In a new study published in PLOS Biology, researchers discovered that human tears have the surprising ability to reduce aggression in men. The discovery not only challenges the long-held belief that tears are simply to protect the eyes, but also suggests that tears play an important role in human social interactions.
The study was motivated by a long-standing mystery surrounding the purpose of human emotional tears. Charles Darwin once thought that tears were an accidental result of evolution, but recent research has shown that tears may function as social chemical signaling, a means of transmitting chemical signals between individuals. It is shown that there is. Rodents use tears in this way, but it is unclear whether humans have a similar system, especially since humans lack the specialized olfactory system that rodents use for this purpose. did.
“We’re interested in human behavior: what makes us do things, why, and how,” said Norm, study author and director of the institute. Sobel said. Weizmann Olfactory Research Group. “Within this very big picture, the small picture that we are studying is a topic called ‘chemical communication.’ Like all terrestrial mammals, humans convey meaningful information through body odor, which influences behavior. We are interested in understanding these chemicals and their subsequent behavior, including brain mechanisms. ”
The researchers conducted a series of three experiments to investigate the potential role of human tears in social chemical signaling, with a particular focus on the effects of tears on male aggression. These experiments were designed to investigate various aspects of this phenomenon, from behavioral responses to the activation of specific olfactory receptors and ultimately the brain’s response to these signals.
“One of the things we like about this study is that it combines three levels of research: behavior, brain imaging, and molecular biology. We are not aware of any previous studies that have combined levels,” Sobel told PsyPost.
The first experiment involved 31 healthy men. To conduct this study, we collected tears from six women who cried easily after watching sad movies. These tears were used as the main stimulus in the experiment. As a control, saline (salt water) was dropped onto the woman’s cheek and collected. Male participants were exposed to either tears or saline in a double-blind setting. That is, neither the researchers nor the participants knew which substances were being tested at any given time.
During the experiment, the men sniffed tears and saline and played a game to gauge their level of aggression. The game, known as the Point Subtraction Attack Paradigm, involved making money-related decisions that could provoke an aggressive response. The results were amazing. Exposure to tears reduced participants’ aggression by 43.7% compared to exposure to saline.
Sobel said he was surprised to see the large effect. “A 40% reduction is not something you typically see in a lab environment,” he explained.
In the second experiment, the research team shifted their focus to understanding how the human body detects and processes these tear signals. They tested 62 different human olfactory receptors (proteins involved in smell detection) using a cell-based system. The aim was to see if any of these receptors specifically responded to collected tears.
Now, the researchers found that four of the 62 receptors responded to tears. These receptors did not respond to control saline. The results show that specific receptors in the human nose can detect signals from tears, even though the tears themselves have no noticeable odor.
“We were also surprised to learn that odor receptors in the olfactory system can respond to tears even though there is no odor,” said study author Shani Agron, a doctoral student at the Weizmann Institute of Science. “While mammals receive chemical signals primarily through the accessory olfactory system, humans only have a primary olfactory system. This discovery is the first of its kind, and suggests that the human primary olfactory system is more complex than previously thought. This suggests that they may also have diverse functions.
The third experiment involved 33 men and used functional magnetic resonance imaging (fMRI) to study the brain’s response to tears. As in the first experiment, participants were exposed to tears or saline and then played a game that measured aggression. But this time, their brain activity was monitored.
The experiment revealed subtle but significant changes in brain activity. Exposure to tears decreased activity in brain regions typically associated with aggression. Furthermore, tears strengthened connections between brain regions involved in odor processing and brain regions involved in aggression.
The results of these experiments suggest that human tears may play an important role in social interactions, particularly in reducing aggression. This is consistent with the concept that emotional crying can serve as a means of nonverbal communication and can significantly influence the behavior of others, especially in close-range interactions. The findings also provide insight into the relationship between the sense of smell and social behaviors such as aggression.
“Tears contain chemical signals that reduce aggression, and this mechanism is common to many mammals,” Sobel said. “We answered a very basic question: What is the functional purpose of emotional tears?”
Despite these findings, this study has limitations. For example, only a subset of human olfactory receptors were tested, leaving open the possibility that more receptors respond to tears. Additionally, this study included only male participants, leaving the effects of tears on women unexplored. Additionally, the discomfort and unique environment of his MRI scanner in the third experiment may have influenced participants’ responses.
Future research could include testing all olfactory receptors, investigating the effects of tears on women, and finding ways to overcome the limitations posed by the MRI environment. This will provide a more comprehensive understanding of the role tears play in human social signaling and behavior.
“We did not study the reactions of female participants,” Sobel explained. “We didn’t do that because this is an incredibly ‘expensive’ experiment to run. It’s expensive in terms of money, but it’s even more expensive in terms of time (which is also money). So we wanted to start where we were most likely to see an effect. We started with testosterone because we knew that smelling tears lowers testosterone, and that low testosterone has a greater effect on aggression in men than in women. However, to get a complete picture of this behavior, it needs to be replicated in women. ”
“A second concern worth mentioning is that from everything we know about the brain mechanisms of such behavior, the hypothalamus plays an important role. But brain imaging studies We did not see any effects on the hypothalamus, which is a concern with our imaging method.”
“Our main focus is finding the active molecules in the tears,” Sobel said. “What influences tears? If that molecule is discovered, it could have significant clinical implications and applications.”
the study, “Chemical signals in human female tears reduce male aggression” is written by Shani Agron, Claire A. de March, Reut Weissgroth, Eva Michor, Lior Gorodisky, Tali Weiss, Edna Furman-Haran, Hiroaki Matsunami, and Noam Sobel.