It’s science: “My farts smell better than yours!”

I don’t get blog requests particularly often. And this request was for a very difficult subject. I wasn’t sure if it was possible, or pleasant. But some questions are so nagging, so universal, so profound, that they need an answer. I need an answer. You need an answer. The Internet needs an answer.

The question: why do people think other people’s farts smell worse than their own? Or, phrased differently, why do my farts smell OK when other people’s are disgusting?

There. I said it, and it’s only going to get more detailed from here. If that makes you squirm, click on the cute bunny below to take you away. Otherwise, get your nose peg and read on.

Forget about flatulence, let’s play in the snow! Source: Madeleine_, Flickr.

Still here? Good. Let’s dive down to the business end.

The first thing we need to establish is what people genuinely think about this. Is it just a lunatic fringe of the population who rate the smell of their own flatulence over others? A quick search revealed that 65-76% of people on the wondrous website “Is It Normal?” say “Yes”, it is normal (on two polls). So, about two-thirds of people agree.

“That’s not scientific!” I hear you cry. Of course not. Perhaps the people on Is It Normal? are a biased sample. Scientific papers to the rescue! Richard Stevenson and Betty Repacholi, in 2005, published a series of 5 studies investigating this intriguing question. Let’s see what they found.

Figure 1 from Stevenson and Repacholi, 2005.

For starters, they identified that the level of disgust people would feel and show upon smelling flatulence and other odours. Flatulence gave by far the largest self vs non-self response. Interestingly, the people surveyed said they feel less disgusted by the armpit sweat of their ‘chosen person’ (their partner/best friend) than by their own!

However, the study was limited: they couldn’t use real smells, and had to ask people to imagine situations where the smell was present by making them read short stories. So, they expanded the study, asking people to keep diaries of when they encountered smells in the real world. The results were much the same. Self-created bodily odours were rated as less unpleasant than those created by other people.

It seems we have a strong basis to believe that people at the very least perceive their various ’emissions’ to smell better than those of other people. The real question is why.

It’s worth making the point here that we don’t get an objective reading of everything we sniff. Our senses are about interpretation and context as much as they are about chemicals. As a result, psychological factors will play in to our perception of how ‘bad’ a fart smells, along with the details of last night’s dinner menu. So, even if the chemical composition of ‘our’ fart and ‘their’ fart is the same, we may still perceive them as smelling different.

A swirling solution of Hydrogen Sulfide: rotten egg gas!

I had to broaden my search for reference material at this point. I genuinely couldn’t come up with a single study that really tackled this question head-on. For starters, I couldn’t find a study in which people actually smell real farts (apparently it’s unethical or unhygienic or something. Pffft). However, after some digging, I’ve come up with five main reasons why this effect could exist.

1. You’re used to the smell of your own brew. Both consciously and subconsciously, you smell yourself more than pretty much anything else in the world. A proportion of the gases made by your intestines dissolve into your blood and get breathed out; it’s not a large or concentrated amount, but it means your sense of smell will be more used to what ‘flavours’ your body has made than other people’s. This also serves to explain why people rated the smell of their ‘chosen person’ as less disgusting than a stranger in the study!

2. When you deal it out, you’re prepared. Being ready for a sensation can make it easier to cope with. For example, we find it almost impossible to tickle ourselves. This might apply to flatulence as well. However, it doesn’t explain the situation when you hear a stranger let one rip. You know something bad is on its way, but that does nothing to lessen the noxiousness when it arrives.

3. It’s more dangerous coming from someone else. This is the currently favoured evolutionary explanation. It’s grosser to smell, see or otherwise experience the bodily functions of other people, because they pose more of a disease risk. Over thousands of years, our ancestors who steered clear of such things were likely to stay healthier, and passed on that innate, psychological ‘yuck’ reflex to us. Think about an example: if you sneeze into your hands, it’s a bit icky, but OK. If a complete stranger on the train leaned over and sneezed onto your hand, that would be WRONG.

4. It’s socially conditioned. There’s a social convention at work here, too. Flatulence is an intrusion. If a stranger intrudes on your experience of the world in an unpleasant way, of course we’re going to react badly!

5. Confirmation bias. We will only really notice a bad-smelling fart. Most people fart multiple times per day, but not all will smell bad (diet etc come into play here). Se we figure, on average, we aren’t too bad. In public, we only smell the bad ones – confirming our suspicion that everyone else smells worse then we do!

It’s clear, though: a proper, definitive, double-blinded study is needed. The difficulty is being able to ensure a blind test when the subject has to provide part of the experimental ‘material’. A quick brainstorm produced ideas ranging from a complex system of tubes connecting multiple subjects randomly, to bags of gas and other such trickery. The mind boggles.

