Question: Why do farts smell bad?
Short answer: Microbial threats.
Long answer: We have previously shown that people find the sound of smelly farts to be slightly less funny than the sound of non-smelly farts. Nevertheless, on balance, most fart sounds are considered funny. This delightful aspect of farts is in contrast to their olfactory effects, which for most people are decidedly unpleasant.
In fact, work by Croy et al. (Int J Psychophys., 2013) shows that chemical compounds that smell like farts (hydrogen sulfide or H2S) elicit a negative emotional response, in contrast to compounds that smell like peaches or roses:
The authors of this paper also used electroencephalography (EEG) to measure brain activity in human subjects while they were exposed to the different odors. As most readers are likely aware, EEG measures electrical activity in the brain via electrodes that are attached to the scalp.
The EEG results revealed that fart-like smells elicited faster and stronger brain responses than pleasant odors.
This was consistent with behavioral data showing that unpleasant odors elicit stronger behavioral and autonomic responses (Bensafi et al., 2002). In other words, there is something very salient about fart smell. Why might this be?
The authors point out that fart-like smells might trigger a behavioral reaction that is meant to warn about microbial threats, by evoking disgust. That is, the unpleasantness of fart smell is likely a way of steering people away from dangerous germs that emerge from the gastrointestinal system, and it might be that this response has arisen through evolution. Indeed, aversion to excreta and decaying organic matter is common throughout the animal kingdom (Hart, 1990).
Of course, an evolutionary explanation for aversion to certain odors was first proposed by Charles Darwin, who wrote about it in his book, The Expression of Emotions in Man and Animals. Darwin himself was very flatulent, describing his symptoms as follows: “For 25 years extreme spasmodic daily & nightly flatulence: occasional vomiting, on two occasions prolonged during months.” His book featured pictures of people experiencing disgust, such as this one:
It is not known whether this subject was reacting to fart smell.
Darwin pointed out that the related emotion of contempt elicited similar reactions, which he characterized as follows: “The nose is often slightly contracted, so as partly to close the passage; and this is commonly accompanied by a slight snort or expiration. All these actions are the same with those which we employ when we perceive an offensive odour.” He further remarked that these reactions are the same everywhere in the world, consistent with an evolutionary origin.
At the same time, recent research has shown that aversion to fart-like smell is stronger in adults than in children, suggesting that some component of the response is learned (Stevenson et al., 2009). Thus, as with so many traits, the human response to fart smell is probably a mix of nature and nurture.
In discussing this research, we wondered why people aren’t more disgusted by the sound of farts. One possible explanation, as previously noted, is that the sound frequencies do not differ greatly between smelly and non-smelly farts, and in fact most farts really aren’t very smelly at all. So one explanation is that there is simply no reliable way to estimate the stinkiness of a fart from its sound.
To test this possibility, we turned to our database of more than 3000 farts. Most of these could be categorized as non-smelly (Type 1) or smelly (Type 2), based on the observed bimodal distribution of stink. We asked whether a three-layer neural network could discriminate Type 1 from Type 2 farts based only on their sound waveforms. Here are the results:
In a typical run, performance was around 60%, with 50% being chance. This is quite a bit worse than the performance of neural networks with similar architecture that successfully distinguished farts from non-farts and individual people based on the sound of their farts.
Thus we conclude that aversion to fart smell is likely caused by fear of microbial contamination, while emotional reactions to the sound of farts are more closely related to the humorous nature of the sounds themselves.
References
Bensafi, M., Rouby, C., et al., 2002. Influence of affective and cognitive judgments on autonomic parameters during inhalation of pleasant and unpleasant odors in humans. Neuroscience Letters 319 (3), 162–166.
Croy, I., Maboshe, W., and Hummel, T. (2013) Habituation effects of pleasant and unpleasant odors. International Journal of Psychophysiology, 88, 104 – 108.
Darwin, C. (1872). The expression of the emotions in man and animals. Chicago: University of Chicago Press.
Hart BJ. 1990. Behavioral adaptations to pathogens and parasites: five strategies. Neurosci Biobehav Rev. 14:273–294.
Stevenson RJ, Oaten M, Case TI, Repacholi BM, Wagland P. 2010. Children’s response to adult disgust elicitors: development and acquisition. Dev Psychol, 46, 165-177.