Eli5: Why do we puke when we *see* something disgusting, although there’s no sign it has actually entered or interfered with our body?
In: Biology
Well firstly, most people don’t. It’s not a normal response, as it isn’t evolutionary beneficial (seeing something disgusting doesn’t harm you, it’s just upsetting), whereas vomiting is quite a harsh process that causes muscle pain, acid reflux issues and can damage your teeth, etc.
So it’s not a good reaction as it doesn’t save you from a real danger, but it can be a bad one because it means simply viewing external things can cause you to damage your own body. Hence it’s not a benefit for evolution, but it can be a bad thing, so it’s not common. Evolution doesn’t care that things aren’t positive, just that they don’t negatively impact you enough to reduce your chances of survival.
It’s a reaction in a small number of people, and as others have said it’s usually a learned behaviour. It’s also something you can become accustomed to via exposure. For example the first time you see blood you may want to throw up, but if got a job in an ER, you’d rapidly become acclimatised to it.
But generally speaking, it’s because your body sees something troubling or worrying and, if that affects you and makes you extremely upset or disgusted, possibly even feeling a bit ill or queasy, it triggers the same ‘my balance is off, mabe I’m poisoned?’ warning in your body which leads to the ‘get this shit out of my body in case it’s poison’ response and it reverts to the natural conclusion of doing a big old vom.
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