The uncanny valley is that place where human-like robots and images turn from acceptable to all-out creepy. What’s that mean? Start with this introduction.

Surprisingly after four years this Extra Credits video is still the best one on covering the uncanny valley:

Interestingly, the reason the uncanny valley exists is not clear but there are theories as to why humans react to the uncanny valley. These range from religious rational to mate selection. The theories that make the most sense to me revolve around avoiding illness (like viruses or diseases) and that the sorties paradox messes our senses up.

For now, we’ll have to keep guessing about the biology behind the uncanny valley while dealing with it when designing games. Luckily there are things we can do now to avoid falling into the valley:

– Be consistent with the look of your design. If it’s a robot keep it a robot and if it’s meant to be human keep it looking human – don’t mix and match.
– Similarly only match photorealism with human facial proportions otherwise our brains will pick up on the strangeness. Just look at this image in Polar Express:

From this great post on The Polar Express: A Virtual Train Wreck (conclusion)

For further reading take a look at:
It’s Uncanny, This Valley: The Ups And Downs Of Cinematic CGI (In)Humanity

Uncanny: L.A. Noire, Blade Runner, and gaming’s quest to capture humanity