The Society for Psychical Research was founded in London in 1882, to study such reports with care. It grew out of a conference held in London in January 1882, and its first president was Henry Sidgwick. Its advice to an enquirer is plain: seek a clean account with as much information as possible, and consider every normal explanation first. The 2003 Wiseman study had two parts: Hampton Court Palace and the South Bridge Vaults in Edinburgh. Visitors reported where they had unusual experiences.

The Hampton Court part used 462 usable questionnaires from members of the public in 36 groups, and they reported 431 unusual experiences. The visitors who took part chose to join, and they were tested in groups walking the rooms. The investigators did not know which areas were labelled haunted until all data had been collected, which prevents their expectations leaking into the results. At Hampton Court Palace, people who already believed in ghosts reported significantly more unusual experiences than people who did not. About two thirds of the unusual experiences people reported at Hampton Court were a change in temperature. Scientists in England and Scotland asked visitors to walk around places with a haunted reputation and mark where they felt anything unusual.

The study also measured the rooms. At Hampton Court the average strength of the magnetic field did not differ between haunted and control areas, but its variation was larger in the haunted areas, with a variance of 12.71 against 2.16. A control area is an ordinary place used for comparison. Ordinary places have their own small effects. Wood, metal and glass get a tiny bit bigger when they warm up and a tiny bit smaller when they cool down. Building materials in a house, such as wood, steel, masonry and glass, expand in the heat of the day and contract again at night when it is cooler. Cold pipes can make noise when warm or hot water runs through them.

The Edinburgh part took place in April 2001 during the Edinburgh International Science Festival, with each visitor spending about 10 minutes alone in one vault. The vaults were ranked from most to least haunted. That ranking matched how many experiences visitors reported, even when visitors who knew the reputation were removed, with a correlation of rho = .87. A correlation near 1 means two lists rise and fall together. The study found significantly more reports of unusual experiences in areas with a haunted reputation, and this was not explained by what visitors knew in advance. The authors said the findings strongly suggest that alleged hauntings may not necessarily be evidence of ghostly activity, and could be, at least in part, people responding to normal features of their surroundings.

Sound was tested too. In 2003 a London concert played infrasound under some pieces, and neither performers nor audience were told which. About 22 percent of the audience reported more unusual experiences during pieces with infrasound. People generally hear sounds between 20 Hz and 20,000 Hz, and infrasound is below that range. It is sound with a pitch below 20 hertz, lower than the sounds people usually hear, and it can be sensed if it is strong enough, mostly by the ear. Infrasound comes from nature too: volcanic eruptions, earthquakes, falling meteorites, tornadoes and lightning make it. A scientist named Vic Tandy traced eerie feelings in a lab to a faulty extractor fan that made very low sound.

The Ganzfeld effect happens when the eyes see a uniform, featureless field and the brain starts filling in the blanks on its own. In a Ganzfeld session, people may see zigzag lines, dots or a blob of color within a few minutes, and after 10 to 20 minutes may lose sight intermittently. In 1998 Tandy and Tony Lawrence proposed that a standing air wave of 19 Hz may create the sensations people take for a ghost. Tandy later reported that infrasound of about 19 Hz had been found in a Coventry cellar where people had reported odd feelings, and he noted that only a minority of people are sensitive to the effects of infrasound.

Reports of hauntings include seeing a figure, odd smells, sudden changes in temperature and a strong sense of presence. Each of these is something a person truly felt. A 2001 Gallup poll found about 38 percent of Americans believed ghosts exist. Seeing faces in things that have no faces has a name: pareidolia. People see faces in clouds, rocks and tree bark. The brain area that notices faces gets busy when we only think we see a face, like one in a cloud. A good question to carry away is how much of a spooky feeling comes from a place, and how much from the person walking through it.

Words to know

• enquirer: person asking
• control: comparison place
• correlation: rising together
• variance: spread of numbers
• Ganzfeld: blank-field effect