The concept was first posed in 1970 by the Japanese roboticist Masahiro Mori. He’d noticed that as robots grow more realistic, people’s attitudes toward them change. When a robot is toylike and capable of only simple, humanlike gestures, we find it cute. If it starts looking and acting a bit more human, we find it even more endearing. But if it gets too human—as with, say, a rubbery prosthetic hand—we suddenly shift allegiance. We find it creepy. Our emotional response plunges into what Mori called the uncanny valley.

Now I know why I am such a big R2D2 Fan.

via Why Do We Love R2-D2 and Not C-3PO? | Arts & Culture | Smithsonian.