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
We have a very vivid sensory imagination, and it’s pretty common that our bodies get their signals crossed a bit and respond in real ways to imaginary stimulus. For some of us, seeing someone get hurt badly can give us ‘sympathy pains’ in the same body part. For a lot of people, looking at or even *thinking* about a lemon, is enough to make their tongue and salivary glands tingle. It’s the same with our disgust reactions.
Neurologically, we don’t have a super deep explanation for this, but we know that there are these things called “mirror neurons” in humans and many other animals, which fire both when we do something *and* when we see someone else do the same thing. We seem to have some part of our brain and central nervous system which is basically hard-wired for the purpose of imagining ourselves in someone else’s place. There are lots of evolutionary reasons why this might be the case.
Well why is someone puking? Perhaps they ate spoiled food, or drank bad water. Now if you’re a human living thousands of years ago, there’s a pretty good chance that you ate the same food and drank the same water as your puking friend. *Maybe* you’d be fine, but evolution has decided that it’s better to play things safe. Worst case, you need to hunter-gather a new meal.
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.
My understanding is that is is also a learned behavior. We see it as the reaction we should have in cartoons, movies, books and phrases. Thus we do it.
I’d like to note something important: The neurologically machinery that causes you to vomit at certain stimuli has no way to know whether it’s entered your body or not. It’s not sophisticated enough to take into account your conscious memories. It’s a (almost literal) knee-jerk reaction to the part of our brain that registers disgust being activated.
As amazing as our nervous systems are, they’re still very, very limited in many important ways. Much to the contrary of what “Intelligent Design” advocates would have you believe, we are very noticeably *not* the product of careful, deliberate design, but are in fact the product of the slow accumulation of whatever random mutations happened to turn out to be useful over the last billion years.
It might be a stress reaction. Blood drains away from the stomach so rather than deal with an acidic nutrient rich mess at body temperature the body chucks it.
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