Phobias are a very simple case of trapped priors. They can be more technically defined as a failure of habituation, the fancy word for "learning a previously scary thing isn't scary anymore". There are lots of habituation studies on rats. You ring a bell, then give the rats an electric shock. After you do this enough times, they're scared of the bell - they run and cower as soon as they hear it. Then you switch to ringing the bell and
not giving an electric shock. At the beginning, the rats are still scared of the bell. But after a while, they realize the bell can't hurt them anymore. They adjust to treating it just like any other noise; they lose their fear - they habituate.
The same thing happens to humans. Maybe a big dog growled at you when you were really young, and for a while you were scared of dogs. But then you met lots of friendly cute puppies, you realized that most dogs aren't scary, and you came to some reasonable conclusion like "big growly dogs are scary but cute puppies aren't."
Some people never manage to do this. They get
cynophobia, pathological fear of dogs. In its original technical use, a phobia is an intense fear that doesn't habituate. No matter how many times you get exposed to dogs without anything bad happening, you stay afraid. Why?
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It makes sense that once you're exposed to dogs a million times and it goes fine and everything's okay, you lose your fear of dogs - that's normal habituation. But now we're back to the original question - how come flooding doesn't work? Forgetting the barbarism, how come we can't just start with the Rottweiler?
The common-sense answer is that you only habituate when an experience with a dog ends up being safe and okay. But being in the room with the Rottweiler is terrifying. It's not a safe okay experience. Even if the Rottweiler itself is perfectly nice and just sits calmly wagging its tail, your experience of being locked in the room is close to peak horror. Probably your intellect realizes that the bad experience isn't the Rottweiler's fault. But your lizard brain has developed
a stronger association than before between dogs and unpleasant experiences. After all, you just spent time with a dog and it was a really unpleasant experience! Your fear of dogs increases.
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I've heard some people call this
"bitch eating cracker syndrome". The idea is - you're in an abusive or otherwise terrible relationship. Your partner has given you ample reason to hate them. But now you don't just hate them when they abuse you. Now even something as seemingly innocent as seeing them eating crackers makes you actively angry. In theory, an interaction with your partner where they just eat crackers and don't bother you in any way ought to produce some habituation, be a tiny piece of evidence that they're not always that bad. In reality, it will just make you hate them worse. At this point, your prior on them being bad is so high that every single interaction, regardless of how it goes, will make you hate them more. Your prior that they're bad has become trapped. And it colors every aspect of your interaction with them, so that even interactions which out-of-context are perfectly innocuous feel nightmarish from the inside.
