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According to my understanding, vomiting happens when the body wants to get rid of something that we have ingested or put in our mouths to get it out of the body.

So why is it that when something smells really bad our bodies make us vomit even if we have not ingested the food, or not even put it in our mouth yet? Is this something psychological?

Thanks!

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    $\begingroup$ If you have a concussion, your body decides that the costs of dropping your last meal are less than the risk of being poisoned, as it cannot decide where the side-effects of the concussion come from. I guess something similar is at work here. $\endgroup$ – Gyro Gearloose Jul 15 '17 at 11:59
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    $\begingroup$ The current hypothesis is anytime what your senses are telling you do not match your brain assumes you are hallucinating and thus want to get rid of as much of the toxic substance making you hallucinate as possible before it kills you. $\endgroup$ – John Jul 15 '17 at 14:00
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It is not really making you vomit not directly, it is making you nauseous if you become too nauseous you vomit.(nausea has multiple triggers but for this I will stick to smell) Nausea exist to keep you from eating toxic food and to help you get rid of it if you do, the reflex is a balance between the biological cost of a meal(minimal) and the cost of being poisoned(extreme) so you can see why it might favor vomiting and losing a meal vs not vomiting and dying of poisoning than it would the two costs were not so drastically different.

Now your body can't really tell the difference between a smell coming from inside your mouth and one from outside, they both reach the nose by a roundabout course, and your mouth cannot smell. The closest your instincts can come is assuming a stronger scent is coming from the mouth since it is close and enclosed. That's why a stronger disgusting smell results in a stronger feeling of nausea. the stronger the smell the better the chance it is coming from inside your mouth.

Combine these two concepts and you can see how the stronger the smell and the more disgusting it is the more likely it is to make you nauseous it is the best protection our anatomy allows for. Now of course this is only the the current hypothesis, Nausea is not easy to study nor is it a in high demand for research.

Additional sources

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  • $\begingroup$ "Now your body can't really tell the difference between a smell coming from inside your mouth and one from outside" That would explain the question, but I really doubt that. At least, it needs more explanation. Maybe this would be worth another question that asks more of the details. As for me, I can clearly distinguish the reflex that makes me close my nostrils and run away for better air and what wakes me up at nights if there is something down in my stomach that needs to be thrown out. $\endgroup$ – Gyro Gearloose Jul 15 '17 at 15:16
  • $\begingroup$ If you need some bodily experience of what I'm taking, visit the nearest swear plant. The stench is unbearable if you hadn't had a chance of previous training. And it's only the smell. No real toxic compounds. $\endgroup$ – Gyro Gearloose Jul 15 '17 at 15:20
  • $\begingroup$ wakes you up at night is not being triggered by smell but by something else, like an illness. nausea does have multiple triggers. $\endgroup$ – John Jul 15 '17 at 15:26
  • $\begingroup$ waking up at nights for a smell of smoke had probably saved the live of millions of people. But what I think you are about to say, I said "what wakes me up at nights if there is something down in my stomach that needs to be thrown out", which matches exactly "like an illness. nausea does have multiple triggers" $\endgroup$ – Gyro Gearloose Jul 15 '17 at 15:32
  • $\begingroup$ @GyroGearloose - Studies have actually shown that very few humans wake up to the smell of smoke, and certainly not milllions. During sleep, the brain mostly turns off processing of smells (unlike sounds, touch, and light, for example. Hence the dire importance of smoke alarms. $\endgroup$ – anongoodnurse Jul 15 '17 at 15:45

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