A new study has revealed that smelling women’s tears decreases aggression in men.
In a recent publication in PLOS Biology, scientists have unearthed that human tears possess an extraordinary ability to diminish aggression in men. This revelation not only challenges the conventional belief that tears primarily serve as eye protection but also suggests a significant role in human social interaction.
The study was prompted by the longstanding enigma surrounding the purpose of human emotional tears. While Charles Darwin once considered weeping an incidental result of evolution, recent research indicates tears might function as a form of social chemo signaling—a way of conveying chemical signals between individuals. Though rodents use tears in this manner, it was uncertain whether humans had a similar system, given the absence of the specialized olfactory system found in rodents.
“We are intrigued by human behavior, understanding what compels our actions, why, and how,” said study author Noam Sobel, the director of the Weizmann Olfaction Research Group. “In this vast exploration, our specific focus is ‘chemical communication.’ Terrestrial mammals, including humans, communicate meaningful information through body odor, influencing behavior. We seek to comprehend these chemicals and the subsequent behaviors, along with their brain mechanisms.”
The researchers conducted three experiments to explore the potential role of human tears in conveying social chemical signals, particularly their impact on male aggression. These experiments delved into different aspects of the phenomenon, from behavioral responses to the activation of specific olfactory receptors, and finally, the brain’s response to these signals.
“One noteworthy aspect of this study is its integration of three levels of investigation: behavior, brain imaging, and molecular biology. We are not aware of any previous study combining these three levels in one study involving humans,” Sobel shared with PsyPost.
Insights from controlled experiments and olfactory receptor discoveries
In the initial experiment, 31 healthy men participated, exposed to tears collected from six women who easily cried while watching sad movies. Tears served as the primary stimulus, with a saline solution as a control. In a double-blind setup, male participants were unaware of whether they were exposed to tears or saline. Results were remarkable: exposure to tears led to a 43.7% reduction in aggression compared to exposure to saline.
Sobel expressed surprise at the substantial effect, stating, “A 40% reduction is not something typically seen in lab settings.”
The second experiment shifted focus to understand how the human body detects and processes tear signals. Testing 62 human olfactory receptors, four receptors showed a reaction to tears, suggesting the human nose can detect tear signals despite tears lacking a noticeable smell.
“It was surprising to learn that the olfactory system’s smell receptors can react to tears despite their lack of odor,” noted study author Shani Agron, a PhD student at the Weizmann Institute of Science. “This discovery, the first of its kind, suggests that the primary olfactory system in humans may have a more diverse range of functions than previously believed.”
Unveiling the neurological impact of tears
In the third experiment involving 33 men, functional magnetic resonance imaging (fMRI) studied the brain’s response to tears. Participants exposed to tears or saline played an aggression-measuring game, with monitored brain activity, similar to the first experiment.
Results showed subtle but significant changes in brain activity, reducing aggression and increasing connectivity between smell and aggression areas.
The findings from these experiments propose a vital role for human tears in social interactions, particularly in reducing aggression. This aligns with the idea that emotional crying acts as non-verbal communication, profoundly influencing behavior in close interactions. The research also provides insights into the connection between the sense of smell and social behaviors such as aggression.
“Tears contain a chemical signal that lowers aggression, and this mechanism is common to many mammals,” Sobel affirmed. “We have answered the very basic question: what is the functional purpose of emotional tears?”
Despite these findings, the study has limitations. For instance, Only a fraction of olfactory receptors were tested, suggesting more receptors could respond to tears. The study only involved male participants, leaving the effect of tears on women unexplored. Moreover, the discomfort and unique environment of the MRI scanner in the third experiment might have influenced participants’ responses.
Charting the course for future tear research
Future research could involve testing the full range of olfactory receptors, exploring the effect of tears on women, and addressing limitations posed by the MRI environment. This would provide a more comprehensive understanding of the role tears play in human social signaling and behavior.
“We didn’t study the response in female participants,” Sobel explained. “This is an incredibly ‘expensive’ experiment to run, both in funds and time. We began where a higher effect was likely, considering tears lower testosterone more significantly in men than women. We must now replicate in women to obtain a fuller picture of this behavior.”
“A second concern worth mentioning is that, based on our knowledge of brain mechanisms, the hypothalamus is a key player. Yet we did not see any hypothalamic effects in our brain imaging. This is a concern regarding our imaging methodology.”