The Uncanny Valley

http://en.wikipedia.org/wiki/Uncanny_valley

Mori’s hypothesis states that as a robot is made more humanlike in its appearance and motion, the emotional response from a human being to the robot will become increasingly positive and empathic, until a point is reached beyond which the response quickly becomes that of strong revulsion. However, as the appearance and motion continue to become less distinguishable from a human being, the emotional response becomes positive once more and approaches human-to-human empathy levels.

This area 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 captures the idea that a robot which is “almost human” will seem overly “strange” to a human being and thus will fail to evoke the empathic response required for productive human-robot interaction.