Human olfaction is fairly sophisticated and sensitive. We can most likely detect approximately one trillion different odorants compared to several million colors and a half million different tones, and we are able to detect odors at very low concentrations, (e.g., 0.2 ppb for some molecules). Further, the nature of the information provided via smell seems unique among sensory modalities. A single scent can consist of hundreds of different compounds, allowing for multiple and complex messages to be packaged in one odor. Smells also linger, allowing for communication across distance and time.
In addition to providing information about the environment, scents also seem important for relaying social information. We can detect someone’s sex, sexual orientation, approximate age, and genetic relatedness simply by smelling his or her body odor. In fact, people most likely have unique olfactory signatures. We can also sense people’s transient states from their aroma. For example, men can detect the menstrual cycle phase in women by smelling their body odor. Further, smelling the sweat from individuals experiencing a particular emotion (e.g., fear, disgust, or joy) has been shown to induce a comparable emotional response in receivers, what can be called olfactory emotional contagion. Such effects of olfaction often occur outside of conscious awareness. Most people are not very good at identifying smells, but, independent of whether scents are recognized or even detected, they still can affect our physiology, feelings, and actions.
Human body odors can impact the sexual and emotional response of others, and sexual arousal can be conceptualized as an emotion. Hence, perhaps, we are able to communicate and detect information about sexual arousal state through body odor. Moreover, we know that sexual arousal increases steroid production in men and in women, which could be the mechanism by which sexual arousal could be olfactorily contagious, since many putative human semiochemicals (pheromones) are derived from steroid hormones. Previous research showed that communication about sexual arousal state via body odor was possible. Zhou and Chen (2008) presented women with the sweat of men who had watched an erotic film versus the sweat of men who had watched a nonsexual documentary. They found that the women showed a distinctive neural response to the sexy vs. control sweat. Gelstein et al. (2011) showed that exposure to female tears caused men to rate female faces as less attractive, to show decreased testosterone, and to report less sexual arousal and show less neural (hypothalamic) activation while watching an erotic film.
I sought to directly examine olfactory sexual arousal contagion. Specifically, I predicted that men, while smelling sweat collected from women when they were sexually aroused compared to sweat collected during a non-aroused state, would show increased subjective sexual and genital arousal to an erotic narrative as well as report more interest in sex and in self-disclosing to a romantic partner (an indirect measure of sexual arousal). I expected this effect to occur independently of women’s menstrual cycle phase, but I thought the effect might be more easily detectable with sweat collected during the luteal (non-fertile) than follicular (fertile) phase, since smelling luteal compared to follicular phase body odor has been shown to induce lower levels of sexual responsiveness in men.
To test this hypothesis, I collected sweat on underarm pads from naturally cycling women under four different conditions, when they were sexually aroused and when they were resting, during both their follicular and their luteal phase. Male participants had the sweat smells delivered to their noses via an oxygen cannula. A temperature- and humidity-controlled air stream was passed over a super-donor sample (the pads from 3-4 women combined) and was fed through the cannula. They smelled odors while listening to erotic narratives. I monitored participants’ genital arousal using a penile plethysmograph, which is a strain gauge-like device that measures changes in penile circumference. I also had them report out on their sexual arousal and other feelings as well as detection, familiarity, intensity, and pleasantness of the body odor.
Men responded differently to women’s body odor depending on the arousal state conditions under which, and on when in the menstrual cycle, their sweat was collected. Participants smelling follicular phase sweat reported greater subjective sexual arousal and an increased likelihood to self-disclose than men smelling luteal phase sweat. The former effect replicated previous findings, while the latter was novel.
Another novel finding was that the men also showed increased genital arousal to sexually aroused (vs. resting) sweat, at least for those exposed to luteal (but not those exposed to follicular) phase body odor. The effect might not have been observed with follicular phase sweat because receptivity indicated by fertile odors might override information about female arousal state. Only about half of the participants reported detecting an odor, but effects were similar whether a scent was detected or not.
Across conditions, women’s body odor was rated as intermediately intense and intermediately pleasant. Interestingly, subjective odor ratings by the men corroborated the distinctiveness of sexually aroused sweat collected during the luteal phase. This type of sweat was less often detected compared to body odor collected under the other three conditions and it was also rated as less familiar than other types of sweat.
This study found evidence for olfactory sexual arousal contagion, even though the effect was subtle and appeared only with the objective measure of genital arousal. However, other research has found that language-unrelated measures might be more sensitive in measuring the effects of olfactory stimuli on behavior. Research on olfactory emotional contagion for positive emotions is relatively new and sexual arousal, like fear which is the emotion most often used in this research, is an affective state with clear evolutionary significance. Also, although most research on olfactory emotional contagion has been done with female participants, the present study confirms that men may also be sensitive to socioemotional information contained in (women’s) body odor.
Being able to detect the scent of sexual arousal (beyond fertility/receptivity) could provide important information on whether to approach and sexually interact with a female (since female arousal is more cryptic than a male’s), could enhance perceiver arousal, and may create conditions for improved sexual satisfaction of a couple.