A while back we had a cat. It originally belonged to Eric Clapton and we inherited it through a friend of a friend, and it was a nice cat, black and white with long hair and it was called Oswald, and we loved him and sometimes he loved us, with the indifference that cats are very good at.
Oswald grew old and in the last months of my marriage, while I was still living with him, he became incontinent. And he began to poo in the bedroom. There is nothing quite like runny cat poo: it is the vilest smell you can imagine. So bad was the smell that if he pooed while I was sleeping it would wake me up. It is worse than changing nappy smell, which is bad enough that you never get used to it, and it takes your breath away, and leaves you in no doubt of the correct functioning of your olfactory senses.
While I was researching my most recent book, Flawless, I though quite a lot about bad smells. Some aromas are appealing; some are aversive. Some we grow to like, while others never grow on us at all. And some smells can be aversive or nice, depending on the context.
There is a phenomenon in psychology known as mere exposure. This is where you grow to like the familiar. It applies in smell: the more familiar the odour, the more we tend to rate it as pleasant. But it only applies up to a point, and this seems to be because of the function of our sense of smell. For bad smells, there is no mere exposure effect. The explanation for this could well be an evolutionary one.
Our chemical senses have a number of functions. The first, is to steer us toward rewarding foodstuffs and away from non-foods. In this case, it’s the sense of flavour, which is multimodal, that is active, combining taste, smell, touch and vision (and to a lesser extent, hearing), plus input from our own internal state (are we hungry? What is our body clock doing? Are we experiencing sensory-specific satiety?). A lot of gastronomy can be explained from a biological point of view. The learning-to-like that is recognized by the concept ‘mere exposure’ is interesting here.
Another function of our chemical senses is more related to the sense of smell on its own, and that is to give us useful information about the environment we are in, and to steer us away from danger. This is where bad smells (malodours) come in. We can find these incredibly aversive. The fecally incontinent cat is a good example.
Consider the evolutionary adaptive environment when we humans were subject to strong natural selection, and where human behaviour was being shaped. The considerable inconvenience of moving some way aware from a residential setting in order to poo would have been strongly influenced by the aversive smell of human faeces. There’s a good evolutionary reason why our olfactory systems should be sensitive to the smell of poo, because of the significant health risk involved. Look at the big Medieval cities with bad sewers: they smelt bad and there were major public health issues. So human olfactory systems have become attuned to certain signature compounds of faeces, such as volatile sulfur compunds like hydrogen suflide, and can detect them at very low levels, and label them as malodours. It is important that we never become accustomed to these, or grow to like them.
These volatile sulfur compounds are produced by microbes in anaerobic environments such as the gut, or sewers, and we don’t like them. Interestingly, this relates to wine, too: microbially produced volatile sulfur compounds cause the wine fault commonly known as reduction. Sticking with the wine theme, the musty taint associated with cork taint is strongly aversive. It’s a similar smell to that obtained by sniffing mouldy bread. While not as intense as the poo smell, it says ‘stop, you don’t want to eat this.’ Fungal contaminants can produce some fairly toxic compounds, so a whiff of this musty smell acts as a deterrent. And we are incredibly sensitive to the compounds involved in cork taint, which suggests some adaptive utility for being able to detect them, even though in the case of corked wines there are no health risks.
[This raises an important point. From a biological point of view, a chemical structure has no ‘smell’. Chemicals smell because our olfactory system has evolved to detect biologically relevant chemicals. We confer smells on chemicals, and there will be some that aren’t biologically significant that we can smell as an artefact of having a system able to detect the significant ones. There are lots of chemical structures that our olfactory systems don’t impart a smell to. But, perhaps there is an adaptive advantage in being able to smell lots of things which aren’t biologically significant now but could be in future contexts?]
Interestingly, we seem to be able to reclassify nice or harmless smells (for which we could experience the mere exposure effect) into malodours because of co-occurrence. Any student who has got drunk on high strength cider and been quite ill will probably be able to tell you about how they have reclassified the smell of cider from ‘nice’ to a malodour. Our olfactory systems work in tandem with memory to enable the exploration of novel foodstuffs and then allow us to grow to like, or grow to dislike certain aromas as a sort of defence mechanism to prevent us repeating bad experiences.
And then there’s a group of smells that can be considered pleasant in one context, and alarming in others. Take wood smoke. If you a sitting in the evening in a country pub, and there’s the smell of the open fire, it’s tremendously comforting and reassuring. But if you are out in a Eucalyptus forest on a hot summer day and you smell wood smoke, this can be quite alarming. Exactly the same olfactory experience can lead to entirely different internal states dependent on context.
So it seems that we can make a rudimentary classification of smells. There are some that are intrinsically nice smells. There are others which can grow on us, and over time we can find them nice. There are smells that we like in one context, but not in another, and there are those that we grow to dislike because of experience. And then there is this group of smells that we hate, that we can never grow to like or ignore, and which remind us forcefully that our sense of smell is still working just fine.