Horror is an ancient art form. We have tried to terrify each other with tales which trigger the less logical parts of our imaginations for as long as we've told stories. From the ballads of the ancient world to modern urban myths, audiences willingly offer themselves up to sadistic storytellers to be scared witless, and they are happy to pay for the privilege. Theories abound as to why this is so; do we derive basic thrills from triggering the rush of adrenalin which fear brings, or do horror stories serve a wider moral purpose.