Why do we see ghosts?

Are ghouls real? That depends. Current science can’t prove that there are spirits walking through walls or screaming below floorboards. Our spooky sightings, however, have certainly felt real. Humans have been spotting specters for as long as we’ve been around, and to some degree we can explain why. These seven mental and physical factors can account for almost any creepy occurrence—including some famous ones ripe for debunking—and help to make sense of our perpetual urge to sleep with the night light on.

You want to believe

“I know that ghosts have wandered on earth.” So says the tormented hero Heathcliff in Wuthering Heights, and he’s not alone: Even for the most grounded among us, there’s something irresistible about haunted houses and vengeful spirits. Sometimes, hoping for a spectral sighting (or, like Heathcliff, dreading one) is enough for us to conjure a wraith.

Thanks to campfire tales and multimillion-dollar horror flicks, spooky notions can infiltrate our subconscious even without any real-life supernatural encounters. Nearly half of Americans think ghosts are real, according to market research company YouGov (bloodsucking vampires scored a measly 13 percent). That preconception primes our minds to run wild whenever we hear a creaky floorboard or feel a sudden chill.

“Believers are a lot more likely to report anomalous sensations, and they’re also more likely to conclude that those sensations indicate a ghostly presence,” says Chris French, head of the Anomalistic Psychology Research Unit at Goldsmiths, University of London as well as a self-described “wet blanket” skeptic.

We have such a tendency because the human mind is highly suggestible, French says. We’ve evolved to take cues from the outside world to escape threats like an animal chasing us, so a well-placed hint can make us see things that aren’t there. In the 1990s, psychologists at the University of Illinois at Springfield gave the same tour of the century-old and long-closed Lincoln Square Theater to two groups of people, telling only one cohort that they were investigating a haunting; sure enough, the visitors who were informed of the excursion’s specifics were far more likely to report intense emotions and strange occurrences. This mental quirk is so powerful that it can deceive us even in real time: In another study, conducted by Goldsmiths’ French, participants were much more likely to report witnessing a key bending of its own accord if someone standing next to them mentioned they had seen the eerie incident happen too.

Our preconceptions can also cause us to find supernatural evidence in garbled noise or blurred images. French says this phenomenon, called pareidolia, can explain many supposed recordings of phantom voices. If a ghost hunter or psychic instructs you to listen for a certain phrase, then your brain (which loves identifying patterns) tries as hard as it can to create those exact words from various bits of random sound.

You’d rather not risk it

It’s easy to disregard the notion of paranormal activity in broad daylight, but everything changes when you head into a dark basement. Unfamiliar and threatening environments kick our survival instincts up a notch.

“If you’re walking in the woods and you see movement, you can make two errors,” says Michiel van Elk, a professor of social psychology at Leiden University. “You can either think it’s nothing, and it could be a potential predator, or you can think there’s a predator, and there’s nothing.” Psychologists suspect humans evolved a cognitive bias toward the latter mistake for good reason: Our ancestors had to keep a constant lookout for stealthy hazards like leopards and snakes, and folks with a “better safe than sorry” attitude were more likely to survive and reproduce. But, van Elk says, this propensity can cause us to sense the presence of another even when we’re alone. That’s why a snapping twig can activate the fight-or-flight reflexes that make us scream.

Ghost tours capitalize on this hereditary paranoia by forcing the mind to wrestle with ambiguity. A good haunted mansion doesn’t shove a spirit right in your face, but encourages you to wonder if you might have just seen one out of the corner of your eye. The uncertainty itself drives up the fear factor. Even quirks of architecture can trigger this primitive terror: In 1975, British geographer Jay Appleton found that, when it comes to our habitats, humans tend to think of places as safe when they offer two things: prospect (a clear view of the outside world) and refuge (the opportunity to hide from danger). A poorly lit old house gives us neither of those two accommodations, blocking our ability to see what’s around the corner and providing plenty of shadows in which malicious entities could lurk.

