All horror stems from the fear of death.
When you are afraid of the dark, it's because you fear that death may come for you unnoticed. When you are afraid of heights, it's because you fear falling to your death. When you are afraid of anything, the root of that fear is always directly centered around a specific concept: oblivion.
Naturally, we have ways to manage this fear. The "fight-or-flight" response is a well-known, well-documented feature of animal thinking. When confronted with a situation where it feels it is in danger of death, an animal makes an intuitive decision: fight back against that which threatens it, or flee to safer territory. Predators tend to lean towards fighting, while prey tend to lean towards fleeing. Even humans have this response, and how somebody reacts in such a situation is usually dependent on their personality.
Horror media, naturally, plays up this response. There is a reason that "jump scares" - startling and threatening acts - are common in media attempting terrify its audience. It's an easy way to trigger that fight-or-flight response.
This doesn't make it good.
A Case Study
Slender: The Arrival is a horror game where you walk around various environments (usually the woods) for a while, picking up pages while a supernatural entity known as Slenderman appears in front of or behind you. If you are caught by Slenderman, you are treated to a jump scare close-up of his (non-existent) face and you get a game over. If you are playing on hardcore, this death forces you to restart the game.
As you can imagine, Slender: The Arrival is a terribly boring game, and the reason boils down to its over reliance on tapping into jump scares.
Slender is a game that relies on the fight-or-flight response almost exclusively in order to drive its scares. You can never fight back against Slenderman; you can only run. As such, the optimal way to approach the game is to run around aimlessly holding down sprint. When Slenderman appears in front of you, turn around and run the opposite way until he appears again. bounce around the environment this way until you collect all of the MacGuffins, and then progress to the next level.
When you rely on primal instinct - known usually as "the lizard brain" - to drive your horror, you essentially give up on truly terrifying the player. You are appealing to a very primal instinct of fear, which is admittedly effective, but when you pull back from the experience you are ultimately left without any take-away.
Playing Slender: The Arrival is spooky even if you know it's a game full of jump scares. But once you are out of that environment of random potential death, you realize that it's horribly boring. It's a game that sits solidly on the "flight" part of fight-or-flight, and the lack of a clear objective besides "find these eight things" means you never feel like you are actually accomplishing anything. Release is important in horror.
Tension and Release
One of the most important concepts in horror is that of the elasticity between tension and release.
These two concepts are simple. Tension is when the player is aware of something - usually because there are hints of its presence - and release is when the fight-or-flight response is actually triggered. Good horror builds tension over time, usually by revealing more and more of its antagonist, and then releases it like a pressure valve once it reaches its highest point.
Take the first-person shooter Receiver. The tension builds when you see or hear an enemy, and reaches a crescendo when the enemy spots you outright. When you disable it with a shot from your pistol, the tension is released, and the cycle can begin again. It's especially effective because dying means starting over from scratch, as Receiver has permadeath. Every encounter is a life-or-death situation, and forces split-second survival thinking on the player.
This principle carries over to all forms of horror. Alien is an effective horror movie - one of the most effective ever made, in fact - because it builds tension effectively and offers only sparing release (seeing the xenomorph). Even though the antagonist is ultimately defeated at the end, the tension remains, because Ripley is left alone and adrift in space. We are left without final release.
Much like how the fear of death is the root of all our primal fears, contemplation of death is the root of all horror. Real, lasting horror is only achieved through forcing the viewer to contemplate their own nature and the inevitability of oblivion.
Fight-or-flight, "lizard brain" horror is abusive to the player. It supposes that players are nothing more than animals whose instincts must be triggered and adrenaline must be exploited. It's a horribly cynical way of looking at a process as complex and subtle as fear, because it bypasses the player's conscious mind to tap directly into instinct. Slender: The Arrival is this sort of horror; a game which relies entirely on adrenaline and nothing else. It's abusive and insulting - without reason - to the audience, which is one of the greatest sins a work of art can commit.
True horror is contemplative and reflective. It encourages players to look at the decisions they make - assuming they actually make any of consequence - and come to terms with their inner self and non-existence. Good horror games - like Receiver, Memory of a Broken Dimension, or Amnesia: A Machine For Pigs - trigger that fight-or-flight response, but they force you to consciously address it. It brings the fear to the surface, which allows you to engage in introspection and better understand yourself and the nature of fear. It's horror that addresses who we are on a fundamental level.
Horror should leave the audience with questions about themselves and the world around them, and should ultimately refuse to answer those questions.
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