The 'Uncanny Valley Effect' is the hypothesis that human replicas that appear almost, but not quite, human elicit feelings of "eeriness and revulsion from some observers." The "valley" part of the hypothesis comes from the following chart.

The chart compares various objects to human likeness and tracks the observers' familiarity. The findings above show that familiarity and sympathy with the object grow, the more similar the object is to a human. It would make sense to assume that this pattern would be constant, but it is not. The dip in the chart above is the "uncanny valley." It covers a specific type of object that is similar to, but not quite, human. Beyond this point are humans themselves, and the fear and revulsion resolves.

But what is it about things that not quite human? What kind of things are included in the "uncanny valley?"

The simplest explanation is death. Death freaks everyone out. Facing the physical evidence of one's mortality (ie. a corpse) or things that are similar invokes a particular negative reaction that's part revulsion and part terror. The chart above mentions corpses and zombies as things that are part of the uncanny valley. Robots like "Sophia" are also firmly in the uncanny valley.

She has the appearance of a human for the most part. Were it not for the exposed machinery in her head, she would be very convincing. However, something about Sophia just isn't quite right. Even as I write this article, her picture makes me do this:

Care to guess another thing that falls into the uncanny valley? I'll give you a hint. It's very relevant to what's been happening in the country over the last few months.

That's right, I'm talking clowns! Even when they're not trying to be intentionally creepy, they are creepy, and why?

They look like corpses. See the pale face and the rigor mortis smile? That is some undead nonsense right there, and most people, on a primal level, want no part of that.