Uncanny Valley/Analysis

The Uncanny Valley may be a deep, instinctual reaction: it steers humans, on an automatic level, away from humans who are dead, diseased, or deformed. In that way, the theory goes, the Uncanny Valley is a protection against sources of infection. Some psychologists believe that this effect (or at least, the instinct that leads to this effect) is a major reason for racism and other forms of intolerance. To early hunter-gatherers, anything different from you was either food, a rival, or a predator. If something is recognizably human, we naturally assume they're either friend or foe. So something that's human but... different in some way creates cognitive dissonance. As said in The Hitchhiker's Guide to the Galaxy, there's something about people with insanity or mental problems, even if not clearly visible, something with the way they speak, move, and react that sets off warning bells in people's heads.

It might also explain the idea of the perfection of the human face. You try drawing a perfectly detailed face, and if there's one thing that's slightly out of place, or if the shadow's just wrong, or there's a small bump where there shouldn't be, the whole face doesn't just look wrong, it just feels wrong, despite it being quite a good drawing nonetheless. Of course, with pieces that are meant to be surreal, for example, surprise, surprise, the surrealist interpretations, you accept them. But with pieces that show forms of realism, like The Scream, the visage is... well, frankly, disturbing.

Pieter Hintjens shows an even darker picture of the mental side in The Psychopath Code. He argues that since eusocial animals (whether ants or apes) come with their food stores and colony members relying on someone else for vigilance against the predation (main advantages of being social animals in the first place), ability to exploit social protocols is also an evolutionary advantage - for example, ants have many known parasites that pass for members of a colony using mimicry. Thus, most complex social behavior must have evolved at least partially as means of authentication, protection from such creatures.. The methods of mimicry, of course, co-evolved with authentication.
 * Applied to humans, this means an adaptation for treating one's species as prey rather than peers, but for the species already having adaptations of a social animal, this requires lack of the "tells" or ability to turn them off. Conversely, the ability to detect this also is an advantage - leading to the "arms race". Starting about 2-3 millions years ago, when Homo Sapiens became one of the main threats to each other and began to adapt for this, and never ending, since while forms of predation have changed, neither the whole setup nor old adaptations vanished. The last great change in co-evolution of bad actors and those trying to keep them out happened when it expanded from "hardwired" protocols and exploits for them to more flexible "software" of changing human societies.
 * Thus if a guy laughs or groans at your joke, you know he's okay. If he doesn't, but after a short delay responds with exaggerated laughter, this makes you uncomfortable. That's how you perceive a blip on the built-in "cannibal detector". Your instincts have detected he is faking it - he isn't who he pretends to be! Why? Maybe he is a sneaky bastard looking for an opportunity to steal your food, or maybe a cannibal patiently waiting until you turn away? Not worrying when something is "off" about another human could have been… strongly selected against.