The Scent of Love, Emotion, and Attraction

It isn’t uncommon to see marketing for fragrances and deodorants that do not show the product or its features. Depending on the intended audience, fragrance advertisements often allude to scantily-clad lovers in exotic destinations, dancing and prancing about while the fragrance bottle merely serves as a motif. Some adverts are more direct, claiming that using their product will equal more dates and increased “manliness” or “femininity”. At first, these pompous claims seem to have no value. However, there could be a physiological basis to their claims.

Enter pheromones, chemical signals used by species for communication. Found across the animal and plant kingdoms, pheromones are used to communicate identity, food, distress, territory, and sexual attraction. While many may believe that pheromone systems are exclusive to plants and animals, the discovery of an organ in humans similar to ones that drives pheromones in other animals suggests that we may also have the underpinnings of a pheromone-based communication system [1]. This is evidence that suggests that humans also have pheromones that are used for sexual attraction, individual identification, mate selection, emotions, and physiological changes [1].

Even though a single isolatable and synthesizable human pheromone has yet to be identified, popular culture has embellished these new ideas and applied them to interpersonal relationships, marketing, and even warfare. There exist so-called “pheromone parties” oriented around purported human sex pheromones that convey sexual attractiveness. Attendees wear t-shirts for numerous days prior to the event without washing and exchange t-shirts with other attendees for them to individually smell and see whose pheromones one fancies. Though bizarre, it is seen as a way to “smell out” a new date for many [1]. In 2007, the Ig Nobel Prize-winning “gay bomb” experiment by the United States Air Force sought to drop a sea of human sex pheromones over enemy forces in order to distract them, or even make them sexually attracted to each other [2]. Though it is hard to ignore the sex-fueled marketing messages put forth by fragrance manufacturers and it is even harder to ignore the frivolity of “gay bombs” and “pheromone parties,” the promise of finding and isolating human pheromones suggests a reimagining of human senses and how humans communicate.

The Underpinnings of a Pheromonal System

The usage of pheromones as communication systems has been observed across a wide set of non-human animal species. Speculation of a pheromone system in humans stems from the identification of the openings for the vomeronasal organ (VNO) in the nasal septum, the cartilage which separates the nostrils [3]. The vomeronasal organ, also known as Jacobson’s organ, is present in many non-human species and is a structural component for the pheromone system in organisms that have it. The VNO conducts nervous signals through axons to the olfactory bulb, the region of the forebrain involved in the sense of smell. Researchers at the French National Center for Scientific Research sampled 1842 individuals without modifications or disease to the septum. The study found that 39 percent of their sampled individuals had a clearly defined opening for the VNO. However, repeated observations of sampled individuals found that the presence of well-defined VNO openings changed over time, from clearly visible to less or not visible. Statistical inference from these repeated observations approximates that 73 percent of the human population exhibits clearly defined VNO openings on some days. Follow-up studies on the VNO in humans found no identifiable nerve bundles connecting the organ to the brain, suggesting that the VNO, if present, does not contribute to the human senses [3].

Despite findings that the VNO is largely useless, researchers have regarded its existence as proof of a pheromone system in humans that was passed down from previous generations and closely related ancestors. Researchers have theorized that the ability to detect pheromones is integrated into the olfactory receptors in humans, for which messages are interpreted in the amygdala, hippocampus, and frontal lobe where they play a role in emotion, learning, and memory.

Pheromones at Play

From our understanding of an evolutionary link between humans and other species that have well-documented pheromone systems, numerous studies have been performed to identify substances to classify as pheromones for humans. The messages communicated through pheromones have been categorized based on research of various insects with pheromone systems, as entomology founded pheromone research. Studies have now found evidence for three types of human pheromones called primer, modulator, and human sex pheromones.

Primer pheromones seek to trigger changes in developmental behavior, kinship, and family order [4]. First documented in 1954 in desert locusts, a study done by researchers at the University of Chicago found evidence of primer pheromones affecting the fertility of women who have never been pregnant in their lifetimes [5]. The study focused on the menstrual cycles of forty-seven women who have never before given birth and exposed them to the odors of infant sweat, breastfeeding women, and breast milk. Prior to exposing them to scents representative of breastfeeding, the forty-seven nonpregnant individuals displayed a typical amount of variance in the length of their menstrual cycles. After exposing subjects to breastfeeding scents for a full menstrual cycle, the variance in their subsequent cycles were reduced, essentially locking in each subject’s cycle length with more predictable timings of the fertile window, ovulation, and menstrual period [5]. The decrease in variance of the menstrual period was concluded to be an effect of the chemical signals of breastfeeding women and infants locking in the cycle lengths of women exposed to these signals. This study ultimately showed the ability of certain chemicals to affect the fertility of other humans, but the exact chemical compound responsible for inciting the stabilization menstrual cycles has yet to be identified.