Conclusion: Why Dutch Ovens Work and Your Brew Smells Better

The effect is real, though not everyone will experience it all the time. It’s most likely a combination of real and psychological effects. Your sense of smell ‘gets used’ to your own chemical mixtures (olfactory fatigue). You’re also more prepared for your own bombs, and feel less affronted by their mere presence. Plus, you’re innately less suspicious of your own gases, because they pose less of a disease risk.

There you have it: the stinkiest blog post I’ve made yet! Feel free to contribute your own hypotheses, experiences and puns on this one. Your inner child is begging to be let loose.
Stevenson, R., & Repacholi, B. (2005). Does the source of an interpersonal odour affect disgust? A disease risk model and its alternatives European Journal of Social Psychology, 35 (3), 375-401 DOI: 10.1002/ejsp.263

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Categories: Problems, Puzzles, Science | Tags: , , , , | 70 Comments

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70 thoughts on “It’s science: “My farts smell better than yours!”

  1. LOL This was excellent! Still trying digest your post – may come back later and comment again.

    So are you taking requests for blog topics now? I ask because I’ve been asked to write about something which I don’t feel I’m really in a position to discuss properly. Would love to pass it onto you! :-)

    • Well, it was mainly a perfect excuse to procrastinate from revising for exams. If it becomes a habit I might be in danger of failure! You should give it a go, unless it’s super serious and way outside your field…

      • The person asked me to write something about funding for scientific research – where does it come from, does private funding compromise research etc. I did a search of your site and see that you did write something back in March (how did I miss it?!) about that. I may still write my thoughts about it (been a while since I was in scientific research though), and then refer to your post, if that’s okay with you?

        I saw a comment you made on one of The Unwitting Traveller’s posts about slaving over certain posts and not getting much reaction. Think it’s like that for all of us, but maybe especially for you as you (normally!) write about more serious science topics. Just speaking for myself, I sometimes find it difficult to keep up with reading posts, and if it’s a more in-depth topic, will need to think about it before responding . . . and then I forget to!

        Good luck with your exams!

      • Cheers Lisa. Yes, we had to write an essay for a Policy module on that very topic. I chose a fairly broad view. Course-mate Anna also has an essay on it:

        Which focuses more on bread. (!?)

        And yes, my drafts folder is littered with good ideas and deep thoughts that have never made it all the way through. There’s so many interesting things out there to see and digest on the ‘net…

      • Thanks for the link!

      • Anonymous

        Any comment on when you smell a fart you are breathing anal vapors and butt particulets ?

  2. While we are onthis subject, why does your pee stink immediately after eating asparagus?

    • Ah, you’re going to have to do some research and answer that one yourself! I look forward to the explanation. I don’t eat asparagus often, myself.

    • Asparagus contains asparagusic acid which is broken down into volatile (smelly) chemicals released in the urine from 30 minutes after eating. These are: methanthiol, dimethyl sulfide, dimethyl disulfide, bis(methylthio)methane, dimethyl sulfoxide and dimethyl sulfone. Fascinating…
      More interestingly, only some people produce smelly wee. And only some people can smell these chemicals. So, some people may have asparagus-ey wee and not be able to smell it. Other people may never be able to make it, no matter how much asparagus they eat!
      No one’s sure why this is, but its likely to be due to genetic variations in individuals that cause some people break down asparagusic acid slightly differently. Other people have a subtly different ‘smelling spectrum’ because of their genetics!

  3. Lillith

    A friend of mine was sitting in a doctor-s office. He was very involved in the subject. Anyway, he raised up and “let one rip” (loud) He immediately looked at the woman
    sitting next to him! Talk about IF LOOKS COULD KILL! lol

    • Jeff

      I saw something similar also in a Dr.’s office. After the man erupted with a brassy fart he looked for eye contact and announced, “top that!”

  4. Lillith

    My very science minded cousin is planning to read here. Sure hope today is NOT the day! Dave, u r a little stinker!

  5. Interesting stuff. I’ve often pondered this question but I think you nailed it with:
    1. You’re used to the smell of your own brew
    From my reading about faecal transplants (Google it – I know, it sounds gross) I found out that each individual has a unique set of intestinal bacteria (which produce gas). People who live in the same family also tend have similar bacterial ‘flora’. Could this explain why each person has their own special brew?
    Could it also explain how my wife KNOWS when it’s me who’s farted…?

    • You’d probably know more than I do, but the gut flora would definitely play a big role, along with diet. Familiarity not just from closeness but also shared bacterial communities, digesting the same meals!

