Japanese roboticist Masahiro Mori stated in 1970 that the more human a robot acted or looked, the more endearing it would be to a human being. For example, most lovable Robot Buddies look humanoid, but keep quirky and artistically mechanical affectations. However, at some point, the likeness would seem too strong, and it would just come across as a very strange human being. At this point, the acceptance drops suddenly, changing to a powerful negative reaction.
When shown as a graph (like in the video to the right), the acceptance on the Y axis and increasing X approaching human normal, there is a slow rise, then a sudden drop, then a sudden peak as "human normal" is reached. Masahiro Mori referred to this as the "uncanny valley". This video explains it extremely well.
Thus, things that look somewhat human, but are clearly not—such as C-3PO (in Star Wars) or a Golem—produce an accepting reaction, while things that are very nearly human, but just a little strange—such as a child's doll, a ventriloquist's dummy, or a clown—produce a negative response. For some reason the resonance is stronger with a moving object, which is why a corpse is creepy but a moving corpse is creepier still. In fact, some say the very lowest point of the valley is the zombie; though others say that zombies are merely another monster, and that slightly-not-right Pod People, for instance, are closer to the nadir.
This might explain why we like Ridiculously-Human Robots, even if they don't make a lot of intuitive sense. They are just far enough out of the Uncanny Valley not to bother us.
This idea has recently been applied to CG effects. While it's become very easy for programs to simulate textures and skin tones, convincing movement and facial expressions aren't always as simple. This can produce an effect where the character comes off as a zombie, if a production company is going for a purely realistic human look.
Similarly, many cartoons nowadays prefer a simultaneously stylized yet simplified character design, versus the realistic look amongst some older cartoons. In the latter, it's more obvious the budget just didn't allow characters to move much. Heavily rotoscoped characters also often seem less real than more stylized animated characters, especially when they're in the same production. See the Fleischer Studios version of Gulliver's Travels for an example.
Your Mileage May Vary though - some people actually don't think things in this page are that uncanny. Sometimes they just say it's more of an art problem. Sometimes though, some people can actually find stuff like unrealistic facial expressions or disturbing movements with a ragdoll model to be funny. Some actually find specific cases of it erotic. And some simply find the effect cool and part of the entertainment of the film or TV show and don't let any "moving corpse" issues affect their enjoyment.
Compare Reality Is Unrealistic, where the poor impression comes less from being 'creepy' as from breaking existing conventions which audiences had come to expect. See also Off-Model, Bishonen Line, No Flow in CGI, and Ugly Cute. And while you're at it, see What Measure Is a Non-Cute? as the scientific study of that trope gave birth to this one. Opposites are Eldritch Abomination, where the unsettling effect is caused by being way too unfamiliar rather than being way too human, yet still produces the same abominable effect (although the two can overlap as a Humanoid Abomination), and Humans Are Cthulhu, where we are the Uncanny Valley.
You'll notice that most the examples below have to do with inadvertently entering the Valley. This trope can also be used to purposely make something creepy, where creepiness is called for. Common cases of such example are: the masks worn by Malevolent Masked Men, Creepy Long Fingers, Eyeless Face and Uncanny Valley Makeup. The Doll Episode uses this purposely as well with too (un-)lifelike dolls. Compare Uncanny Atmosphere.
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