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What makes a ghost story unforgettable? Maskot/Maskot via Getty Images

Marcus Librizzi, University of Maine

Along with the pumpkins and costumes, the candles and cobwebs, Halloween is all about storytelling. From tales of haunted houses to accounts of demonic possession, ghost stories are part of the holiday's appeal.

As a professor at the University of Maine who specializes in ghost stories, I've thought a lot about what makes one unforgettable. My field research has taken me from the coast of Maine to the Amazon jungle, but some factors remain constant in an unforgettable tale.

One factor is what I call "creep value," which includes the scare factor but...