      There was a flurry of media around ‘bacteria type’ a week or so ago, suggesting there are very distinctive groups (as with blood types). I wonder if there are between-stranger differences mediated by ‘gut type’?

      • Genius! Perhaps you could tell if someone shared your ‘bacteria type’ by whether you enjoyed the smell of their fart? Maybe dating agencies could extend their matching systems to – ‘do you like the smell of this fart?’
        Aha! And I guess that’s why dogs spend so much time smelling each other’s backsides…

      • Well, there’s the MHC and its supposed influences on body odour and attractiveness… perhaps gut flora has a similar effect! I like it. Let’s break down the stigma. Or…

        … not.

  6. It’s true then, you really are what you eat. I’ll have to remember to add better scent appealing culinary essential oils to my cooking. Great post! :)

  7. Jess

    I thought origin was a big factor in determining how you perceive a smell? Like, get someone to smell a compound, and they will tend to rate it as more or less disgusting depending on where you tell them it was sourced (e.g. ‘a chunk of cheese’ vs ‘someone’s sweaty sneakers’). There’s a key word I can’t think of here, meaning essentially ‘origin’ or ‘source’, when I think of it I will probably be able to link to something meaningful. There was a doco about it, where they constantly used the euphemism ‘roses’…
    Great post!!

  8. LOL! If this is the type of research taxpayer’s money is paying for, I want my money back! ^_^
    Something to ponder — vegetarians’ farts don’t smell as ‘badly’ as of those who eat meat. Meat products putrify and fester, especially if one doesn’t evacuate the bowel as regularly as one should, ie every morning. Vegetables don’t putrify so much, hence much pleasanter smells are emitted. Become a vegetarian, if you wish to retain your friends, marriage partners, etc!

    • Haha… it might depend on what veggies they’re eating though, because according to some of the stuff I looked at, cabbage/sauerkraut was particularly dangerous!

      • Oohh, forgot about that one, ‘tho sauerkraut doesn’t have that effect on me as I was brought up on it. However, now that you mention it, chickpeas are notoriously fart-producing and very quickly too! Avoid them wherever possible. ^_^

    • Kenya W.

      Meat farts can definitely be gross, and in the lifting community we joke about the farts you get after a particularly potent protein shake. But vegetarians tend to get a lot of fiber. Makes for a healthy bowel but I definitely passed way more gas when I was a vegetarian. And it was pretty uh, rich.

      On a side note, my partner is not a very gassy person unless he takes certain medications. But the farts he passes on his sleep are oddly not bad smelling. Maybe the compatibility based on similar gut flora isn’t that farfetched after all. Hmmmm

    • Anonymous

      I love all farts

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  10. I like to drink peppermint tea..and use some great smelling essential oils in my food and drink…helps to alleviate the odors….

    Funny post … we do get used to our own smells..

  11. Here’s a theory I really like: your own farts are smell-memory linked to a sensation of relief. So in the same way that the lingering smell of a lover in your bed can be arousing days later, fart smells are bound up with thousands of occurrences of one particular sensation.

  12. man to doctor… my farts don’t smell any more
    Doc… can you give me a sample
    Man… sure, and lets one rip
    Doc puts poker in fire till red hot
    Man… hey Doc you ain’t gonna put that up my ass are you
    Doc… NO… its for your nose

  13. Pingback: My 5 best posts of 2011 « David Robertson

  14. carl doehring

    The only way to truly tell, if possible, would be to somehow be able to preserve farts in their original state; extract a fart from the subject without having him smell it, and see if he can identify it amongst others.

    • You’re right, a blinded experiment would be the only way, but strangely enough no scientist has either been motivated enough or given permission to do the test! I wouldn’t like to be the subject in the experiment either…

  15. Shannon

    I’m so glad you posted this blog. I was just proposing the question “Is it biological or environmental aversion?” and this was the best resource I could find. Thank you!

  16. I agree about getting used to it. A lot of what you said makes sense. I’ve always thought of it as something like when cats mark their territory. They smell their own scent, and they’re like, “Oh, it’s okay, this is ME. This is mine. It’s okay.” But they smell that other cat from down the street, and it’s all, “Oh HELL no! Some other cat is in my zone! This smells bad!”

  17. Jake the snake

    I believe it’s much deeper than what is said in this article. Olfactory sense is the most primitive of all, I believe, and thus most closely connected to the primitive parts of the brain. I believe the body uses flatulence to check on its condition and respond accordingly, on various levels to the sample. It’s a test.

