It’s a reaction we all have experienced when a doll, clown, robot, or animated person triggers a reaction of repulsion and disgust for no other reason then being not quite right. Something in our brain screams that there is something wrong with what we are seeing but we can’t fully explain why. Enter the Uncanny Valley, back in 1970 Japanese roboticist Masahiro Mori stated that the more human a robot acted or looked, the more endearing it would be to a human observer. However, when they become too closely human either in appearance or action it triggers a negative reaction. We get creeped out. If you were to graph the phenomenon it would look like this:
Not sure this is true? Consider the following images:
Believe me now? Thankfully, we as writers can use this to our advantage, especially in the genres of sci-fi, fantasy, and horror where robots, strange dolls, clowns, or otherwise altered humans might show up. By acknowledging this reaction in a character it emphasizes their humanity in contrast to whatever thing they might have found. Readers will instantly relate because they have felt it before.
Want to learn more? The Wiki article on the Uncanny Valley explains the psychology behind the reaction along with lots of other interesting information.