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Toy dolls are having a Halloween moment. And it's anything but pretty or cute.

You can purchase made of baby doll heads to unsettle trick-or-treaters or shell out bigger bucks for a 2-foot animatronic version of scary doll from the "The Conjuring" movies.

If you go , there are a plethora of online tutorials about how to turn a standard-issue doll into a ghoul with cracked, greenish-gray skin and blackened eye sockets.

The trend gives a whole new meaning to the term "dolled up."

Delightful as they may be to their young owners, dolls have a long history of creeping people out due to something called the "uncanny valley" effect, professors at Northeastern University say.

As to how the beloved toys have become such popular and spooky Halloween props—there are theories. And they involve baby boomers.

The uncanny valley of the dolls

The "uncanny valley" effect was an observation by scientist Masahiro Mori that people's empathy for human-looking objects initially climbed as the objects grew more lifelike before dipping into revulsion.

Mori came out with his essay in 1970 at a time robotics was really taking off, says Christie Rizzo, Northeastern professor of applied psychology.

"When robots were looking very robot-like and not very human-like, people had a very positive kind of connection. They thought they were cool," she says.

"But when robots started looking more human, there was a point where people started to be put off by the robot's look and maybe creeped out," Rizzo says.

"That reaction took place in this really narrow place scientists called the 'uncanny valley," between where the robot was sort of human-looking but things weren't quite right," she says.

"When something looks almost human but is not quite, sometimes we find it repulsive. Sometimes we just find it strange," says Mathew Yarossi, an expert in biomechanical engineering and rehabilitation at Northeastern.

One theory holds that the uncanny valley effect creates uncomfortable mental conflict by violating our expectations, says Ajay Satpute, Northeastern associate professor of psychology and director of the Affective and Brain Sciences Lab.

"There are strong relationships between uncertainty and fear," he says.

"We place a high priority around being able to predict or categorize and know what's coming next," says Leanne Chukoskie, director of the Playful Mind Lab at Northeastern.

"We try to predict to make sense of the world," she says.

The split second between recognizing the human-like visage of a doll and realizing it is not a person can lead to an almost unconscious sense of unease.

This tendency has not gone unnoticed by pranksters playing which is when they freak out friends, family and roommates by placing dolls in unexpected places and moving them around when no one is looking.

Annabelle and Chucky vs. Raggedy Ann

The uncanny valley effect has been documented in images of people's brain waves, Rizzo says.

"There's an area of the brain that is specific for the recognition of the faces of humans," says Yarossi, jointly appointed assistant professor in physical therapy, movement and rehabilitation sciences and electrical and computer engineering.

"The firing is different for things in the uncanny valley," he says.

Rizzo says there's also social learning that comes from viewing horror movies that feature demon dolls.

"There's been lots of depictions of evil clowns and evil dolls. There's the Chucky doll that many of us grew up with," she says.

The red-headed Chucky first burst on to movie screens in 1988 and went on to star in seven more feature films and two shorts, even getting his own TV series and video game.

Then there's Annabelle, the possessed doll from "The Conjuring" movies, of which the fourth in the series, "Last Rites," is currently streaming on platforms including Amazon Prime Video.

"If you see a movie like that at the right time, you can actually develop an ongoing fear of those things," Rizzo says.

The story of a haunted doll that inspired a book and the first "Conjuring" movie featured a with a sweet painted smile, shoe button eyes and an innocent appeal.

In the film version, however, Annabelle gets hair and makeup treatment that turns her into a toy that no parent in their right mind would tuck into a child's bed at night.

"She's got huge eyes that are sunk in low, a weird button nose and super sculpted cheekbones," which amplifies the horror story, Chukoskie says. Then there are the eyes that sometimes seep blood tears.

Also striking a discordant note is Annabelle's mouth, which looks like that of an adult more than a child, Yarossi says. "There's something off about that."

It's only a leap of the imagination—OK, a big one—to visualize the weird dolly chasing you with a knife or hammer.

Boomers and baby dolls

Interestingly, children don't experience the uncanny valley effect until the age of 9 or so, according to research involving human-looking robots, Satpute says.

Children may be terrified of inanimate objects like frightening scarecrows, but when it comes to baby dolls, it seems they take them at face value.

All of this raises the question of why Halloween horror doll decor has only gotten bigger over the past several years. "For a while it was alien abduction, then it became scary dolls," Satpute says.

It may be fair to blame the baby boomers, at least this time.

The rising tide of economic prosperity that accompanied boomers' childhoods saw , from Mattel to Madame Alexander, involved in the production of toy dolls.

Now these dolls are piling up in thrift stores and vintage boutiques where Gen Z and millennials love to shop, prompting an outburst of macabre creativity.

Dolls are so prolific that they have inspired not just individual decorations but whole tableaus, such as carrying a "corpse" wrapped in black plastic across lawns.

Like many things on Halloween, the trend may say something about laughing in the face of what really scares us, Satpute says.

"Some have argued that the uncanny value between humans and robots is really about recognizing our own humanness, which might make us feel our sense of mortality and lead to a fear of death," he says.

Rizzo says exposure therapy can reduce symptoms associated with pediophobia, the name for an intense irrational fear of humanoid forms such as dolls.

But the rush of adrenaline that comes from viewing a Chucky movie or running into a roommate's "doll in the hall" is more fun than fearful for a lot of people, she says. "It's a safe scare."

This story is republished courtesy of Northeastern Global News .