"No live organism can continue for long to exist sanely under conditions of absolute reality; even larks and katydids are supposed, by some, to dream. Hill House, not sane, stood by itself against its hills, holding darkness within; it had stood for eighty years and might stand for eighty more. Within, walls continued upright, bricks met neatly, floors were firm, and doors were sensibly shut; silence lay steadily against the wood and stone of Hill House, and whatever walked there, walked alone."

— Shirley Jackson, "The Haunting of Hill House," 1959

Of all the paranormal phenomena that surround Halloween, the haunted house may be the last to inspire real fear. Witches? Haven't been scary since the days of Salem. Zombies? Fun makeup, sure, but a bit campy. Vampires? Blame sparkly Robert Pattinson for taking the bite out of those bloodsuckers.

But the haunted house can send chills up the spine of the staunchest nonbeliever. Ghost stories tend to happen to the unsuspecting; who's to say they might not happen to you? They're also passed around by word of mouth, often by seemingly trustworthy sources. Nowadays, the Internet expands this oral tradition to almost anyone: Witness the website Jezebel's annual spooky stories contest (and then try to sleep soundly tonight).

Science, of course, counsels skepticism toward the idea of spirits and spooks. So if real ghosts aren't to blame for things that go bump, what might be? Though researchers have investigated culprits like electromagnetic fields and infrasound below the range of human hearing, the ultimate source of hauntings may just be that 3-lb. organ between the ears. [10 Ghost Stories That Will Haunt You for Life]

Seeking ghosts in sound

One plausible explanation for haunted houses is that people are responding to something in the environment — but that the "something" is far more mundane than restless spirits.

A possible culprit is infrasound, or sounds just below the typical human hearing threshold of 20 hertz. In 1998, Vic Tandy, a researcher at Coventry University in England, joined with fellow Coventry professor Tony Lawrence to write a paper based on Tandy's own spooky experiences at a medical equipment manufacturing shop. On occasion, employees reported spooky sensations and the feeling of a presence in the room; Tandy dismissed all of this until one night when he began to feel cold and gloomy. After checking that none of the medical gas bottles were leaking, he sat back at his desk, only to see a gray figure emerge in the corner of his vision. When he summoned the courage to look at the apparition directly, it faded away. [Infographic: Belief in the Paranormal]

A subsequent experience while cutting metal led Tandy to wonder if sound energy was causing his and his colleagues' inexplicable experiences. After a particular fan in the building was switched off, the "ghosts" disappeared, the researchers wrote in 1998 in the Journal of the Society for Psychical Research.

Proving this notion has been more difficult. Lots of things create infrasound, from the wind gusts of air conditioners to earthquakes. In one experiment, researchers used hidden infrasound generators during ghost tours given at Mary King's Close in Edinburgh, United Kingdom. The close is now underground, but in the 1600s, it was a series of narrow alleyways and passages through tall buildings; local legend tells of plague victims bricked into the walls. During a city ghost festival in 2007, some unsuspecting tour groups were blasted with infrasound as they roamed these creepy passageways. [10 of the Spookiest Haunted Houses in America]

The results revealed no difference in the number of people who reported a paranormal experience whether they'd been exposed to infrasound or to ambient noise. However, the infrasound-exposed groups did report a greater overall number of spooky experiences, with more people reporting multiple such experiences. Meanwhile, 20 percent of those in the infrasound groups reported feeling the temperature rise during their tours, compared with only 5 percent in the ambient-noise group, the researchers reported in their preliminary results.

It was hardly a ringing endorsement of the notion that ghosts and ghouls are simply sounds below the threshold of the human ear; after all, people in haunted houses usually report cold spots, not feelings of excessive warmth. And it's not clear why infrasound would lead to an increase in spooky experiences per person, but not more people reporting spooky experiences.

Electrifying chills

Another natural explanation for ghosts might be electromagnetic energy. Spirits may not surround us, but electromagnetic fields generated by power lines and electronic devices certainly do. Could electronics be giving off ghostly vibes?

A few small experiments suggest electromagnetic fields might have this effect. In 2000, cognitive neuroscientist Michael Persinger of Laurentian University in Canada and his colleagues used magnetic fields to stimulate the brain of a 45-year-old man who'd reported previous ghostly experiences; they managed, with the magnetic fields, to "conjure" an apparition similar to what the man had seen years before, along with a corresponding rush of fear, the researchers reported in the journal Perceptual and Motor Skills.

The next year, in the same journal, Persinger and his colleagues reported on the strange case of a teenage girl who said she'd been impregnated by the Holy Spirit and felt the invisible presence of a baby on her left shoulder. The girl had experienced a brain injury earlier in her life, the researchers wrote, but the trauma wasn't the sole reason for the religious visitation: Next to the girl's bed was an electric clock that generated magnetic pulses similar to those used to trigger seizures in epileptic rats. Once the clock was removed, the feelings of a presence vanished. Persinger and his colleagues argue that some people are particularly prone to disruption of the temporal lobes, which happen to be where the brain synthesizes information.

Patients undergoing brain surgery reveal how important the temporal lobes can be to the experience of reality, said Christopher French, a psychologist at Goldsmiths College of the University of London who researches the roots of paranormal experiences. When surgeons stimulate the part of the brain where the temporal and parietal lobe meet — the temporoparietal junction — they "can actually switch out-of-body experiences on and off," French said.

In your head

But French and his colleagues have found little evidence that infrasound and electromagnetic fields explain ghostly apparitions. He and his team tried to create scientific hauntings by building a chamber in which participants were exposed to 50 minutes of infrasound, complex electromagnetic fields, both or neither. The participants then reported the sensations they'd experienced during their time in the chamber.

