A chemical signal in women’s tears reduces men’s aggression

What are tears for? To answer this question and know precisely their role, researchers studied this question closely, in order to determine if human tears could, as in certain animals, serve as a medium for chemical communication.

A 40% reduction in aggression!

For this work, the scientists therefore developed a series of three experiments to understand the role of human tears in the transmission of chemical signals, particularly in cases of male aggression. Research around this property is linked to the fact that it is already known that the aggression of male rodents is blocked when they smell female tears. This is an example of social chemosignaling, a process common in animals but less common – or less understood – in humans.

In the first experiment, 31 healthy men were chosen. To carry out the study, tears were collected from six women who could easily cry while watching sad films. These tears were then used as the main stimulus for the experiment. For comparison, a salt water solution was poured onto the women’s cheeks and then collected. Then, in a double-blind setup, male participants were exposed to tears or saline.

After sniffing the tears or saline solution, the men were asked to play a game designed to measure their aggression levels. This game involved making money-related decisions that could provoke aggressive responses.

The results were striking: exposure to tears led to a 43.7% reduction in aggression among participants compared to their exposure to saline. Results that surprised the researchers. “A 40% reduction is not something you typically see in the lab” says Noam Sobel, director of the Weizmann Olfaction Research Group and author of the study.

The nose has receptors capable of “sniffing” tears

For the second experiment, the team of researchers was interested in how the human body detects and processes these tear signals. They tested 62 different human olfactory receptors – which are the proteins responsible for detecting odors – using a cellular system.

The goal was to see if any of these receptors responded specifically to the collected tears. This time, researchers found that 4 of 62 olfactory receptors showed a response to 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 if the tears themselves have no noticeable odor.

“It was also surprising to learn that odor receptors in the olfactory system can respond to tears despite their lack of odor” added Shani Agron, doctoral student at the Weizmann Institute.

Brain activity altered by tears

The third experiment involved 33 men and used functional magnetic resonance imaging to study the brain’s response to tears. As in the first experiment, participants were exposed to tears or saline and then played the aggression measurement game. However, this time their brain activity was monitored.

Results: This experiment revealed subtle but important changes in brain activity. Indeed, tears increased connectivity between areas of the brain involved in processing odors and those involved in aggression.

The authors state: “We discovered that, just like in mice, human tears contain a chemical signal that blocks the aggression of males of the same species. This goes against the idea that human tears are solely the result of emotion“.

In conclusion, these first results could reveal the important role of tears in social interactions, particularly in reducing aggression. However, the researchers recognize that this initial work has limitations and must be continued, in order to further establish these initial conclusions.