The phenomenon can be explained by the notion that, if an entity is sufficiently non-humanlike, then the humanlike characteristics 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. In sum, a robot stuck inside the uncanny valley is no longer being judged by the standards of a robot doing a good job at pretending to be human; but is instead being judged by the standards of a human doing a terrible job at acting like a normal person.
Another possibility is that affected individuals and corpses exhibit many visual anomalies similar to the ones seen in humanoid robots and so elicit the same alarm and revulsion. The reaction may become worse with robots since there is no overt reason for it to occur, whereas distaste for the sight of a corpse is a feeling easy to understand.
It is possible that the Uncanny Valley effect evolved as a means of instinctively identifying and ostracizing human individuals carrying illnesses or mental problems that might render interaction (specifically breeding and long-term care) detrimental to the group. |link|