The Human Evolution Blog

That sounds like a stupid question, but it’s actually not because most other species are not as grossed out by vomit as we are. Humans have a particular aversion toward the sight or smell of puke. We find it utterly revolting. And if we actually witness someone tossing their cookies, our reaction can be so strong that we may actually join them.

But why? What purpose does that serve? Is there an evolutionary explanation for why we feel sick watching someone else be sick?

A Human Aversion

Most animals are not revolted by vomit. Anyone with a dog can confirm this. (Gross.) Even in our fellow primates, the consumption of vomitus is a somewhat common occurrence.

For humans the mere thought of puke can stop us cold and have an immediate and profound impact on our mood and behavior. That we are so different from other animals in this regard calls out for an explanation.

Nausea

While only a fraction of people actually barf when they witness someone else barf, most everyone becomes a little nauseous.

Nausea is the feeling that one is about to vomit. When someone is nauseated, they may or may not actually upchuck, but they definitely feel like they might. Nausea is brought on by many things, not just witnessing someone else hurl. Many medicines, illnesses, injuries, infections, and toxic agents can bring on nausea. Nausea and vomiting are really the same symptom, with nausea being the mild form of the symptom and vomiting is like “nausea, but more so.” You can bet that if nausea is listed as a possible symptom or side effect, vomiting will be listed as well. They go hand in hand.

One of the key features of nausea, besides being a good warning that vomiting may be imminent, is that it is accompanied by a complete loss of appetite. Even if you don’t actually retch each time you are nauseous, you definitely lose any interest in eating. This is the biological purpose of nausea: it is our body’s way of stopping, or at least pausing, our desire to eat.

Nausea is pretty easy to understand in the case of ingested poisons or spoiled food. If one has just ingested a large load of potentially harmful bacteria, nausea is the appropriate physiological response to prevent further consumption. Nausea is rather clever because, often, the low pH of our stomachs can destroy the toxin or the infectious bacteria. There is no sense in tossing a valuable meal when a short soak in an acid bath might do the trick. Nausea is like hitting the pause button on our appetites while we assess how serious the infection or intoxication is.

Why contagious nausea?

So why do we lose our appetites when others are sick? Maybe they ate something spoiled, but we didn’t. Why do we feel nauseous when we are around someone that is heaving?

For millions of years, our hominid ancestors lived in small tight-knit communities. Communal eating was the norm and in that context, contagious nausea is adaptive. When a community takes all their meals together, if someone has ingested something toxic or spoiled, then probably everyone else has, too. If someone starts throwing up, it’s best if everyone stops eating for a while as their stomachs attempt to neutralize the bacteria or toxin.

Think about the next most disgusting thing that can come out of the human body. Humans also have a particular aversion to feces and the reason is probably the same. Diarrhea is another common symptom of food poisoning.

The property of “grossness” is purely in the mind of the beholder. We find things gross because evolution has trained us to. In the case of vomit and excrement, the resulting nausea is a protective mechanism designed to mitigate the effects of food poisoning or intoxication.

I should also say that other things that we find gross have their own explanations. Most of us have an aversion to snot, phlegm, and spit because they are potential vectors of contagious disease. We are repulsed by bitter tastes because most toxins are bitter. Very sour food and drink are usually repugnant (unless we’ve acquired a taste for them) because food spoilage generally results in the release of carboxylic acids, which are very sour. Even the thought of spoiled foods, such as eggs or milk, is enough to turn our stomachs. And good thing for that!

Just like our biological drives, our biological aversions have a biological benefit.

Why are only humans affected by contagious nausea?

Humans are most definitely not the only species that is known to eat communally. Therefore, the reasoning behind contagious nausea should apply to countless other species. But it doesn’t. Most species are not revolted by puke and in fact are known to re-ingest their own vomit and that of others. What gives?

This is indeed a conundrum and three ideas have emerged as possible explanations. Firstly, it could be that the particular eating habits of early humans were especially vulnerable to food spoilage and so the issue was a clear and present danger in strong need of adaptive defense mechanisms.

This is a plausible notion because hominid and early human communities of hunter-gatherers were known to organize complex hunting strategies. A “hunt” might have gone on for days. This means that the meat that was caught might have been transported back many miles over several days to be shared with the rest of the community. That is not something typically seen in other animals and creates a scenario where food spoilage would have been common (especially in the warm climate of the African savannah). Perhaps the combination of hunting, food transport, and communal eating of our ancestors resulted in a lot of food poisoning and contagious nausea evolved as a result.

A second factor that may explain why humans are grossed out by puke, but other animals are not, is the power of our memories. Humans have exceptional memories compared to other species and we can often recall the minutest details of events in the distant past that we haven’t thought about in years. We even have the ability to remember past emotions and other mental states very vividly.

With this in mind, at least part of why we experience nausea when other people throw up could be that it induces intense memory recall of our own history with being sick. Seeing someone else toss her cookies could boot up the emotional memory of nausea. Just like you can close your eyes and generate images of places, people, and things in your mind, emotional memories can be stored and recalled as well. This might be part of the mechanism of contagious nausea and help explain why other animals don’t have it.

Of vomit and empathy

A final reason why humans are uniquely revolted by throw up is that the mental system believed to be responsible for contagious nausea is quite pronounced in humans: empathy.

Empathy is identifying with the emotional experience of someone else so intensely that you actually begin to feel their emotions as if you were experiencing them yourself. Empathy is why we are saddened when a friend’s grandma dies, why we cringe when someone gets a paper cut, and why we are exuberant when the protagonist finally wins in our favorite movie. Empathy is like emotional contagion.

Empathy is even why we yawn after seeing someone else yawn. There are systems in our brain designed to mirror the mental states of others, which has helped our species attain social cohesion, work together, and understand each other.

Empathy has been documented in many animals, especially birds and mammals, the most social of animals. (There is a whole chapter on empathy in animals in my new book.)

Of all the animals, mammals show the most empathy. Of the mammals, primates show the most. Of the primates, the apes show the most, and so on. In other words, in the evolutionary lineage that led to humans, empathy has been on a steady rise. Empathy is a key aspect of sociality and goes hand-in-hand with attachment to others.

What does this have to do with our being grossed out by vomit? Empathy may very well be the mechanism of that response. While the evolutionary reason that we get nauseous around puke is probably to protect ourselves from food poisoning, the mechanism is likely empathy. Empathy was the tool with which natural selection was able to evolve this self-defense.

Our ancestors had a neural system in their brains designed to observe, understand, and then mirror the emotional states of others. When a mutation emerged that extended this mirroring phenomenon to nausea, the descendants of the mutant had an advantage: they were less likely to die from food poisoning because they were more likely to stop eating when a comrade got sick and possibly even puke themselves.

In summary, contagious nausea was an adaptive response in our ancestors, whose lifestyle of transporting food and communal eating made it necessary. Humans were able to exhibit this unique quality because of our impressive memories and heightened sense of empathy.

You may have even felt nauseous while reading this post. Take heart. If you did, that’s just a sign that you have a healthy sense of empathy.

-NHL     (@nathanlents)