  18. dil

    In an evolutionary sense, perhaps we are configured to like our own scent and feel comfortable around it. Whereas if others enter our territory they are repulsed. Perhaps survival strategies to keep enemies/ the unwanted away om

  19. Amanda

    Just a little addition of information and experience:: mama’s like their own baby’s poo smell more than other baby’s poo smells. I breast feed, so my son and I essentially share the same food. I’ve found our gas smells extremely similar… and I’m not put off by it at all:: in addition to this, I can tell when my son has a dirty diaper in a room of babies.

    Another thought:: maybe, when nomadic people ate something that produced bad smelling gas, it gave them incentive for moving onto a new site (and by and by: a new set of food less likely to contain bad smelling fart causers).

  20. Ryan in L.A.

    Although most people won’t talk about it, this is a very interesting subject from a scientific standpoint. Unfortunately, the article doesn’t really answer the question.

    The reason you like the smell of your own flatulence (or poop, for that matter) is rooted in sense-memory and it goes back to your earliest days on Earth. As a tiny baby, you associate the smell with relief from the expulsion of gas and the coinciding relaxation of muscles. The sensation is one of the very first things you experience, and it becomes hard-wired into your subconscious and never changes.

    The topic is interesting because it demonstrates the power of early childhood memories and how they can affect the rest of your life in ways you do not readily understand.

    • jkautz43

      You are offering up a Classical Conditioning explanation. The “relief” you mention occurs whether their is a smell or not. Moreover, evolutionarily, such an infancy bound explanation serves no purpose in reproduction (passing genes) without further development. The argument below for marking territory is (pleasant=mine. Bad=foreign) is compelling and might dovetail with your argument. Maybe u r onto something….

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  22. On a tangent to the main topic: I’m particularly impressed by your use of the ‘random article‘ feature as a way for the reader who arrives but would rather not proceed with the chosen topic. Doubly clever, as it both recognises what many content creators fight against (you have no control over where your visitors go), while at the same time offering an ‘exit’ that gives you a chance of retaining your audience, albeit elsewhere.

    I think I might steal that :)

  23. percy

    my guess would be an evolutionary trait from when we needed to mark our territory and routes with pee and poop,…. smells good must be mine so im going the right way home or smells bad thats not me, better get out of here!!

  24. Parker

    I would be interested to see a scientific experiment that highlights what parts of the brain are activated when smelling own vs. some one else’s farts and comparing it to other smells… (Fresh bread, rotting food etc etc)

  25. Hmm is anyone else encountering problems with the pictures on this blog loading?
    I’m trying to determine if its a problem on my end or if it’s the blog.
    Any responses would be greatly appreciated.

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  28. Ghkphg

    So, my initial guess would be that when we are babies, we smell our waste and when we do, we relate that to a sense of ‘relief’ thus, over time, we relate our smell to a good thing.

  29. jkautz43

    Totally wrong. Your controls are meaningless and you leave out obvious cases that don’t conform. I applaud you for taking on the subject. I suspect that our “old brain” uses cues in our gas and in others to judge fitness and health. Diarrhea gas has special notes peculiar to that issue. We would tend to avoid partners pooting that type out into the air. This is an important question that deserves real scrutiny. Why isn’t my cat bothered by even my worst gaseous offerings? I believe the root of the issue is lies in how our “old brain” (hypothal. Etc) parses out the ancient chemical messages our farts contain. A fart Rosetta Stone of sorts would be a real breakthrough in the annals of flatulence and gut health science.

  30. Peter Lindenmayer

    Interesting set of comments, however there’s another perspective that seems to have been missed. I suspect the distaste we feel for other people’s farts comes from knowing where they came from and feeling (more) disgust with other’s bowels than our own.

    For example certain marshy areas sometimes produce smells that resemble farts, and I’ve noticed that I find them less offensive when driving past than I would if someone else in the car had emitted a similar odour.
    A test might be to give subjects a range of smells with varying apparent origins and see if there was a correlation between disgust and smell or disgust and alleged origin.

  31. Anonymous

    Hey all,

    great article (in terms of people not trolling constantly, like on some of the other posts I’ve read), but I have a few pieces of input. The author also misses the tendency for couples to be eating the same foods, and therefore producing very similar smells. Granted, I would rather smell my girlfriend’s farts than some stranger’s, but I am still disgusted by the smell of her nasty ass (which incidentally is lighting off horrible cheese/wine farts at the moment – hence my interest in the subject). The farts, however, are very similar to ones that I have lit off in the past. This may lend to the evolutionary point of view, in which it is a natural response to avoid disease (eating another’s feces can kill you, since they may have a very different bacterial ecosystem living in their guts).