You need a little company

The apparitions in movies like The Grudge and The Amityville Horror will stop at nothing to chase down their human victims, but ghosts aren’t innately terrifying. Research suggests that the brain may summon spirits as a means of coping with trauma, especially the pain of losing a loved one. Just as most amputees report what’s known as “phantom limb,” the feeling that their detached appendage is still there, surviving spouses frequently report seeing or sensing their departed partner. One 1971 survey in the British Medical Journal found that close to half the widows in Wales and England had seen their mates postmortem. These vivid encounters, which psychologists call “after-death communication,” have long been among the most common kinds of paranormal experience, affecting skeptics and believers alike.

Experts think that such specters help us deal with painful or confusing events. A 2011 analysis published in the journal Death Studies looked at hundreds of incidents of supposed interaction with the deceased. The paper concluded that some occurrences provided “instantaneous relief from painful grief symptoms,” while others strengthened preexisting religious views.

Death isn’t the only trigger for a friendly ghost encounter either. Studies suggest kids who are bullied or exposed to dangerous situations are more likely to have paranormal fantasies, a trend psychologists also found in adults with a history of childhood trauma.

There’s also evidence that sightings have other mental benefits. In a 1995 survey in The Journal of the American Society for Psychical Research, 91 percent of participants said their encounter had at least one upside, such as a sense of connection to others. So if you do see a shroud down the hallway, you might not want to run.

Your brain is unwell

Ghostly occurrences can be the result of larger problems in our gray matter. For some, hearing voices or experiencing a vision can be an early indicator of medical conditions such as schizophrenia. Some evidence even suggests that people with underlying brain disorders tend to have paranormal confrontations that are more intense and negative than the average brush with the beyond.

Even in those without mental illness, temporary changes in brain activity can lead to run-ins with wraiths. People who experiment with psychoactive drugs like LSD and magic mushrooms frequently report spiritual fantasies. Furthermore, psychiatrists have deemed many visions the result of sleep paralysis, a poorly understood condition in which the afflicted wake up and find themselves unable to move. Scientists have yet to pinpoint the roots of this phenomenon, but some think it occurs when the brain crosses wires between conscious awareness and the dream-filled REM stage of slumber. This mixup is almost always accompanied by a sensation of entrapment, floating, or detachment from one’s body—and in many cases sleepers see an accompanying demon or hag. According to a 2018 survey in the International Journal of Applied and Basic Medical Research, at least 8 percent of the general population and around 30 percent of people with psychiatric illnesses have reported having one of these nighttime episodes at some point in their lives. Many cultures even have a specific name for the ghoulish occurrence. In Cambodia, for instance, the freakish event is called “the ghost that pushes you down”; in Nigeria, meanwhile, locals have another name for it: “the devil on your back.”

You’re getting some bad vibrations

Sometimes people experience an otherworldly encounter simply because something in their environment is making a strange noise that sends their bodies into disarray.

In the early 1980s, British engineer Vic Tandy was working in the research lab of a medical supply company when a strange feeling came over him. All at once he felt frigid and overwhelmed with a sense of impending doom. As he paced around the room to calm down, he suddenly sensed an ethereal presence. Moments later, he was sure he saw a gray apparition in his peripheral view. When he whirled around, the specter was gone.

Tandy’s colleagues had warned him the facility might be haunted, but the engineer was a skeptic by nature, so he scoured the place for an explanation. The culprit turned out to be a fan that hummed at a rate of 18.9 Hz. Though we can’t sense their quivering, our eyeballs vibrate at a very similar frequency. The sound threw Tandy’s vision for a loop and caused him to see a vague spook. The rogue fan may also have triggered his momentary panic, as studies suggest that certain noises can cause a person’s organs to shake, which makes them hyperventilate.

Waveforms that dwell around this acoustic sweet spot and below are known as infrasound. Though they’re inaudible to human ears, whose range bottoms out at 20 Hz, the interval creates some fairly insidious side effects. In fact, after Tandy published his findings in 1998 in the Journal of the Society for Psychical Research, 18.9 Hz got a reputation as the “fear frequency.”

Most of us don’t regularly carry around audio gauges, so it’s hard to know how many hauntings might be explained by a buzzing fan or a rumbling fridge. For Tandy, the fright left him more curious than ever about ghosts. “When it comes to supernatural phenomena,” he told a reporter some years later, “I’m sitting on a fence.”

