The valence of a smell thought according to the theory of evolution

«It is not the strongest of the species that survives, nor the most intelligent, but the one most  adaptable to change.» 

This citation was attributed to Charles Darwin and to his evolutionary theory. Indeed,  Darwin explained the mechanism of natural selection, a key mechanism of evolution that postulates that generations are formed according to the characteristics of the most fit individuals to survive and reproduce in previous generations. For example, we can imagine that between a species of cat that has a good sense of smell and knows how to use it to react to potential dangers and another species that does not know how to use its sense of smell, the first one will have a better chance to survive and reproduce. Indeed, the cat that uses its sense of smell could more easily detect a potential predator or find food or even more, choose the best partner in order to reproduce. This selection will lead to the disappearance of the anosmic cats and the multiplication of the number of cats with good smelling abilities. 

Chemical senses, such as smell, are essential and are involved in a wide variety of adaptive behaviors (e.g., sexual partner selection, protection from potential predators, or foraging). Olfaction is a sense that is often difficult to express : unless we are professional, we don’t always know how  to describe what we are smelling and struggle to communicate about the odors surrounding us. To counterbalance our lack of vocabulary, odors are generally evoked according to four aspects: their hedonic perception (whether the odor is pleasant or unpleasant), their intensity (whether the odor is strong or weak), their familiarity (whether I've smelled this odor before or not) and their effect on our body (whether the odor relaxes us or activates us). Among these dimensions, the hedonic  perception is most associated to the perception of an odor (Yeshurun & Sobel, 2010). When an  individual smells a scent, he or she may usually think, «Oh, that smells good!» or, conversely, «I don't like that smell». In psychology, this pleasant or unpleasant character attributed to a smell is known as valence. An odor that attracts and pleases us has a positive valence, while an odor that has a repulsive effect and that we don’t like has a negative valence.  

Research in cognitive psychology has shown that when the brain processes an odor,  physiological and behavioral levels differs according to the valence of the odor. For instance, a recent EEG study demonstrates a difference in the temporality of odor processing depending on whether the odor was judged as positive or negative (Iravani et al., 2021). Brain processing of the odor would occur in 2 stages: one short and one long. In this study, it was shown that negative odors were mostly processed over a short period of time, whereas the effect of positive odors occurred later and was of longer duration. The hypothesis underlying these results would be that the processing of a negative odor requires rapidity in order to prepare a motor response in case of danger (avoidance response). On the contrary, a positive odor does not require a rapid response and would be processed over a long period of time in order to allow the pleasure that we feel when we smell this odorant to last over time. 

At the behavioral level, negative odors are known to generate faster motor responses than neutral or positive odors (Bensafi et al., 2003). We could imagine that negative and positive odors are processed in two different systems but this theory remains unlikely and has not been confirmed. On the other hand, another theory explaining these results would be that negative odors trigger a defense system (to protect oneself from the associated negative event). This theory can be defended since it has been proven that negative odors lead to a stronger activation of the amygdala, a brain structure that plays a critical role in the fear conditioning and defense system (Zald & Pardo, 1997).  This speed difference in the processing of negative odors is also found for food odors. Orange or fish odors, indicative of the presence of food, elicit faster behavioral responses than rose or dirty sock odors (Boesveldt et al., 2010). However, the fish odor, rated as negative, results in an even faster response time than for the orange odor, rated as positive.

From an evolutionary perspective, it makes sense to imagine that food odors implement a rapid response; feeding being essential for human survival. Ingesting a food that smells bad or is out of date could, on the other hand, be dangerous for humans and would require the rapid avoidance of contact with such food. The difference between the effects that positive and negative odors can have on behavior and brain activity would, from an evolutionary point of view, come from the use of these odors as potential signals of danger (quick and succinct reaction) or pleasure (slow and  prolonged reaction). 

This evolutionary perspective may explain why children between five and eight years old  across 18 different country make similar evaluation of the valence of 17 odors (Oleszkiewicz et al.,  2022). It has indeed been showed that the perception of odor valence is independent of cultural factors (Arshamian et al., 2022). Knowing that some odors can elicit in all people the same response is essential for cognitive psychology. It could help us to detect, to choose and to use these kind of odors in research. In everyday life, these odors could be helpful to elicit behaviors : use of negative odors to help someone move faster, use of a positive odors to boost the productivity or to make someone feel at ease. However, still a lot of work needs to be pursue on detecting which odors are best to elicit different behaviors.  

References  

Bensafi, M., Rouby, C., Farget, V., Bertrand, B., Vigouroux, M., & Holley, A. (2003). Perceptual,  affective, and cognitive judgments of odors: pleasantness and handedness effects. Brain and  Cognition, 51(3), 270-275. 

Boesveldt, S., Frasnelli, J., Gordon, A. R., & Lundström, J. N. (2010). The fish is bad: negative food  odors elicit faster and more accurate reactions than other odors. Biological psychology, 84(2),  313-317. 

Iravani, B., Schaefer, M., Wilson, D. A., Arshamian, A., & Lundström, J. N. (2021). The human  olfactory bulb processes odor valence representation and cues motor avoidance  behavior. Proceedings of the National Academy of Sciences, 118(42), e2101209118.

Yeshurun, Y., & Sobel, N. (2010). An odor is not worth a thousand words: from multidimensional  odors to unidimensional odor objects. Annual review of psychology, 61, 219-241.

Zald, D. H., & Pardo, J. V. (1997). Emotion, olfaction, and the human amygdala: amygdala  activation during aversive olfactory stimulation. Proceedings of the National Academy of  Sciences, 94(8), 4119-4124.