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Key points
- Animals rely heavily on the sense of smell. But what about humans?
- Research on olfactory chemical communication has been growing, and the results are fascinating.
- Contrary to myth, human pheromones have not yet been detected.
- Contrary to myth, women who reside together do not synchronize their menstrual cycles.
Many animals, mammals included, are heavily reliant on their sense of smell in dealing with the world. Yet people often assume that this is not the case with humans.
Indeed, research on how smell factors in people’s judgments, decisions, and behaviors has lagged behind research on other senses like sight, touch, and hearing. This fact is surprising given that, as German researcher Helene Loos and colleagues write, “the human body is richly endowed with apocrine and sebaceous glands that produce hundreds of chemical compounds in individually variable amounts, bestowing each individual with a unique chemical signature.”
Our understanding of smell has advanced of late. A fascinating recent article (2023) by Loos and a team of European scientists sought to summarize the growing body of work exploring the effects of smell in humans. In the process, the authors debunk several myths about olfaction.
The first myth is that the human sense of smell is underdeveloped and unimportant. Indeed, the authors cite work that has shown how human olfaction is a powerful and sensitive sense. For example, research has shown that adult partners or child peers can recognize each other solely based on their body odors; mothers can likewise identify their infants (this ability is hindered in depressed mothers). Neonate infants have been shown to orient to their mother’s breast based on smell.
Moreover, research has found that volatile emissions (molecules in the gas phase emitted from a given source that may reach a receiving organism through the air) from multiple sites of the human body “are chemically differentiable along perceptually valid social dimensions such as age, sex, individuality, and health.” In other words, we can reliably distinguish old from young, men from women, sick from healthy by smell.
Third, “although the most impactful compounds in these chemical signatures still need to be identified, they were shown to be clearly perceivable and learnable and to bear multiple meanings for receivers.”
Humans have been shown to reliably recognize their own body odors as well as those of their kin and friends. In fact, research has suggested that persons who experience a "click" feeling upon meeting “smelled more alike than expected by chance.” Similarity in body odor was also “correlated with better social interaction between strangers.”
Research using dual-EEG recording of a mother-infant (versus stranger-infant) interactions has shown that the higher inter-brain mother-infant synchrony disappeared when the mother’s odor was brought into the the stranger-infant interaction. Moreover, the typical fear response to the presence of strangers disappears with the introduction of their own mother’s odor. The mother’s odor, in other words, appears to be more essential than previously believed to the formation of healthy attachment.
In addition, contemporary research has demonstrated how olfaction is involved in human mate selection and romantic relations. Men, for example, perceive female body odors as most attractive during the fertile phase of the women's menstrual cycle. Preference for spousal odor has been shown to positively correlate with relationship duration.
The authors discuss the role of smell in the priming individuals for certain adaptive emotional responses. Individuals with high levels of social anxiety, for example, show “more pronounced withdrawal-related behavior (startle reflex) when exposed to anxiety-related sweat samples than individuals with low levels of social anxiety.” Moreover, individuals with high social anxiety and those with a history of childhood maltreatment “show an altered responsiveness to anxiety-related sweat samples.”
Human beings, in sum, are capable of subtle and substantial chemical communication via the sense of smell.
The second myth is that human "pheromones" are active in controlling human mating behavior. Pheromones are defined as “substances which are secreted to the outside by an individual and received by a second individual of the same species, in which they release a specific reaction, for example, a definite behavior or a developmental process.” Pheromones have been identified in many species, and the search for the human version has been intense, buoyed in part by studies of so-called menstrual synchrony: the reported tendency of the menstrual cycles to synchronize in groups of women who reside together. Such synchronicity was thought to have been caused by pheromones.
Alas, while widely circulated, this notion is inaccurate. In fact, multiple studies have tried and failed to replicate the synchronicity effect. And the effect, when found, is often explained best by simple chance, given the inherent overlap in women’s cycles.
The search for human pheromones beyond this famous finding has likewise come to naught. The authors conclude: “The quest for a human pheromone still remains unfulfilled. No compound(s) in the human volatilome has yet been isolated and ascertained to unconditionally and reliably release stereotyped behaviors or to prime physiological responses.”
In sum, research has established that the human sense of smell is in fact highly developed. We know that one’s emitted airborne volatile compounds are readily detected and processed by others to myriad conscious and covert effects.
The authors conclude: “Smell is a potent influence on multiple life domains, including cooperation, mate choice, parenting, and emotional state.” They note that individual reactions to volatiles is shaped by multiple internal and external variables including personality, mental state, social context, expectations, and evolutionary preparedness. They further note that olfactory reactions are integrated with the other senses and may at times confirm, contradict, overrule—or be overruled by—inputs from the other senses.
Finally, volatiles are “processed in social and emotional brain areas rather than purely olfactory brain areas, carefully sculpted over the course of evolution but still malleable to present-day psychological factors.” In other words, as with our other senses, biology designs the hardware, yet psychology programs the software.
The challenges ahead for the field are considerable. For one, sorting through the multiple scents emitted by humans in varying degrees and combinations, and relating them to specific behavioral or bodily reactions is a daunting task. Moreover, the work on olfaction will need to remain sensitive to the influence of context and avoid rushing to generalize from small samples to the whole and from one culture to the next.
“Nevertheless,” write the authors, “major breakthroughs may be forthcoming because of the increasing availability of advanced techniques that enable the measurement of dynamic changes in volatiles and the identification of active compounds in normally behaving individuals.” The authors conjecture that the principles of chemical communication could, in the future, be used to “regulate individual emotional states, such as reducing anxiety and depression by specific olfactory safety cues.”
Aromatherapy, in other words, may at last become a real, empirically based therapy.
