Have you ever been creeped out by a wax figure of a celebrity?

Or gotten an unsettling feeling while playing a video game that looked sort of real but still seemed a little … off?

That unfamiliar feeling is what is known as the uncanny valley.

The theory was first posited 40 years ago by Masahiro Mori, a robotics professor at the Tokyo Institute of Technology. Although it can be traced back to Sigmund Freud’s 1919 essay Das Unheimliche.

Or, arguably all the way back to Charles Darwin in 1839:

“The expression of this [Trigonocephalus] snake’s face was hideous and fierce; the pupil consisted of a vertical slit in a mottled and coppery iris; the jaws were broad at the base, and the nose terminated in a triangular projection. I do not think I ever saw anything more ugly, excepting, perhaps, some of the vampire bats. I imagine this repulsive aspect originates from the features being placed in positions, with respect to each other, somewhat proportional to the human face; and thus we obtain a scale of hideousness.”

— Charles Darwin, The Voyage of the Beagle

Simply put, the uncanny valley is the theory that as something becomes almost human, it becomes less aesthetically appealing to humans until the difference is virtually imperceptible.

For example, if you’ve seen the movie Polar Express, you might’ve felt slightly creeped out by the life-like characters in a way that you might not have watching a super-stylized animated film like Frozen or Up.

Still confused? Watch this scene from from the latest season of Silicon Valley.

Like a campfire, Fiona is sad and exciting at the same time. She’s amazing and also creepy. That’s because she’s in the uncanny valley.

Although the ambitious animation in Polar Express is more lifelike than its more abstract contemporaries, something doesn’t quite settle right with most viewers. Another example is the famous CGI scene from The Sopranos. After the actress who played Tony’s mother (Nancy Marchand) passed away during the season, the filmmakers decided to use existing footage and superimposed her face on a body double.

See? Not quite right.

That technique has been improved (see Sean Young in Blade Runner 2049), but it still hasn’t found its way out of the uncanny valley.

While movies like Up and Frozen exaggerate the features of their human characters, media that attempts to opt for accurate renderings of people often fall just short … into the valley.

Let’s not pick on Christmas classics, though.

Another good example: ZOMBIES.

The living dead are ubiquitous these days — and for good reason. Filmmakers employ the uncanny valley to their advantage throughout the horror genre. The aesthetic displeasure of zombies is due, in part, to the fact that they look like us — or used to be us.

Pixar, Dreamworks, and many Japanese animators have been actively avoiding the uncanny valley for years. Their stylized characters are intentionally less human than they could be.

On the other hand, the commercial success of the Planet of the Apes series may be partially due to the films’ obviation of the valley altogether. Since most people don’t get to spend a lot time with primates — and since they don’t actually talk — the film works as a piece of hyperreal cinema. The apes become human-like characters while maintaining a sense of realism without having to look exactly like us in every way.

To take it even further, a film like Anomalisa can blend the whole spectrum together into a delightful cocktail of perception. Leave it to Charlie Kaufman to completely turn a concept on its head. In Anomalisa the characters are clearly depicted as puppets, but they’re also exceptionally lifelike. In a way, Kaufman is playing with the uncanny valley as a tool to get the viewer to examine precisely what makes them, well, canny. And (sorta spoiler alert) by the end of the movie the concepts of representation, self, and form are literally disassembled. It’s great. Go watch it!

The uncanny valley presents itself in many fields beyond filmmaking, like advertising, artificial intelligence, and robotics.

But how will it affect immersive technologies like virtual and augmented reality?

Well for starters, VR interfaces will probably lean heavily toward cartoonish avatars.

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This will prevent the user from being jarred out of the immersive experience by slight irregularities. Another option is to rely on 360 video as much as possible — at least until the tech catches up to the lofty aspirations of graphic artists.

But as far as lifelike VR experiences go there’s still some work to be done to produce completely lifelike experiences. However, there will be a race to cross the valley; and eventually, technology like foveated rendering — which compresses only the polygons the user is looking at by tracking their eyes — could help developers get there sooner than later.

In a few years, the valley might be (effectively) gone. And when designers and developers bridge the gap, the possibilities will be endless.

For now, VR and AR developers have to employ their talent and creativity to create high fidelity immersive experiences.