The Uncanny Valley is a principle of robotics concerning the emotional response of humans to robots and other non-human entitities. It was discovered by Japanese roboticist Masahiro Mori in the late 1970's through psychological experiments in which he measured human response to robots of varying "humanness".
The principle states that as a robot is made more humanlike in its appearance and motion, the emotional response from a human being to robot will become increasingly positive and empathetic until a point is reached at which the response suddenly becomes strongly repulsive. Thenceforth, as the appearance and motion are made to be indistinguishable to that of human being, the emotional response becomes positive once more and approaches human-human empathy levels.
This gap of repulsive response aroused by a robot with appearance and motion between a "barely-human" and "fully human" entity is called the Uncanny Valley. The name harkens to the notion that a robot which is "almost human" will seem overly "strange" to a human being and thus will fail to evoke the requisite empathetic response required for productive human-robot interaction.
The phenomenon can be explained by the notion that if an entity is sufficiently non-humanlike, then the humanlike characters will tend to stand out and be noticed easily, generating empathy. On the other hand, if the entity is "almost human", then the non-human characteristics will be the ones that stand out, leading to a feeling of "strangeness" in the human viewer.
Although originally applied only to robotics, the principle has been applied increasingly to computer-animation characters. The Uncanny Valley is considered to be the reason behind the difficulty in creating computer-animated characters which seem human-like enough to be "real" without arousing revulsion. The principle leads to the logical conclusion that in order to generate a positive emotional response in human beings, it is often better to include fewer human characteristics in the entity, lest the entity fall into the Uncanny Valley.
The principle also furnishes an explanation of the phenomenon in movies (particularly animation) by which anthropomorphic cartoon animals (which have only limited humanlike features) are perceived as cute and cuddly, whereas such creatures as zombies (which are more humanlike without being fully human) incite disgust, since the latter fall into Uncanny Valley [1]