Modulator pheromones, chemical signals that affect individual mood and emotion, have also been researched in humans. Researchers at Stony Brook University studied the effects of sweat odors caused by stress on the behavior and stress responses of individuals exposed to them [6]. Saliva and sweat samples were collected using absorbent pads on the armpits and tongues of volunteer donors who were gearing up for skydiving. Novice skydivers were specifically targeted to ensure that the act of skydiving would elicit a bonafide high-stress and high-emotion response. Separate individuals were then exposed to the stress sweat and saliva and had their brain activity scanned using functional magnetic resonance imaging (fMRI). Brain imaging analysis primarily focused on the region of the brain called the amygdala, which is associated with human emotional processing, fear, aggression, and anger. Results from the brain imaging experiment found higher activity in the amygdala when exposed to the sweat and saliva samples, indicating that chemicals released by the body during stress can affect the emotional processing of other humans who are exposed to them. Related studies have found that exposure to fear-related chemical signals, such as stress sweat, can cause increased interpretations of fear and stress in response to ambiguous stimuli in separate individuals [7]. Despite these results, the exact secretions responsible for affecting the emotional processing and heightened amygdala activity have yet to be identified.

Lastly, human sex pheromones have gained the most recognition due to ease of marketing and our general gravitation toward sex-fueled messages. Sex pheromones function by affecting an individual’s feelings of sexual attraction and desire towards another, as well as affect mate preferences. One study has found that copulins, a group of fatty acids secreted in the vagina before ovulation, affects feelings of sexual confidence in men [8]. Researchers at Rutgers University exposed a sample of one hundred heterosexual men to a synthetic copulin-like compound, with its composition similar to that of naturally-occuring copulins during ovulation. Men who were exposed to the synthetic copulins had rated themselves as more sexually desirable to other women and rated other women as more attractive on average compared to men who weren’t exposed to any copulins [8]. This finding supports a theory that men seek out fertile and ovulating women through pheromones in order to increase reproductive success. This study is also one of the only pheromone studies using a synthesized test compound, which puts copulins ahead of the pack as a potential chemically identifiable and synthesizable compound.

In regards to mate selection, studies have also shown that our natural scents communicate the makeup of our immune systems, and that we tend to prefer the scents of people who have a different immune system composition to our own [9]. The makeup of our immune systems are distinguished by the type of major histocompatibility complexes (MHC) present in our genes. These MHCs are found in human DNA and allow the body to identify its own cells from foreign cells. In essence, MHCs are like molecular fingerprints. Allowing one person to be distinguished from another based on the antigens that their MHCs encode for. Researchers at the University of Bern sampled and identified the MHCs for 137 male and female students and asked them to rate 36 different perfumes and scents. They found that by and large, subjects with the same MHC type preferred the same perfumes [9]. One further study sought to directly correlate the effects of MHC type on attraction [10]. Often referred to as the “sweaty T-shirt study”, researchers at the University of Bern sampled and labelled 49 female and 44 male participants by MHC type (HLA-A, -B, and -DR). Male participants were asked to wear the same T-shirt for two days without washing and the T-shirts were then put into boxes for female participants to smell. Results showed that the female participants preferred the scents of T-shirts belonging to men who had a different MHC type than their own [10]. This study is usually cited by the “pheromone parties” as proof of their efficacy in setting up potential mates. A theory has also arisen from this study, stating that our preference for the scents of partners with different MHCs and immune system composition suggests a sort of inbreeding avoidance within humans. As it is advantageous to have a diverse set of genes to create offspring, our MHC-based scent preferences may be for this purpose.

What’s Next for Pheromones?

Modern neuroscience interest has generally moved away from pheromone studies in pursuit of other topics, such as neurotechnology and pharmaceuticals. However, the implications of pheromones should not be underestimated. If found and proven true, pheromones could reshape our understanding of human communication. The pompous claims made by fragrance and shampoo manufacturers may be true — using their products may get you more attention by the opposite sex. Pheromone parties, though they seem strange, may actually be effective and become the preferred method of speed dating. Therapies can be made available for individuals with low sex drive using human sex pheromones. Modulator and primer pheromones can be used to regulate the cycles of women with irregular menstrual cycles.

More “left-field” approaches include weaponizing pheromones. In 1994, a United States Air Force biotechnology laboratory proposed a nonlethal bioweapon, dubbed the “gay bomb”, based on human sex pheromones. The idea was to drop a sea of human sex pheromones over enemy soldiers, making them all distracted with sexual thoughts, potentially even becoming sexually attracted to each other. Friendly troops would then be able to move in on the sexually distracted soldiers and take them out with ease [2]. The project was quickly scrapped and later declassified. Despite the futile attempt to weaponize pheromones, the “gay bomb” project won the 2007 Ig Nobel Prize, a satirical prize mirroring the Nobel Prize, celebrating the most unusual and impractical in science [2]. A common shortfall for most pheromone experiments is that we cannot pinpoint one chemical compound solely responsible for an effect on human behavior and cognition. In other words, while researchers believe that human pheromones are present and can affect other humans, they do not yet know what they are. Many pheromone studies also have small sample sizes and are prone to observer bias, as it is very difficult to have experimental controls with “no odor” at all. The results of many pheromone studies, therefore, cannot be generalized to all humans and may be due to external factors not controlled in experiments [1]. However, modern pheromone research, such as the copulin study, has been more targeted on certain naturally occurring and synthetic compounds, allowing for more specific conclusions to be made. Moving forwards, pheromone research must be more focused on the effects of certain compounds rather than the effects of general scents and odors, as well as emphasize synthetic recreation and testing of potential pheromones.

References

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2. Improbable Research. (2020, October 9). Ig Nobel prize winners.  https://www.improbable.com/ig-about/winners/#ig2007
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