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Olfaction, which is the sense of smell, plays a significant role in many aspects of human life, ranging from everyday human behaviors and interpersonal communication to being a diagnostic tool for various pathologies. Ever since scientists began studying olfaction at the turn of the 20th century, they have been interested in how the sense of smell differs between the two sexes.
The importance of smell
Between 1997 and 2017, the publication of scientific studies on the olfactory system has increased by more than three-fold. This rapid growth rate is largely due to three different discoveries about the significance of the olfactory system, the first of which is the identification of olfactory nervous systems to exist throughout the central nervous system (CNS).
Second, several studies have found that a loss of smell is an early marker of several neurodegenerative diseases, most notably of which include Alzheimer’s disease and Parkinson’s disease.
The third discovery that has increased the attention on the olfactory system is the finding that sex effects on the olfactory system can determine the heterogeneity of certain psychotic disorders.
How is olfactory performance measured?
Many scientific studies determine an individual’s olfactory performance by observing their olfactory identification, discrimination and/or odor detection thresholds. By definition, odor identification is the ability of an individual to recognize and identify a specific smell and is often assessed in either uncued or cued identification tasks.
An individual’s olfactory discrimination abilities are typically assessed by providing the subject with two or three different smells and subsequently asking the subject to determine whether two odors are different when a pair of odors is presented, or which of the three presented odors is different.
The third type of olfactory performance test is olfactory threshold, which can also be described as an individual’s overall smell sensitivity and is defined as the lowest concentration of an odorant that can be reliably detected by a subject.
The battle of the sexes
Some of the earliest studies that investigated sex differences and the human sense of olfaction determined that the odor detection, identification and discrimination abilities of women were superior to those of men.
The female superiority in olfactory abilities, which is most evident in odor identification assessments, appears to be consistent across virtually all tested age groups and cultures. This phenomenon remained supported during the 1990s after the “Smell Survey,” which was one of the largest olfactory endeavors to ever be conducted, as it involved over 1.5 million people.
Many different factors have been used to support the notion that females, on average, have superior olfactory skills, which include neuroendocrine agents, social and cognitive factors, as well as structural differences that have been assessed by functional magnetic resonance imaging (fMRI).
The role of neuroendocrine agents
One of the primary factors that has been proposed to be responsible for the female superiority in olfaction capabilities is neuroendocrine agents and their interaction with the olfactory system.
While circulating levels of gonadal hormones do not appear to have a direct effect on olfaction, direct links between these agents and the sense of smell have been recorded. More specifically, females appear to have the highest odor sensitivity to androstenone, which is an odorous steroid-structure pheromone that is secreted from the auxiliary region, and other similar musk odorants.
Many of the observed fluctuations in the olfactory sensitivity of women have been observed during their normal cycling events; however, these same fluctuations appear to remain consistent in women who are taking oral contraceptives.
This observation thus points to the possibility that ovarian hormones may not be the primary neuroendocrine factor responsible for this difference in olfaction capabilities. While this may be true, other studies have found that women who are in their late stages of pregnancy or those who have just received estrogen injections exhibit changes in their threshold level of olfactory sensitivity to certain stimuli.
The wide range of potential conclusions for how sex hormones play a role in olfactory function are complex and therefore require further analysis to determine the potential endocrine-related influences on smell perception.
Social factors
Differences in the olfactory performance of both sexes have been observed at every stage of life, beginning with newborn female babies who show more interest in olfactory cues as compared to their male counterparts. Increased odor awareness has also been found to vary between male and female children at an early age.
Many researchers believe that since many female-stereotyped activities such as cooking, cleaning and the use of cosmetic products are often encouraged from a very early age, female children have a greater long-term exposure to a wider variety of odors. This greater amount of exposure to odors has subsequently led many women to also exhibit an increased sensitivity to certain odorants as well.
Cognitive factors
Many of the olfactory assessment tasks that have been used to determine the superiority of the olfactory capabilities of females often involve the use of various verbal components. Since men are generally associated with having lower verbal skills than women, it has been postulated that these higher verbal skills might make it easier for women to determine the correct descriptions for odors during these assessments.
Health-related factors
Throughout the world, industrial jobs remain dominated by men. Since many of these industrial jobs either require workers to handle or indirectly expose them to a wide range of potentially harmful chemicals like soot and cadmium, olfactory impairment is often inevitable. However, this type of occupational exposure will typically only affect olfactory threshold tasks, as compared to olfactory identification or discrimination tests.
Another factor that might be responsible for the sex differences in olfactory capabilities is the fact that men generally age faster than females; therefore, olfactory abilities will deteriorate more rapidly in men.
Structural differences
To determine structural differences that might exist between men and women following odor-induced activation tests fMRI has been widely used. In a 2018 study, fMRI imaging was taken of thirty healthy subjects, of which included 17 women, who were stimulated with mint and butanol.
This study found that the most prominent sex differences appear in the right middle/superior temporal lobe, which is particularly active in olfactory processes. Aside from studies utilizing fMRI results, researchers have also analyzed the microcircuitry of the olfactory bulbs of men and women and determined that while the male olfactory bulb is typically less dense than those of women, women tend to have smaller olfactory bulbs.
In terms of the total inhaled volume capacity through the nose, it appears that while women tend to have smaller nose openings, the intranasal volume of women does not appear to be any different than that of men. While some small differences between men and women have been observed, the results have not been uniform and must therefore be studied further in order to make a definitive conclusion.
References
- Sorokowski, P., Kawowski, M., Misiak, M., et al. (2019). Sex Differences in Human Olfaction: A Meta-Analysis. Frontiers in Psychology. doi:10.3389/fpsyg.2019.00242.
- Melero, H., Borromeo, S., Cristobal-Huerta, A., et al. (2018). Sex Differences in the Olfactory System: a Functional MRI study. Chemosensory Perception 12; 50-58. doi:10.1007/s12078-018-9250-1.
- Nováková, L., Valentova, J. V., & Havlíček, J. (2014). Engagement in Olfaction-Related Activities is Associated with the Ability of Odor Identification and Odor Awareness. Chemosensory Perception 7;’ 56-67. doi:10.1007/s12078-014-9167-2.
- Verhaeghe, J., Gheysen, R., & Enzlin, P. (2013). Pheromones and their effect on women’s mood and sexuality. Facts, Views & Vision Issues in Obstetrics, Gynaecology and Reproductive Health 5(3); 189-195. PMID: 24753944.
Further Reading
Source: Sex Differences and Smell