You’re in the wrong place at the wrong time

Situational quirks can easily manipulate our senses into seeing what’s not there. Consider the rural town of Anson, Texas, where locals long believed that if you drove out to the crossroads nearest the local cemetery and flashed your headlights, a mysterious flicker would bounce back at you. Legend held that the blink came from the lantern of an ill-fated mother searching for her son. In 2011, a group of skeptics armed with iPhones and Google Maps confirmed a less evocative explanation: Cars coming around a bend on a nearby highway cast the eerie beams of light.

A far more troubling circumstantial peculiarity is the notion that mold and other pollutants—often found in old buildings—can mess with people’s minds. Over the past few years, environmental engineering students at Clarkson University in Potsdam, New York, have been searching supposedly haunted structures across the Empire State for evidence of funky microbes; while it’s too early to draw conclusions, the places they’ve visited seem to have higher spore counts than your average inhabited building. Believers often cite the smell of rotting food (which fungi and mildew gather on) as a solid indicator of a phantom visit, and there’s some evidence that microscopic growths can trigger anxiety, depression, or even psychosis. Some historians believe that rye bread contaminated with ergot fungus (the same microbe from which LSD is derived) may have triggered the presumed possessions that led to the Salem witch trials of the late 1600s. Further, a dermatologist and known fungal expert at Guy’s Hospital in London has theorized that moldering books could induce enough mental weirdness to have inspired some of literature’s best works.

The same way scientists can potentially identify natural agents to explain “the devil’s magic,” known geologic phenomena may influence seemingly ghostly happenings. For example, some out-there theorists say that more sightings happen on days when Earth’s geomagnetic activity takes a sudden plunge. Disturbances in the planet’s magnetosphere, which are usually caused by anomalous outer-space events like solar flares, might mess with the inner workings of the brain, scrambling our perceptions in strange ways. So far, the evidence supporting this hypothesis is pretty thin.

Your mind is playing tricks on itself

In recent years, neurologists have identified potential bases for the feeling that someone or something is haunting us.

Research suggests seizures in the temporal lobe—the area of your noggin that processes visual memory and spoken language—might trigger ghost sightings. Electrical disturbances in this brain area could make us feel connected to otherworldly realms. Patients who have a history of such problems are more likely to report paranormal beliefs; furthermore, supernatural experiences tend to cluster between 2 a.m. and 4 a.m., which some studies suggest is when these seizures occur most often.

Gray matter researchers have also spotted similar activity in controlled laboratory settings. A 2016 case study by doctors at a Jerusalem hospital described a patient who had a spontaneous religious experience as physicians stimulated his temporal lobe while treating him for epilepsy. And a 2008 paper published in the International Journal of Yoga found that people with supposed telepathic powers exhibited unusual activity in a section of the lobe called the right parahippocampal gyrus—one of a pair of regions that handle memory—when they tried to complete a mind-reading task.

Other sections of our headspace can also fall victim to phantom confusion. In a 2014 study, Swiss neuroscientists blindfolded a group of participants, then hooked up their hands to a machine that tracked finger movement. When the subjects moved their arms, a robotic appendage behind them simultaneously touched their backs in the same fashion. But when investigators delayed the mimicking movements of the animatronic device by just a few milliseconds, several people reported sensing an intelligent presence behind them, as if a spirit were poking them in the back. The researchers think the stalled movements wreak havoc on how the brain times incoming signals in the frontoparietal cortex, which controls inbound sensory and motor cues. Later imaging on folks who reported sensing paranormal shadows in the past found many had lesions in that exact area of gray matter, affecting its normal functioning.

This “feeling of a presence” phenomenon has more general implications for the hard-to-study field of the paranormal too. If a tiny movement delay is enough to conjure up spirits, perhaps our brains are predisposed at some deep level to imagine ghosts are walking among us. We might grow up, but those feelings never go away.

This story appears in the Fall 2020, Mystery issue of Popular Science.

Correction: The story previously stated that the Salem Witch Trials took place in the late 1800s. It’s been changed to reflect that the real date was the late 1600s.