Listening to a recent episode of the wonderful radio show Perfume on the Radio, discussing the ability of dogs to detect and signal very specific scents when trained—from diseases to explosives to cadavers—got me wondering whether humans, also, had some reward system that fed into our tendency to point out certain smells more than others. For example, commenting that someone’s cooking smelled good might make them happy and more prone to offer you food. On a more juvenile level, calling out that someone farted could help you bond with other presumed non-farters over a few giggles while asserting momentary superiority over the embarrassed offender. Otherwise, with the exception of interesting, polarizing, or potentially dangerous smells, many scents get ignored routinely.
I haven’t found any research on this topic. Most studies of human brain reward circuits and smell relate to food and eating disorders, which have far more consequential implications. However, work has been done to show that people’s associations and judgment of whether a smell is “good” or “bad” are learned, not intrinsic. We may think a scent is pleasing based on our own positive experience with the source, or because someone told us it is when we were very young.
Or perhaps not so young, if the subject is not one for or against which we’ve already formed an opinion or bias. When I was in my early twenties, a friend told me that he loved to bury his nose in the ears of his dog, a very large Newfoundland mix. The ears and the crevices of the paws. It had never occurred to me before to do such a thing, and I thought it was very strange, but eventually I tried it. The inside of a dog’s (non-infected) ear smells quite rich and ripe—very warm and animal. I’m not enamored with it the way my friend was, but it’s pleasant because of the positive mental associations I’ve made with this idea and because I love dogs.
Interestingly, different regions of the brain are involved when we need to make “decisions” about a smell versus allowing our perception of pleasantness/unpleasantness to simply be. In this study, when participants were asked to compare 2 scents and decide which was more pleasant or more intense, activity was higher in the medial prefrontal cortex area 10 (medial PFC). When they were given the same 2 scents and asked to rate the pleasantness and intensity of each, without comparing them, the mid orbitofrontal cortex (OFC) was more engaged.
Personally, I am all for experiencing smells and enjoying the ones I happen to like, without them contributing to any kind of decision fatigue! (Not that they are.) Neutrality may be one of the hardest things to learn.
Nose Prose 
It’s an interesting reading, thank you. But I have some doubts about the “learned” part: from my childhood I remember at least a couple of strong (mostly negative) reactions to smells without additional social clues or with those very similar/same to the ones that didn’t produce the same reaction. It has happened later in life as well, but those from the early years are harder to explain by any social or previous experience bios. But of course I agree that a potentially neutral scent can be “colored” in our brain as more positive or negative based on a suggestion (explicit or implicit). And even a scent previously loved and considered as pleasant might get a strong negative connotation if associated with something unpleasant or traumatic.
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Completely agree with that last part, which leads to the behavior of avoiding using beloved scents in stressful situations. I wish I could remember smells I disliked that didn’t have the reinforcement of popular opinion. I may have liked certain smells that other people would consider “gross” by association. It took a while for me to even get used to the idea that smell preferences were different between people—the first time a coworker told me she liked the smell of marker pens, I was unreasonably surprised because to me, it always smelled “toxic.” Now I realize it’s not that uncommon.
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