A majority of people reported some sort of weirdness during the experiment: Almost 80 percent said they'd felt dizzy, half said they felt like they were spinning and 23 percent felt detached from their bodies, the researchers reported in 2009 in the journal Cortex. Notably, 23 percent also said they felt a presence, and 8 percent felt sheer terror.

Some of these experiences certainly mimic the feeling of a haunting; others, not so much (5 percent of the participants reported becoming sexually aroused, for example). But when the researchers analyzed the data, they realized it didn't matter which experimental condition the participants were in. It made no difference if the electromagnetic fields were on or off, or if the infrasound was booming, French told Live Science. However, they did find that the participants' individual levels of suggestibility influenced the results.

"The most parsimonious explanation is just if you say to suggestible people, 'Go in here, and you may have some weird experiences,' some of them do," French said.

As French's work suggests, the real cause of hauntings may simply be the human brain. In one study, published in 1996 in the journal Perceptual and Motor Skills, two participants asked to keep a diary about "poltergeist-like" activity in their home for a month suddenly started seeing evidence of potential poltergeists all over the place. In a follow-up paper in the same journal, the study researchers hypothesized that haunting events happen because people misperceive slightly ambiguous events as paranormal and then become primed to look out for even more weird stuff. [Spooky! The Top 10 Unexplained Phenomena]

Basic personality traits could make people particularly likely to attribute a bump in the night to a ghost or ghoul. A survey released recently by Chapman University in California found that the more generally fearful a person, the more likely he or she is to believe in the paranormal. Another study, published online in the journal Consciousness and Cognition in August 2013, found that paranormal believers are more likely to believe in the illusion of agency, or that there was a deliberate entity behind an event.

That study was based on a theory that people have evolved to see patterns where none exist. Imagine walking in the woods at night, said study researcher Michiel van Elk, a psychologist at the University of Amsterdam. You hear a rustle in the trees. Do you keep going, or run away? If you keep going, you might be attacked. If you run away, no harm is done.

"It's better to be safe than sorry," Van Elk told Live Science.

Evolutionary theorists suggest that this tendency to attribute events to an entity with agency might explain beliefs in ghosts, angels, demons — and even God. To test the idea, Van Elk went to a psychic street fair and asked psychic believers to watch computer animations of moving points of light. Some of the points were arranged to look like the joints of an invisible stick figure walking; other dots moved at random. The participants were asked to determine whether the dots were moving randomly or whether a deliberate agent (a walking person) was behind the motion. In some cases, additional dancing dots were added in, to obscure the random or deliberate dots, making the task trickier.

Both paranormal believers and nonbelievers were good at telling the difference from movement with agency and random movement when the distinction was clear. But in more ambiguous cases, people with greater paranormal beliefs were more likely to jump to an explanation involving agency than the nonbelievers were.

"Even when there were only randomly moving dots, the psychic believers would say they saw a human figure moving in the dots," Van Elk said. [Optical Illusions: A Gallery of Visual Tricks]

The study suggests that falling prey to the illusion of agency could explain belief in the paranormal; a draft in an old house or the creak of wood settling could easily be misconstrued as a ghost. It's not clear, however, whether this illusory agency bias is genetic or learned, Van Elk said.

"This is still one of the key challenges of the field: to see if it is possible to come up with a good study to tease these two explanations apart. What is the nature part of the story, and what is the nurture part of the story?" he said.

Your lying eyes

Indeed, it's tough to even know how much to trust people's own reports of their experiments. In a follow-up, Van Elk wasn't able to replicate his 2013 study. He suspects the reason might be that his original psychic-believer participants might be more eager to please than other groups of people. In other words, they might not be hallucinating a person in the moving dots on a perceptual level. Instead, they could be interpreting information overly generously in order to meet what they think are the experimenter's expectations. It's not that they're lying, Van Elk said; rather, their interpretation feels as real to them as an actual perception. It's just that the mistake arises at a different level of brain processing. [7 Odd Hallucinations]

The psychic problem isn't the only evidence that suggests people overestimate their own trustworthiness. In one study, French and his team had participants watch a video of a psychic supposedly bending a metal key with his mind. In some versions of the experiment, the psychic (actually a sleight-of-hand magician) concluded by putting the key down on the table and saying, "If you look closely, you can see it's still bending."

The key was not still bending. But 40 percent of people who heard the verbal suggestion that it was bending reported seeing it move. By contrast, no one in the group that didn't hear the verbal cue said it had moved, French said.

And multiple witnesses don't necessarily make a report more believable. When another person in the room said they'd seen the key move after hearing the psychic suggest that it was still bending, the percentage of people who said they'd saw movement jumped from 40 percent to 60 percent, French said.

"If you've got one very confident but actually inaccurate witness, it can influence the memory of other witnesses," he said.

Studies find that people with paranormal beliefs tend to have particularly rich imaginations and are inclined to become easily engrossed in tasks, French said. They're also more prone than average to false memories. For example, people prone to false memories might say they remember clearly where they were and who they were with when they saw a video of the Bali nightclub bombing of 2002. But that bombing was not captured on video.

"Presumably, what they're doing is, they're remembering the time they imagined it," French said. "[And] when they're imagining something, it's so much like the real thing — maybe more so than it would be to the likes of me — they're more likely to make a false memory."

In other words, the possible explanation for spooks and spirits might be scarier than actual ghosts: You can't even trust your own mind.

Follow Stephanie Pappas on Twitter and Google+. Follow us @livescience, Facebook & Google+. Original article on Live Science.