"If you see something that... looks human and isn't, you keep your eyes on it and you feel for your hatchet."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 seems too strong, and it just comes across as a very strange human being. At this point, the acceptance drops suddenly, changing to a powerful negative reaction. The Uncanny Valley doesn't necessarily have to invoke fear either, for some people the reaction is more similar to Narm or unintentional comedy. If shown as a graph (like the one 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 people 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 people that don't have a problem with things like zombies and consider them merely another monster may still be creeped out by things like unnatural movement. This may also apply to sound as well. For example, a voice speaking words, but at a higher or lower pitch than is humanly possible, or a recording of a human voice, but played backwards. Or maybe a computer voice like Microsoft Sam. Though some people just find the effect comical and/or silly. 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. 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. This also happens in video games, especially western ones, because audiences in the western world tend to demand photorealistic graphics for their games (or at least that's the perception, sales numbers don't always back this up), while scorning many of the stylized games that get released. Eastern gaming companies tend to still do sprite-based games, although there are some eastern 3D games that enter the valley with too detailed graphics as well. (You'll be very hard-pressed to find a western studio that's not Indie or developing for handheld systems that still uses sprites. They're even rarer than studios that use stylized graphics, although some western developers do use stylized graphics, like Valve and Team Fortress 2.) But this is also extremely indicative of a huge generation gap, where 2D was the ONLY type of video game for two decades, and video games only went from niche to mainstream after 3D was implemented and became a standard, for a multitude of reasons. So sometimes you will get people who have been playing video games for decades missing 2D graphics, the Nostalgia Filter in effect, or thinking a series that started in 2D graphics feels very wrong to them when switching to 3D graphics, creating a total Uncanny Valley. Rather unfortunately, this trope can be applied to real life people and may be in part an explanation (through not an excuse) for things like racism when other groups of people inspire this reaction in certain people. People with social disabilities tend to be hurt hardest by this reaction as people usually don't try to see past the "unnatural" behavior of the individual and may have the same negative reaction that this trope describes. On the bright side, because people can often learn to accept other groups of people if they are around them enough this reaction can eventually be averted. See also Reality Is Unrealistic, where the poor impression comes less from being "creepy" as from breaking existing conventions which audiences had come to expect. 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. An opposite is 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). This trope can also be used purposely, to make something creepy when creepiness is called for. Some examples of particular ways to produce this effect are listed under Creepily Long Arms, Creepy Long Fingers, Malevolent Masked Men, and Uncanny Valley Makeup. This should not be confused with the 5th anniversary of That Guy with the Glasses.