Margee Kerr leads a unique double life. By day, she teaches sociology at the University of Pittsburgh. By night, she works for ScareHouse, a renowned haunted house in Pittsburgh, where she analyzes data from customers and employees to make the attractions as terrifying as possible.

Kerr’s new book Scream: Chilling Adventures in the Science of Fear explores how fear works in our bodies and societies, and why many of us intentionally seek it out—especially at this time of year. We interviewed her and several other professionals working at haunted house attractions to find out more about how these places work, and why we love them like we do.

1. FEAR IS SUBJECT TO TRENDS.

Just like Halloween costumes, haunted house characterscycle in and out of popularity. Since 2008, Kerr has been asking ScareHouse visitors to rate which types of characters they find most frightening, whether it’s zombies, ghosts, witches, demons, serial killers, or other nightmares stalking the house's halls.

Kerr says that when she first started the interviews she saw a rising fear of zombies, a trend that has yet to entirely fade. But the characters scoring the highest on questionnaires this season are ones that seem plucked from recent American Horror Story storylines: circus sideshow oddities dripping with nostalgia, face-paint, and worse.

Kerr says movie serial killers like Jason and Freddy are still popular, even with kids who were born long after the franchises got their start. Other perennialfavoritesinclude creepy kids and creepy dolls. (Although people are only scared of porcelain dolls, Kerr notes, never stuffed animals or Raggedy Ann types. She blames that on the uncanny valley effect.) Ghosts, Kerr says,never seem to get to the very top of the rankings.

Amy Hollaman, Creative Director at Terror Behind the Walls at Eastern State Penitentiary, says they’ve found that their scariest characters are the ones people can’t quite interpret. “We have a kind of cult that has taken over a ‘haunted machine shop,'" she explains. "They all have shaved heads, workers uniforms, and the same tattoo. It’s not a character that everyone is familiar with, so people are trying to constantly assess it—is this threatening? Can I trust this? We find that people think that’s very scary."

2. PROGRESSION IS KEY.

Well-designed haunted houses will take into account the entire flow of the experience, much like a symphony—or even just a good workout.

Several haunted house experts noted the importance of starting out strong. Hollaman says they used to have a slower warm-up sequence at the start of their experience, but visitors didn’t remember it. Today, they go straight for the high intensity, high-startle scares and then get further into the story.

Ben Armstrong, co-owner of NetherworldHaunted Housein Atlanta, Georgia,agrees:“You always want to hit them hard at the beginning and hard at the end."

Kerr confirms that starting out with high intensity is important. At ScareHouse, she says, the initial attractions are designed to get your body in a state of high arousal. They “activate the threat response and get the chemicals flowing,” she notes. “Startle scares”—think a zombie jumping out from the corner—accomplish this effectively. The startle response is hard to shut off, Kerr says, and it gets you into high-alert mode.

ScareHouse, like other haunted houses, employs a series of peaks and valleys as visitors wander through the attraction. Their final room ends on a comedic note, one that tries to leave visitors “laughing, pumped-up, and excited.” There'salso acool-down at the end, where visitors check out their belongings, and staff ask how they’re doing. But Kerr notes that not all haunted houses pay attention to the winding-down part of experience. The ones that do tend to be more business-oriented—they’re concerned about the liability issues of sending terrified people off into the night.

3. SOME OF THE CHEAPEST TRICKS ARE THE MOST EFFECTIVE.

“There are a lot of old-school hack tricks that haunters discovered and now science is confirming,” Kerr says.

Many of these have to do with altering visitors’ sense of space and time. Strobe lights induce what Kerr calls "a feeling of depersonalization" by messing with our proprioception—our sense of our own bodies and movement—and our ability to locate ourselves visually from one moment to the next. Blackout rooms also interfere with our ability to orient ourselves, which triggers a fear response.

Other tricks involve subtle movements. At ScareHouse, some of the walls are on springs and move when you lean against them—not enough to make visitors fall and hurt themselves, Kerr says, but enough to make you question what just happened. Golf balls wedged tightly underneath the floors are another classic trick, since they make the ground move just enough to be alarming. "If you put the golf balls really close together in a very tight space and then put a board on top of it (also secured), so there is only about a half an inch worth of movement, it creates just enough disorientation to put you off balance (and set up you for the next scare), but not enough to actually make you fall," Kerr explains.

Still, startles remain one of the most effective tools. “Startles might be cheap, but they’re required to keep that high level of arousal,” Kerr says. “You can have great sets, but you need to have these fundamental things in your pocket to create that feeling of chaos. Otherwise you’re just walking through really cool sets.”

4. DON’T GET TOO COMFORTABLE.

“I always look for ways to establish a pattern or rhythm,”Elizabeth Harper, an LA-based lighting designer who’s worked on several horror-themed attractions, says. “If you subvert the pattern, the audience has a moment of relief where they feel like they've escaped unscathed—and that's your opportunity to really scare them.”

