What is horror?

I've been in several discussions recently about what defines horror. I have some fairly specific ideas on what makes horror what it is and since the conversation seems to be coming up frequently, I might as well write up a rant so that I can just link it for any initial conversation and bear any details on out from that for the individual conversation. Which is probably an inspiration for my other rants now that I think about it.

Anyway, I feel that the most central aspect of what makes horror into horror is the rocking of a world view. Horror doesn't have to be a matter of good vs evil. It doesn't even have to deal with anything that intends harm or even anything that might cause harm. What it has to include is being confronted with things that do not fit in with what you view to be possible in your understanding of the world.

By this I mean how you understand the world to be on an instinctive level not an intellectual one. Most of us have been exposed to the sort of horrors humanity is capable of, but only in a distant manner. We have read about or been told about the actions of serial killers, rapists and abusive parents as well as being aware of things like the Khmer Rouge, Stalin's Purges, the March of Tears and the Holocaust. But we are exposed to these things at a distance. Just like we are told about the negative impact of persistent combat and war, but most of us haven't lived it. We have an intellectual awareness of such things but many of us don't really know them. They're stories we hear about, not realities that we face. When we encounter such a thing in reality we often have trouble adjusting to it.

Horror is about the conflict between perceived reality and assumed reality. Now, for the audience to experience a bit of that horror you have to conflict the perceived reality of the movie with the assumed reality of the audience. As a result, most of the best horror stories are ones where the setting closely matches what we understand to be reality and there is a sort of creeping plausibility to the source of the horror. The story starts off rather ordinary enough and then slowly builds the disjointed elements up until you have a twisted vision that is not so different from your assumed reality that you immediately recognize it as unreal, but far enough apart that you are at the very least uncomfortable. Basically, horror exists when reality as a whole is your uncanny valley.

Most of the time, when the setting is significantly different from what we accept as reality, it is difficult to have actual horror. The Aliens franchise works around this by keeping most of the action in the bowels of the ships or facilities filled with industrial set pieces giving us the subconscious impression of a factory and keep us within that area of familiarity where discomfort and horror can survive. Compare the first two movies with the later movies and how the settings diverged much more from what we are at least basically familiar with in reality. The similarity of Giger's designs to certain pieces of human anatomy is another facet that puts us within that region of uncomfortable familiarity. Alien Resurrection especially hyped up the setting in space which pushes us outside the uncanny valley, making it more of an action movie than a horror movie. The familiarity that we have with the xenoforms themselves by that time does not help with establishing the sensation of horror.

Compare the appearance of zombies in fantasy stories versus zombies in a Romero movie or one of its spiritual successors. There isn't really all that much difference between D&D zombies and Romero zombies, but the stories where some necromancer is raising the undead in Faerun aren't as likely to provoke a sensation of horror from us as the Romero zombies. Most fantasy settings are just too far different from what we know of as reality to keep us within that area of familiar enough but still alien enough to experience a high level of discomfort.

All the other discussions of what makes different monsters terrifying is related to how those monsters distance us from our assumed reality while keeping us close enough to be uncomfortable. The fear of a loss of humanity, the fear of sex, the fear of isolation, fear of death and so many other things are various flavors of how to get into that area of balanced familiarity and alien-ness.