Prosthetic hands that look more human than their mechanical equivalent elicit an eerie feeling in people, a study finds.

By showing photographs of human, robotic, and prosthetic hands, researchers at the University of Manchester discovered that people preferred to look at actual human hands or robotic hands instead of prosthetic hands, which struck them as eerie. But this sense of revulsion lessened when prosthetic hands closely resembled human hands.

“Our findings show hands are viewed in a similar way to previous experiments which have looked at faces and bodies,” explained lead researcher, Ellen Poliakoff, in a press release. “Finding out more about this phenomenon, known as the uncanny valley, may help with the design of prosthetic limbs.”

"Uncanny valley" is the sense of disgust, fear, or dread that occurs when people encounter something that looks and behaves similarly to humans but falls just short of being convincingly human.

A Japanese robotic professor first made this association between things that are almost human and a sense of revulsion in 1970. Masahiro Mori noticed that people's emotional responses to robots grew more positive and empathetic as they became more human-like until a certain point where the visceral reaction switches to repulsion. 

The uncanny valley effect can be seen on a graph that shows an observer’s sense of familiarity dropping, and then rising again as a robot’s anthropomorphism becomes increasingly human. Apparently, the sense of revulsion becomes more extreme (i.e. the valley deepens) when movement is also used to affect the degree of humanness.

The Manchester study required 36 females and seven males, all of whom were right-handed, to look at photos of mechanical, prosthetic, or human right hands and rank them on eeriness, which produced a rise and then a fall, as photos transitioned from mechanical to prosthetic to human hands.

“We hope this and further research will allow us to learn more about social perception and what is special about perceiving another human being,” co-author, Emma Gowen, commented in the press release. “Determining what factors contribute to eeriness can help us to understand how we interpret and respond to other people.”

In addition to the field of prosthetics, the idea of uncanny valley is taken very seriously by the entertainment industry, such as Pixar, who wish to avoid creating characters that make moviegoers uncomfortable. The 2001 movie, Final Fantasy: The Sprits Within, apparently flopped because the all-too-human, photorealistic characters gave viewers a feeling of creepiness.

But as Ross Eveleth pointed out in his BBC article, the term that describes the sense that is lost when observers experience the uncanny valley is nebulous. “Mori used the Japanese word "shinwakan" on the y-axis, a word that has no direct translation into English,” he wrote. “The most common interpretation is ‘likeability,’ but not all translators agree about that. Other suggestions include ‘familiarity,’ ‘affinity,’ and ‘comfort level.’”

The shape in the graph that inspired the term, "valley," is also being debated. Christoph Bartneck, a robotics researcher at Canterbury University in New Zealand, told Eveleth that he would describe the phenomenon more as a cliff because, according to his findings, likeability actually plummets as robots increasingly look human but are still discriminately not human. Likeability is regained only when a person can’t tell that it is a robot anymore.