What a Haunted House Usually Turns Out to Be

The Survey does not attend hauntings. It attends buildings, and it measures them, and then it writes down what it found. In fifty-two years the great majority of what it found has been on the following list.

This is not written to debunk anybody. It is written because the list is genuinely useful, because several items on it are things people are living with and can fix, and because the first one is a gas that kills people in their homes every year in every country that burns fuel indoors.

1. Carbon monoxide

There is no honest way to write a note like this without putting that first, and no investigation of any building with a fuel-burning appliance should proceed without it.

2. Infrasound

Sound below about 20 Hz is largely below the threshold at which we hear a tone, so it arrives as sensation instead — pressure, unease, nausea, and a difficulty in settling that people describe in strikingly consistent language.

The famous account is Vic Tandy's. In 1998, in the Journal of the Society for Psychical Research, he described his own experience in a laboratory — a grey figure at the edge of vision, a cold sweat, the certainty of being watched — and traced it to a standing wave produced by an extractor fan whose mounting had failed. He fixed the fan and it stopped.

The sources are ordinary: extraction fans, boiler pumps, ventilation plant, machinery in an adjacent unit, traffic, wind across a chimney or a courtyard. Structures with long corridors and hard surfaces support standing waves readily, which is why the effect turns up in exactly the kind of building people already think is haunted.

Two honest caveats. Tandy proposed that the visual part came from the eye's own resonance being driven, and that specific mechanism has been repeated far more often than it has been tested. And the frequency universally quoted online is not quite the figure in the paper, which tells you how many people repeating it have read it. One of our own files is a straightforward instance of the whole thing, including the part nobody mentions — that telling a man his hallway is a fan does not stop the hallway working on him.

3. Sleep

An enormous proportion of "something in my bedroom" reports are sleep phenomena, and they are far more common than most people realise because they are rarely discussed.

Sleep paralysis is the ordinary muscle atonia of REM sleep persisting into wakefulness. You are awake, aware of the room, and unable to move or speak, usually for seconds to a couple of minutes. It is frightening in a way that is difficult to convey and it very frequently comes with a sensed presence, pressure on the chest, and a figure in the room or at the foot of the bed.

Hypnagogic and hypnopompic imagery are the vivid perceptions at the edges of sleep, which can be fully formed, in the room, and indistinguishable at the time from waking perception.

Both are commoner with irregular sleep, sleep deprivation, shift work, stress and sleeping on your back. All of which describe somebody who has been frightened in their house for a month and is not sleeping well — which is why this one compounds.

4. The building

Buildings move. A structure that has been at 20 °C all evening and is falling to 12 °C by 3am is contracting: timber, plaster, pipework, floorboards and roof structure all release stress as they cool, and they do it in cracks and knocks with no obvious source. The rate of these events peaks in the small hours, which is one reason the small hours have their reputation.

Add to that: water hammer in pipework, expansion of central heating pipes against joists, loft tanks refilling, birds and rodents in voids, extractor backdraught, and doors moving because a building under stack effect has genuine pressure differences between its floors.

5. Air

Air movement accounts for most cold spots and a great many touch reports. The stack effect makes a heated building into a chimney, drawing cold air in at low level and moving it along floors and up stairs. Chimneys, letterboxes, loft hatches, floorboards and service penetrations are all inlets. There is a whole note on measuring this properly, and the summary is that a tissue on a thread answers in four seconds what an infrared gun cannot answer at all.

6. Us

The last category is the largest and the least comfortable, and it is not about credulity.

Pareidolia is the brain's willingness to resolve noise into faces, figures and speech. It is not an error; it is a threshold set deliberately low by evolution, because the cost of missing a real face is much higher than the cost of seeing one in a curtain.

The ideomotor effect is small muscular movement produced without the sense of having produced it. It is the entire mechanism of dowsing rods, pendulums and planchettes, it has been demonstrated cleanly since the nineteenth century, and the striking part is not that it happens but that the person doing it is completely and honestly certain they are not.

Expectation shapes perception before it reaches you rather than filtering it afterwards. Told that a room is cold, people feel it colder. Told what a recording says, they hear it. Told a building's story on the way in, they interpret every sound in its terms for the rest of the night, and they do so in good faith.

Attention. You cannot hear the fridge you have been sitting beside for ninety minutes, and when you stop being able to hear it you stop being able to attribute anything to it.

What is left

The Survey has been doing this since 1974 and it does not close every file. Roughly six per cent of them end with the sentence cause not established, and that sentence means exactly what it says: we measured, we excluded what we could, and we did not work it out.

Those files are worth something only because of the other ninety-four per cent. An organisation that finds the extraordinary in every building it enters is not finding anything. One that finds a fan, and a draught, and a fridge, and a flue, and then one November writes down that eleven bearings converge on an empty cupboard, has at least earned the right to be taken seriously when it says it does not know.