    Nevertheless, she thinks the whole ordeal is hilarious, and I think that aspect is very psychological. She really is prepared for the rancid smell, and since she is at the epicenter, her nose is the first to become used to the smell, and therefore, she can sit back and enjoy the disgust inflicted on others.

  32. ed

    I think that when l appreciated that a fart was essentially a sh&t particle in the air it made things worse. I was actually ingesting, in small part, the foetid bi product of someone else’s waste system. We become so used to our own waste that it would be neurotic not to become, at least, comfortable with it – if you become comfortable with something then you can grow to like it. Farts smell as good or bad as a hamburger or bacon on a grill. The problem is if it is someone else’s dirty grill.

  33. Alex

    What about when old people fart and don’t even notice and then get a whiff of it? (provided their olfactory receptors still function properly) Does the bias still remain?

  34. Pingback: Things You Should Know About Farts : AweSci

  35. Doug

    There was an episode of Myth Busters were they rigged up with system where one of them could fart sitting in a bathtub full of water, and then they “captured” the fart when the air bubbles raised to the surface. They used the fart to try to determine what made the smell dissipate fastest by testing things like burning a candle and using air freshener. If anyone would be willing to do this experiment it would definitely be them.

  36. Anonymous

    I think there is another evolutionary reason. All secretions of the body give you information about your health status. Checking their color, odor, etc. is a very simple way to find out if somethings wrong. For example people instinctively look into their handkerchiefs after they blow their nose, to see if the snot is bloody. Or when you scratch your head you probably will smell your fingernails after doing so. You only have to think of medicine: First thing they do is checking your blood, your poo, your urine,…

    • Anonymous

      That is it, exactly. It is why we prefer our own, it tells us about our health status. Mothers sometimes say they “like” the smell of their babies doodoo, if it smells healthy. Toxins in the body are released through the GI tract. anyone with a massive hangover taking a dump will testify with all their heart and pounding head. Their doo smells worse when they’re sick, in this case, self-inflicted. This is a quick check, like when people look at their business before flushing. They’re seeing if the colour and texture are healthy. I’m surprised more comedic acts do not include this human tendency, it’s actually interesting and pretty damn funny all at the same time.

  37. Dr. DCJ

    Dude, you have tackled _the_ big question in neuroscience. FWIW, I favor a Bayesian scenario with a strong prior on “liking” the smell of your own flatus. In order to dislike the smell of your own, it must be profoundly epic in order to shift perception to the “unpleasant’ category. There is only one way to find out: a carefully controlled, double-blind experiment where people are exposed to odours emanating from unknown sources.

  38. Pingback: Book Review: On “The Origin of Feces” by David Waltner-Toews | cats and squirrels and other important things...

  39. george

    I scoop-cup mine and send em away into unknown territories. :-D

  40. Anonymous

    i had my own theory on the evolution aspect. the physical act of defecation is made easier by proper breathing. being better able to tolerate your own smell meant that you would finish sooner and reduce the risk of being eaten by a tiger, thus increasing your odds of breeding.

    only half joking. :p

  41. Great discussion! Is no one going to mention the humor aspect, or should I say, ass specks? I’ve traveled the world, and I believe for a wide range of cultures, farts are a source of not only foul smells but great mirth. Then, there is the pride factor involved, when one releases the infamous “silent but deadly” variety and quietly waits for others in the area to unexpectedly encounter the fetid odor. Just how wrong is it for Uncle Charlie to tell a 4 year old to pull his finger? Why is it so hard not to laugh when that happens? When our dog farts and then looks around, we all laugh! When grandma sneezes and simultaneously farts, we refer to that (in our family) as “achoops!” What a rich topic!

  42. mary smith

    What about eating habits? It is the food you eat and if one person hates onions while one person loves them it could make a big difference. You are what you eat and your farts are from what you eat.

  43. Pingback: Why Do We Sometimes Like To Smell Stinky Things? – Phenomena: Gory Details

  44. Anonymous

    Thank you for this. I am going to add some additional subjective observations because I believe a similar mechanism is at work. Forgive me for being crude, but I’ve noticed that I rarely notice an unpleasant odor when I have a bowel movement. I’m sure there is one, I’m just unaware of it, as I believe that many people are, in the majority of cases. However, and this is where it gets interesting, if I have to return to ‘the scene of the crime’ within a short period of time, I immediately notice that it seems much worse. In my mind, on the return visit I’m getting something very close to the actual scent, while my ability to perceive it is blunted while in progress. It’s as if my brain has turned of my ability to perceive it objectively during the act, but forgets it’s mine if I step away for a short period of time… Would love to see more investigation, not because I’m fascinated by the subject, but the ability of the mind to block out or blunt a sense is intriguing.

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