David Hufford's work exploring the experiential basis for belief in the supernatural, focusing here on the so-called Old Hag experience, a psychologically disturbing event in which a victim claims to have encountered some form of malign entity while dreaming (or awake). Sufferers report feeling suffocated, held down by some force, paralyzed, and extremely afraid.

The experience is surprisingly common: the author estimates that approximately 15 percent of people undergo this event at some point in their lives. Various cultures have their own name for the phenomenon and have constructed their own mythology around it; the supernatural tenor of many Old Hag stories is unavoidable. Hufford, as a folklorist, is well-placed to investigate this puzzling occurrence.