The Uncanny Valley

Of course, human beings themselves lie at the final goal of robotics, which is why we make an effort to build human-like robots. For example, a robot's arms may be composed of a metal cylinder with many bolts, but to achieve a more human-like appearance, we paint over the metal in skin tones. These cosmetic efforts cause a resultant increase in our sense of the robot's familiarity. Some readers may have felt sympathy for handicapped people they have seen who attach a prosthetic arm or leg to replace a missing limb. But recently prosthetic hands have improved greatly, and we cannot distinguish them from real hands at a glance. Some prosthetic hands attempt to simulate veins, muscles, tendons, finger nails, and finger prints, and their color resembles human pigmentation. So maybe the prosthetic arm has achieved a degree of human verisimilitude on par with false teeth. But this kind of prosthetic hand is too real and when we notice it is prosthetic, we have a sense of strangeness.

Mori, Masahiro, Bukimi no tani [The uncanny valley], trans. K. MacDorman & T. Minato, Energy, Vol 7 No 4 (1970), pp. 33–35.