There are things that are scary because they're threatening/dangerous, like chainsaws and rabid dogs. These are authentically scary because they pose a clear physical threat and everyone agrees that they're dangerous. The part of your brain most engaged is the reptilian portion concerned with survival. Psychological horror brings in a mental component, plays with your fears, makes you doubt your own sense of reality. The threat may or may not be real. A lot of times when you're reading a book with psychological horror you don't know what's real or not either, if the main character is an unreliable narrator or not, if you're going to find out the house is haunted or the main characters is delusional. That character has no reasonable expectation that if they tell someone they won't be locked up, and it's quite possible the people around them are messing with their sense of what's real or gaslighting them. Answer from BetPrestigious5704 on reddit.com

There are things that are scary because they're threatening/dangerous, like chainsaws and rabid dogs. These are authentically scary because they pose a clear physical threat and everyone agrees that they're dangerous. The part of your brain most engaged is the reptilian portion concerned with survival. Psychological horror brings in a mental component, plays with your fears, makes you doubt your own sense of reality. The threat may or may not be real. A lot of times when you're reading a book with psychological horror you don't know what's real or not either, if the main character is an unreliable narrator or not, if you're going to find out the house is haunted or the main characters is delusional. That character has no reasonable expectation that if they tell someone they won't be locked up, and it's quite possible the people around them are messing with their sense of what's real or gaslighting them. Answer from BetPrestigious5704 on reddit.com