungeons & Dragons games or heavy metal in the 1980s, or Ouija boards long before that. Sometimes these are real but isolated incidents; often they are rumors, themselves a part of the folklore.

Internet horror memes are no more likely to motivate violence or insanity than any other aspect of contemporary culture. But like other examples of the folklore of horror and the supernatural, they can lead us to question distinctions between “everyday life” and the unreal or the numinous — distinctions that may be made even more ambiguous by the bewildering variety of alternative realities found online.