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You might think something is so "icky" that you try avoid it, and scientists think there's a reason humans, and even some other animals, do this.

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It's a dirty world out there, like, there's literal dirt.  There's a lot of dirt, but I'm not talking about dirty dirt, I'm talking about the stuff that makes us sick, pathogens, parasites, and because they're basically everywhere, there's a good chance that they might end up inside you.  So all these little jerks evolved to exploit us, it makes sense that we also evolved a sense to avoid stuff that could be chock-full of them.  Things like poop or other bodily excretions and rotting meats and vegetables.  Basically, everything we all agree seems gross. 

That's where humanity's universal sense of disgust comes from, or so one idea goes.  Disgust protects us from catching diseases, and two new studies published this week in the journal Philosophical Transactions of the Royal Society B dug a little deeper into that feeling.

One team of researchers broke the sense down by surveying around 2,700 people, mostly from the UK, US, and Canada.  These participants had to rate their level of disgust on 75 different scenarios from hearing someone sneeze to a friend showing off a big oozing lesion, and based on these data, the team identified six main categories of disgust that humans seem to experience, for better or for worse.

There's hygiene disgust, so displays of unhygienic behavior or evidence of that behavior like snot-filled tissues and animal disgust, such as dead mice, which can carry disease or squishy slugs.  Sex disgust includes prostitution and promiscuity, while atypical appearance disgust includes body irregularities like a missing thumb or apparent homelessness, and finally, there was lesion disgust, like pus and other goopy medical junk and food disgust, like lumpy spoiled milk.

The researchers argued that each of these categories lines up with the parasite avoidance theory.  That's the idea that disgust evolved in animals to encourage behavior that reduces their risk of catching diseases, so like avoiding potentially sick people or stuff that could make you sick, and the team found that these categories of disgust corresponded more to signs of sickness instead of ways diseases can be passed on.  

They admittedly focused on infectious disease rather than moral disgust, the idea that we might judge people and feel disgust when they don't fit social norms, even though there's probably an intersection between those ideas.  Overall, there was a lot of variation in peoples' total sensitivity to these scenarios, but on average, women were more likely to rate things more disgusting than male respondents, and they found risky sexual behavior and animals carrying disease extra gross, and the older the participant, the less disgusting they rated scenarios, except for hygiene.  So while all humans experience disgust, there were lots of different trends and there's still a lot of work to be done before we can develop new ways to keep our environments and ourselves healthy.

For example, the research did not study how participants' levels of disgust changed over time or like, if food disgust decreased the more hungry people got, because I don't know about you, but sometimes when I get real hungry, that old hard pizza starts to look kinda good, and humans might not be the only animals that experience disgust.  

According to the other paper published this week, one of our closest genetic relatives could, too.  You might be wondering how that's possible, 'cause chimps are known for like, grabbing a piece of poop and just throwing it at you, but we're not talking about chimps, we're talking about bonobos, which are basically smaller chimps with different social norms, and yeah, they can also throw poop, but that's generally because they're in captivity and stressed out and don't have much else to throw.  

Researchers from Kyoto University's Primate Research Institute exposed their bonobo test subjects to a series of experiments.  One set involved offering them three pieces of apple, one uncontaminated, one smeared with soil, and one doused with unfamiliar bonobo poop, and maybe this isn't a huge surprise, the bonobos much more readily ate the uncontaminated fruit and they avoided both the contaminated ones equally.  

Several of them tried to clean the gross fruit by rubbing it on grass and/or spat out the pieces after tasting them, which, like, I wouldn't have gone that far, but I'm not a bonobo.  Another round out experiments involved presenting bonobos with an apple just beyond arms' reach while exposing the area to different odors.  Three were averse: poop, spoiled banana, and rotten chicken.  One was a chemical detergent, which has a strong smell but acts as a non-gross control, and one was just water.  The researchers wanted to see how much effort the bonobos would put into trying to obtain the apples depending on what the area smelled like.  Turns out, yeah, they put less effort into getting food that they suspect may be contaminated, but the water and detergent were about the same, so they seemed to only avoid the disgusting smells.

In both cases and in other tests, the younger the bonobo, the less they were concerned with contamination, which is basically in line with human children.  The researchers suggest this behavior could be tied in with exploring the world and possibly it helps them build up their immune systems, but there are a couple of caveats here.  The researchers acknowledged that these bonobos are used to being fed by humans and the food samples weren't discovered in their enclosure, so it's not clear how generalizable the results are to how bonobos would normally act in the wild, and while the results are also in line with the parasite avoidance theory, they didn't test it directly.  They just tested behavior and basic decision making, so they could not conclude that bonobos actually show evidence of feeling disgust.

Now, if you'll excuse me, I have to go show the pus-filled lesion on my foot all around the office during lunch break.  For science.  

If you want to share non-disgusting science with your friends, we are adding more finds to today.  Some of the favorites are still available, like Mars socks and the strongest refrigerator magnets in the world, and I'm excited that we have more copies of The Story of Earth by Robert Hazen.  But we've also found and made some new items like mosasaur teeth and ammonite pairs and the wandering womb pin, which some of you requested after seeing our episode on the history of hysteria.  SciShow video-maker Sarah (?~5:51) designed these and you can wear them anywhere on your body or on your backpack or wherever you have your own wandering womb.  Look at them!  They're so angry and disapproving of bad science!  

Just like before, we've found these things, we've made them, and they're available until we run out of them, or if you really like them, we might make or get more, so head over to to get your own scientific artifact of our amazing universe.