Article by Masahiro Mori. Published in the Japanese journal Energy in 1970. Reprinted in IEEE Spectrum.
IEEE Spectrum Editor’s note: More than 40 years ago, Masahiro Mori, then a robotics professor at the Tokyo Institute of Technology, wrote an essay on how he envisioned people’s reactions to robots that looked and acted almost human. In particular, he hypothesized that a person’s response to a humanlike robot would abruptly shift from empathy to revulsion as it approached, but failed to attain, a lifelike appearance. This descent into eeriness is known as the uncanny valley. The essay appeared in an obscure Japanese journal called Energy in 1970, and in subsequent years it received almost no attention. More recently, however, the concept of the uncanny valley has rapidly attracted interest in robotics and other scientific circles as well as in popular culture. Some researchers have explored its implications for human-robot interaction and computer-graphics animation, while others have investigated its biological and social roots. Now interest in the uncanny valley should only intensify, as technology evolves and researchers build robots that look increasingly human. Though copies of Mori’s essay have circulated among researchers, a complete version hasn’t been widely available. This is the first publication of an English translation that has been authorized and reviewed by Mori.
A Valley in One’s Sense of Affinity
The mathematical term monotonically increasing function describes a relation in which the function y = ƒ(x) increases continuously with the variable x. For example, as effort x grows, income y increases, or as a car’s accelerator is pressed, the car moves faster. This kind of relation is ubiquitous and very easily understood. In fact, because such monotonically increasing functions cover most phenomena of everyday life, people may fall under the illusion that they represent all relations. Also attesting to this false impression is the fact that many people struggle through life by persistently pushing without understanding the effectiveness of pulling back. That is why people usually are puzzled when faced with some phenomenon this function cannot represent.
An example of a function that does not increase continuously is climbing a mountain—the relation between the distance (x) a hiker has traveled toward the summit and the hiker’s altitude (y)—owing to the intervening hills and valleys. I have noticed that, in climbing toward the goal of making robots appear human, our affinity for them increases until we come to a valley (Figure 1), which I call the uncanny valley. [ . . . ]