In a new study published in PLOS Biology, researchers have discovered that human tears possess the remarkable ability to reduce aggression in men. This finding not only challenges the long-held belief that tears are merely for eye protection but also suggests that they play a significant role in human social interaction.
The study was motivated by the long-standing mystery surrounding the purpose of human emotional tears. While Charles Darwin once thought weeping to be an incidental result of evolution, more recent research has indicated that tears could serve as a form of social chemosignaling – a way of conveying chemical signals between individuals. While rodents use tears in this manner, it was unclear if humans had a similar system, especially since humans lack the specialized olfactory system rodents use for this purpose.
“We are interested in human behavior, in what makes us do things, why, and how,” said study author Noam Sobel, the director of the Weizmann Olfaction Research Group. “Within this very big picture, the smaller picture we are studying is a topic referred to as ‘chemical communication.’ Humans, like all terrestrial mammals, communicate meaningful information in body odor, and this effects behavior. We are interested in understanding these chemicals and ensuing behaviors, including their brain mechanisms.”
The researchers conducted a series of three experiments to investigate the potential role of human tears in conveying social chemical signals, particularly focusing on their impact on male aggression. These experiments were designed to explore different aspects of this phenomenon, from behavioral responses to the activation of specific olfactory receptors, and finally, to the brain’s response to these signals.
“One thing we like about this study is that it combines three levels of investigation: behavior, brain imaging, and molecular biology. We are unaware of any previous study that combined these three levels in one study in humans,” Sobel told PsyPost.
In the first experiment, 31 healthy men were involved. To conduct the study, tears were collected from six women who could easily cry while watching sad movies. These tears were then used as the primary stimulus in the experiment. For control, saline (a saltwater solution) was trickled down the cheek of the women and collected. The male participants were exposed to either tears or saline in a double-blind setup, meaning neither the researchers nor the participants knew which substance was being tested at any given time.
During the experiment, the men sniffed the tears or saline and then played a game designed to measure their aggression levels. This game, known as the Point Subtraction Aggression Paradigm, involved making money-related decisions that could provoke aggressive responses. The findings were striking: exposure to tears led to a 43.7% reduction in aggression among the participants compared to when they were exposed to saline.
Sobel said he was surprised to observe the large effect. “A 40% reduction is not something typically seen in lab settings,” he explained.
In the second experiment, the team shifted focus to understand how the human body detects and processes these tear signals. They tested 62 different human olfactory receptors – the proteins responsible for detecting smells – using a cell-based system. The goal was to see if any of these receptors responded specifically to the collected tears.
This time, the researchers found that four out of the 62 receptors showed a reaction to the tears. These receptors did not respond to the control saline solution. This result indicates that certain receptors in the human nose are capable of detecting signals from tears, even though the tears themselves do not have a noticeable smell.
“It was also surprising to learn that the olfactory system’s smell receptors can react to tears despite their lack of odor,” said study author Shani Agron, a PhD student at Weizmann Institute of Science. “Mammals mostly receive chemical signals through the accessory olfactory system, but humans only have a primary olfactory system. This discovery is the first of its kind, and it suggests that the primary olfactory system in humans may have a more diverse range of functions than previously believed.”
The third experiment involved 33 men and used functional magnetic resonance imaging (fMRI) to study the brain’s response to tears. Like in the first experiment, the participants were exposed to tears or saline and then played the aggression-measuring game. However, this time, their brain activity was monitored.
This experiment revealed subtle yet significant changes in brain activity. When exposed to tears, there was reduced activity in brain areas typically associated with aggression. Additionally, tears increased the connectivity between areas of the brain involved in processing smells and those involved in aggression.
The results of these experiments suggest that human tears could play a vital role in social interactions, particularly in reducing aggression. This aligns with the concept that emotional crying can be a means of non-verbal communication, significantly impacting the behavior of others, especially in close-range interactions. The findings also provide insights into the connection between the sense of smell and social behaviors like aggression.
“Tears contain a chemical signal that lowers aggression, and that this mechanisms is common to many mammals,” Sobel said. “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 the human olfactory receptors were tested, leaving the possibility that more receptors could respond to tears. Also, the study only involved male participants, leaving the effect of tears on women unexplored. Additionally, the discomfort and unique environment of the MRI scanner in the third experiment might have influenced the participants’ responses.
Future research could include testing the full range of olfactory receptors, exploring the effect of tears on women, and finding ways to overcome the 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. “We didn’t because this is an incredibly ‘expensive’ experiment to run. Expensive in funds, but more so expensive in time (which is also funds). Thus, we wanted to start where we had higher chances of seeing an effect. We knew that sniffing tears lowers testosterone, and lowering testosterone has a greater effect on aggression in men than in women, so we started with them. We must, however, now replicate in women to obtain a fuller picture of this behavior.”
“A second concern worth mentioning is that from all what we know about the brain mechanisms of such behaviors, 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.”
“Our key focus is to now find the active molecules in tears,” Sobel said. “What is it in tears that drives the effects? If we find the molecule(s), this may have significant clinical implications and applications.”
The study, “A chemical signal in human female tears lowers aggression in males“, was authored by Shani Agron, Claire A. de March, Reut Weissgross, Eva Mishor, Lior Gorodisky, Tali Weiss, Edna Furman-Haran, Hiroaki Matsunami, and Noam Sobel.