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From the ghost stories of ancient Mesopotamia, which surviving loved ones need to care for, to Charles Dickens' "A Christmas Carol," humans have a long tradition of ghost stories.
But why does our fascination with ghosts persist?
Professor Christine Simmonds-Moore, a psychologist at the University of West Georgia in the United States, says people undeniably have experiences that they consider paranormal, a term generally used to describe a phenomenon that cannot currently be explained by science.
“They are subjectively real…and they can have a huge impact on people,” she says.
But that doesn't mean they have a supernatural cause.
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“Altered consciousness”
According to Simmonds-Moore, many processes can take place in the brain when people have a paranormal experience.
A possible explanation is linked to the so-called hypnagogic state, that is to say the period between sleep and wakefulness.
“Many of these experiences occur when people are in slightly altered states of consciousness,” explains Simmonds-Moore.
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During this period, inner imagination and outer perception can become confused, making it difficult to distinguish between the two.
This is also when sleep paralysis can occur, of which, according to a 2011 study, 7.6% of the general population suffered.
This study, published in the journal Sleep Medicine Reviews, involved an international sample that did not include students or people with psychiatric disorders.
It's when "your body wakes up but your brain doesn't", according to Professor Chris French, of Goldsmiths University London, where he founded the Anomalistic Psychology research unit which studies the scientific causes of apparently paranormal experiences.
Sleep paralysis is associated with a range of symptoms, including the strong feeling of another presence in the room.
“Even if you don’t see or hear anything else in the room, you feel like there’s something there,” French says.
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People may also be prone to actual hallucinations during sleep paralysis, including physical sensations.
“It can be absolutely terrifying,” French says. “But it’s a hallucinatory experience. It's not real. »
Pareidolia – or the tendency to perceive patterns in randomness – could also be the cause of paranormal experiences, experts say.
“[Due to] the way our visual system works, it's quite easy for us to see something out of the corner of our eye and misattribute it to something that looks like a face when it's not a face,” says Professor Rodney Schmaltz, of MacEwan University in Edmonton, Canada.
According to Simmonds-Moore, we're even more likely to spot these patterns when we're stressed.
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Some psychologists believe that we may also experience a sense of presence when we are alone due to a phenomenon called dissociation.
According to Simmonds-Moore, dissociation occurs when certain parts of the self are perceived as distinct.
She explains that at one end of the spectrum, this can happen during times of extreme stress, such as an accident, when a person feels like they are "detaching from themselves."
But it can also happen in everyday life, for example by acting “on autopilot” when performing a familiar task such as driving home.
When experiencing a sense of presence, a person may perceive an aspect of themselves “but as another being,” explains Simmonds-Moore. She adds that these dissociative states are more common in times of stress or lack of sleep.
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Some research has linked reports of paranormal experiences to a psychological trait called transliminality.
According to Simmonds-Moore, this trait is best assessed using a questionnaire. People scoring high would generally be "very creative, with a great imagination and a mind fertile in fantasy", and would be more likely to be "deeply affected by art or music", as well as to "remember their dreams and derive meaning from them".
They're also more likely to feel an invisible presence beside them, she says.
They may also exhibit synesthesia, where information transmitted through one sensory pathway causes stimulation of another.
For example, if a person with synesthesia saw the color pink, they might unconsciously associate pink with the color of roses and, therefore, feel like they smell the scent of roses, Simmonds-Moore says.
For some people, this intersensory experience could explain certain paranormal phenomena, she suggests.
But psychological phenomena are not the only possible explanation.
The role of the environment
Some places are “inherently scarier” than others, French says.
“If you're going from a high-light area to a low-light area, another factor to consider is ceiling height: people tend to feel a little more anxious, a little uncomfortable when they go to those areas,” he explains.
But other environmental factors are a little more complex. We can cite the example of infrasound, sounds whose frequency is less than 20 hertz.
They are produced by various elements, such as machinery, traffic and even plumbing and heating systems, especially in older buildings.
We can't consciously hear infrasound, but that doesn't mean it doesn't impact us, according to Schmaltz.
He conducted a study that found that the presence of infrasound led to a slight increase in cortisol (a hormone associated with stress) in participants' saliva. This increase was also correlated with feelings of increased irritability.
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The link between infrasound and paranormal experiences is controversial, and a study conducted by French himself found no evidence of such an association.
Schmaltz acknowledges that the presence of infrasound alone is not enough to trigger a paranormal experience, but says it could be "a small piece of the puzzle," especially since psychological phenomena such as pareidolia are often compounded by anxiety.
Feeling anxious or having the impression of having a premonition of something is one thing, but what leads us to attribute this something to the supernatural?
Truth versus survival
Some suggest that the answer could lie in our evolution.
“We are built for survival, not for truth,” says folklorist Deborah Hyde, who studies why people believe in the supernatural.
“A very large part of our cognitive and sensory systems are actually focused on our survival rather than on finding the absolute truth of a situation. »
In the past, it may have given us an evolutionary advantage to favor the worst-case scenario – for example, interpreting a shadow as that of a predator, or an unexplained noise as a potential threat.
Added to this is a cognitive bias in humans, known as agency detection, according to Hyde.
This means that when we have a strange experience, "our instinct is to assume that someone or something with consciousness caused it with specific intentions toward us," according to French.
“It's not a big step to get to the idea that there could be these kinds of threats and that these threats could sometimes be invisible beings of all kinds,” he says.
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Confirmation bias may also help explain why some people attribute unusual experiences to supernatural causes.
Hyde states that although exact beliefs vary from culture to culture, religion and superstition "seem to be built into our nature."
"If you already have a belief system that assumes that ghosts actually exist, then it's not irrational to say, 'Well, here's the explanation,'" French says.
This could be related to the fact that humans may want to believe in ghosts.
“When there is something that we already believe to be true or that we would like to be true, we pay more attention to the evidence that seems to confirm it,” French explains.
“Ghosts are a form of evidence for the existence of an afterlife, and so if we want to believe in the idea of an afterlife, we will be more open to the idea that perhaps ghosts actually exist. »
We used AI to help translate this article, originally written in English. A BBC journalist checked the translation before publication. Learn more about how we use AI.
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