Why Do People See Ghosts? Sleep Paralysis, Grief and the Brain That Finds Faces in the Dark

The commonest ghost is met in bed. Sleep paralysis happens when a person wakes, or half-wakes, while the body is still in the paralysis that stops it acting out dreams, and for a few seconds or minutes they lie unable to move, often with a crushing weight on the chest and a vivid sense of a presence in the room, frequently a figure at the door or the foot of the bed. About eight percent of people have it at some point and far more in the sleep-deprived, and the experience is so consistent across cultures that each has named the figure: the Old Hag of Newfoundland, the Japanese kanashibari, the incubus of medieval Europe, the ghost that sat on the chest. The dreaming brain is awake enough to know the room is real and asleep enough to fill it, and the result is a haunting with a mechanism.

The second commonest ghost is a dead relative. Studies of widows and widowers find that around half sense the presence of the dead spouse in the months after the death, a third feel they have been touched or spoken to, and a smaller number see them, usually briefly and in a familiar place; the experiences are mostly comforting, are reported by people of every belief and none, and fade over a year or two. Grief keeps a model of a person running in the brain long after the person has gone, and the model, expecting them at the kitchen door, occasionally supplies them. Psychiatrists no longer treat these as symptoms unless they persist, and the anthropologists who study mourning describe them as one of the ordinary ways a mind lets go.

Outside bed and bereavement, ghosts come from the brain's ordinary tendencies, sharpened by darkness, expectation and fear:

• •Pareidolia: the face-finding reflex that sees a figure in a curtain's fold, a face in a window and a shape at the edge of vision, where the eye's resolution is poor and its sensitivity to movement high
• •Infrasound: sound below 20 hertz, from wind, machinery or a long pipe, which is felt rather than heard and produces unease, chills and, at 19 hertz, a flicker at the edge of the eye; an engineer who found a haunted laboratory had a fan producing it is the classic case
• •Carbon monoxide: a leaking heater produces headache, dread, hallucinations and the sense of a presence, and a 1921 medical report of a family haunted for months ended with the furnace
• •Suggestion: a room described as haunted is experienced as haunted, in controlled tests, by people who feel more cold spots and presences than those told nothing
• •Hypnagogia: the images and voices at the threshold of sleep, which are hallucinations and are normal
• •Temporal lobe activity: stimulation of one region of the brain produces a felt presence at the person's shoulder, and seizures there produce visions and visitations

Ghosts have been looked for with instruments since the Society for Psychical Research was founded in London in 1882, by scientists who took the question seriously and mostly concluded, case by case, that the phenomena were fraud, error or the mechanisms above. The photographs of the nineteenth century were double exposures; the spirit mediums were exposed by Houdini and others; the electromagnetic field detectors and thermal cameras of modern ghost-hunting television pick up wiring and draughts. The Enfield poltergeist of 1977, the most investigated case in Britain, produced two girls caught faking and no event that occurred when someone was watching. The absence is the finding: two centuries of looking by people who wanted to find something have found nothing that survives a second observer.

Belief in ghosts has risen in the rich world as religious belief has fallen, which suggests it fills a need rather than following an argument. The brain that evolved to detect agents, to assume that a rustle is something rather than nothing because the cost of missing a predator was higher than the cost of a false alarm, sees intention everywhere, and the same bias that finds gods finds ghosts. The death of someone loved leaves a mind that cannot yet model their absence, and the culture supplies the form the experience takes. Ghost stories, meanwhile, are among the oldest stories there are, told in Mesopotamia and in Pliny's letters, and the pleasure of them, a safe fear in a lit room, is its own reason. None of this makes the experiences false. The presence in the dark is really felt; what is not there is what the feeling is about.

People see ghosts because the brain has several ways of producing a person who is not present: sleep paralysis, in which a half-woken dreamer sees a figure by the bed; grief, in which the mind's model of the dead supplies them for a time; the face-finding reflex in dim light; infrasound, carbon monoxide, suggestion and the hallucinations of the threshold of sleep. The experiences are real and common, no ghost has survived investigation, and belief persists because a mind built to detect agents finds them in the dark.