It's a pattern that's often used in horror movies. “The killer's theme music will prime you to get scared," she explains. "Then one time it turns out to be something normal—it was only the wind or a cat ... you can bet that right after that moment of relief they're going to unleash something terrifying.”

Hollaman uses similar tactics in what she calls her “scareography”—the choreography of a scare. “It’s a sequence of steps, like with a dance move,” she explains, and one that incorporates both physical movements and dialogue. The principles of Hollaman’s scareography includes first scanning the customer and getting a read on their body language, which can determine the sequence or the tempo of what happens next. The second step often involves a distraction, often with a prop.

“For instance, in the morgue, visitors see a rib cage slowly lifting up,” she explains. “While they’re turned and looking at that, it allows the actors to slowly creep out of their scare pockets and slide right to the middle of the group.” That’s when the real scare happens.

“We’re using the principles of redirection,” she notes. "The scare isn’t where you expect it to be.”

5. THAT SMELL MIGHT JUST BE ANIMAL URINE.

Entire companies are devoted to designing haunted house smellscapes, withthe scent of rotting corpses a particularly popular choice.

ScareHouse has used a variety of scents, Kerr says, but this year there’s a boar’s urine smell in a haunt designed to look like a butcher room. “It’s awful, but in a way that isn’t completely repulsive,” Kerr says.

Finding the right balance between scary and just plain disgusting is important when it comes to smells. “Something like bad breath is just going to make people disengage and move away," Kerr says. "But there’s other smells that are weird and gross, yet that don’t take you out of the experience.” Boars’ urine seems to fit the bill, for some reason—perhaps because most of us have never encountered it before and don’t have a script for how to deal with it, Kerr says.

Harper, however, says she’s not a fan of using smell in a haunted house, and “not just because the smell gets in your clothes and hair if you're working all night.” She says the unfamiliar smells can actually take people out of the moment. “Unlike sound or lights or actors, you can't hit someone with a smell and make it disappear, so it overstays its welcome. People start wondering how we did that, which is the least scary thought in the world.”

6. THE SCARES MAY CHANGE ACCORDING TO YOUR RESPONSE.

At ScareHouse, “if a group comes in that’s not jumping at all, the actors will abandon any dialogue and go right to startle scares,” Kerr says. In other words, plot goes out the window, while the basic physiological triggers come back into play.

On the other hand, if people who seem too scared, the actors will take it in a more comedic direction. Armstrong at Netherworld agrees: “If someone is too scared we train our actors to back off … We want them to have nightmares for a week, not the rest of their lives!”

According to general manager D. Brandon LeJeune, the goal at House of Torment in Austin is "to scare first and foremost, but when that doesn't work out, we fall back to entertaining. Groups are instructed before entering the attraction that if they are too scared to continue, they can inform a monster and they will be escorted out. This happens on a very regular basis.”

Hollaman, however, says her actors aren’t supposed to improv very much. They might have to make some of their scares shorter or longer, but basically need to stick to the script.

7. SIMPLE IS BETTER.

Not every scary setup works. Around 2009, ScareHouse created a haunted house built around a detailed universe with good soldiers fighting an evil overlord. Visitors were supposed to be able to choose sides, but people didn’t get it.

“The complexity of the story didn’t allow people to go into primal, no-thinking scare mode. That was a lesson in how simple is better—narrative thread good, narrative cobwebs bad,” Kerr says. Or as Harper puts it, “a little bit of narrative goes a long way."

8. THEY PRODUCE A NATURAL HIGH.

Kerr says there can be surprising benefits to visiting a haunted house. For some, they induce a kind of natural high. “The adrenaline, the dopamine, the endorphins that course through your body—the scary material is just a trigger for that kind of response with some people,” Kerr says.

Others seem to enjoy haunted houses because they genuinely like scary material. “Some people have a positive response to a negative picture,” Kerr explains, “and there’s not necessarily a pathology behind that.” The reasons behind such responses are complex. “It could be because some people have linked scary stuff with the feel-good endorphins, so the negative image takes on a positive feeling.”

And some people, like Kerr herself, go to haunted houses as a way to enforce feelings of competency. Going through a set of thrilling experiences can be a safe way to challenge oneself and emerge feeling great. “It’s a confidence boost,” Kerr says. She actively seeks out safe-but-scary experiences on days when she’s feeling low.

9. THEY MAY GET US CLOSER TO OUR ANCESTORS.

Humans evolved in environments in which they were facing constant physical threat. For a lot of us, life is significantly more cushy now. That could mean we’re missing out on dangerous but potentially exhilarating experiences our ancestors were more familiar with, whether that’s running from a bear or fighting a battle.

“For many Americans, their emotional expressions on a day-to-day basis are very narrow,” Kerr says. “We’re not having many highs or lows. We’re living a more restricted emotional life. I think that’s why we go out to scary movies and haunted houses—we evolved to have this massive range of emotional experiences, and we still want them." In other words, haunted houses might just be a way to recapture the sense of thrill our foremothers and forefathers knew—but instead of ending up as a snack for a wild animal, we can go off laughing into the night.