From ancient Mesopotamia's tales of ghosts who needed to be cared for by living relatives to Charles Dickens's "Christmas Carol," humans have a long history of telling ghost stories.
But why does people's fascination with ghosts remain so enduring?
Christine Simmonds-Moore, a professor of psychology at the University of West Georgia (University of West Georgia), said that people do experience experiences that they think are paranormal.
The term is often used to describe phenomena that currently cannot be explained by science.
"For the person involved, these experiences are real... and can have a huge impact on them," she said.
However, this does not mean that these experiences necessarily have supernatural origins.
altered consciousness
Symonds-Moore said that when people experience phenomena that are considered paranormal, there may actually be many activities happening in the brain at the same time.
One possible explanation has to do with the so-called "hypnagogic state," the stage between wakefulness and sleep.
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"Many of these experiences occur in slightly altered states of consciousness," Symonds-Moore said.
During this time, the lines between inner imagination and outer perception may become blurred, making it difficult to distinguish between the two.
This is also when sleep paralysis (commonly known as "ghost paralysis") may occur. A 2011 review study published in Sleep Medicine Reviews estimated that approximately 7.6% of the general population experiences sleep paralysis. The study used an international sample but did not include students or people with mental illness.
Prof Chris French, a professor at Goldsmiths, University of London, said sleep paralysis is when "your body wakes up, but your brain doesn't fully wake up."
Franzi founded the Anomalistic Psychology Research Unit, which specializes in studying the scientific causes behind seemingly supernatural experiences.
Sleep paralysis is often accompanied by a variety of symptoms, including an intense feeling that there is another presence in the room.
"Even if you can't see or hear anything else in the room, you still feel like there's something there," Franzi said.
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Some people even experience complete hallucinations, including physical sensations, during sleep paralysis.
"The experience can be extremely scary," Franzi said.
"But that was a hallucinatory experience, not a real thing."
Experts point out that another phenomenon that may cause people to have paranormal experiences is "pareidolia," which is the tendency to recognize specific patterns or patterns in random things.
"Because of the way our visual system works, it's easy to see something in the corner of our vision and think it's a face when it's not," said Prof Rodney Schmaltz, a professor at MacEwan University in Edmonton, Canada.
Symonds-Moore added that this tendency to look for patterns and meaning is more likely to occur when people are under stress.
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Some psychologists believe that the reason why people feel there are other people around them when they are alone may also be related to a phenomenon called "dissociation".
Simonds-Moore stated that dissociation is a state in which an individual experiences parts of the self as separate from each other. She points out that on one end of the spectrum of dissociation, a person may feel "separated from themselves" when experiencing severe stress, such as after an accident.
But dissociation may also occur in daily life, such as when driving home and entering "autopilot mode" because the route is too familiar.
Symonds-Moore points out that in feeling present, a person may be experiencing a part of themselves but perceive it as "another presence."
Such dissociative states may be more likely to occur during periods of stress or sleep deprivation, she adds.
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Some studies have found that people who claim to have had paranormal experiences often have a psychological trait called "transliminality."
According to Symonds-Moore, this trait is best suited for people who score higher through questionnaires. People who score higher are usually "highly creative, imaginative, and dreamy people." They are also more likely to be deeply moved by art or music, and are more likely to remember their dreams and find meaning in them.
She said that such people are also more likely to have the feeling that there is some kind of invisible presence around them.
In addition, they are also more likely to experience synaesthesia, in which information received by one sense stimulates another sense at the same time.
For example, Symonds-Moore said that when people with synesthesia see pink, they may subconsciously think of roses, and further feel that they smell the fragrance of roses.
She believes that for some people, this cross-sensory experience may explain some experiences that are considered supernatural.
However, psychological phenomena are not the only possible explanation.
environmental impact
Franzi said some places are inherently "eerie" than others.
"If you go from a well-lit area to a dark area, it will be affected. Another factor is high-floor spaces. People tend to feel more anxious and uncomfortable when entering these places," he said.
However, other environmental factors are more complex. One example is infrasound, sounds with frequencies below 20 Hz. Infrasound waves can be generated by a variety of sources, including machinery, vehicles, and even pipes and heating systems, especially in older buildings.
Schmalz said that although humans cannot consciously hear infrasound waves, this does not mean that they do not have an impact on the human body.
One study he conducted found that infrasound waves slightly increased cortisol levels in subjects' saliva. Cortisol is a hormone associated with stress.
Studies have also found that this is also related to subjects feeling more irritable.
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However, the connection between infrasound waves and paranormal experiences has been controversial. A study conducted by Franzi himself found no evidence of a link.
Schmalz agrees that the presence of infrasound waves alone is not enough to trigger paranormal experiences, but he thinks they may be "a small piece of the puzzle," especially since psychological phenomena such as pareidolia are often exacerbated by anxiety.
However, it's one thing to feel anxious or to feel as though there's a presence around you, but why people attribute such feelings to supernatural forces is another entirely.
truth and survival
Some speculate that the answer may be related to human evolution.
"Our brains are built for survival, not for the pursuit of truth," says folklorist Deborah Hyde, who studies why humans believe in the paranormal.
“A large number of our cognitive systems and sensory systems are really geared around survival rather than exploring the absolute truth of things,” she said.
Over the course of human evolution, the tendency to judge worst-case scenarios may have given us a survival advantage. For example, mistaking a shadow for a predator or interpreting an unexplained sound as a potential threat.
Hyde pointed out that humans also have a cognitive bias called "agency detection."
What this means, says Franzi, is that when we encounter something strange: "Our default reaction is to assume that there is someone or something conscious that is causing that event with a specific intention."
He said: "From this idea, it is not difficult to further infer that there may be some kind of threats around, and these threats may sometimes be various invisible existences."
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Confirmation bias may also help explain why people attribute some unusual experiences to supernatural causes.
Hyde said that while specific beliefs about related phenomena vary from culture to culture, religious beliefs and superstitious concepts "seem to be built into the human mindset."
"If you already have a belief system that ghosts exist, it's not irrational to say, 'Well, this is the explanation,'" French said.
He noted that it may also have something to do with humans wanting to believe in ghosts.
"When something is something that we already believe to be true, or wish to be true, we pay more attention to the evidence that seems to support that belief," French said.
He added: "Ghosts are a form of evidence of life after death. If we want the afterlife to be real, then it becomes easier for us to accept the idea that ghosts might actually exist."
This article was originally written in English. We used artificial intelligence to assist in translation and was reviewed by BBC journalists before publication. Learn more about how we use artificial intelligence.
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