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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