Scientists at the Weizmann Institute of Science discovered that tears activate human olfactory receptors, even though they are odorless, affecting the brain activity associated with aggression - and leading to more peaceful and less vengeful behavior
All land mammals have tear glands in their eyes, but human crying is seen as unique; After all, we are the only animal that sheds tears in front of a series on Netflix. However, a new study by Weizmann Institute of Science scientists reveals that, contrary to popular belief, our tears are not so different from those of other animals: they also contain molecules that are used to reduce aggression in others, just like the tears of a mouse or rat. the research, published today in the scientific journal PLOS Biology Shows that exposure through the sense of smell to women's tears reduces the brain activity associated with aggression in men and leads to a significant behavioral change and reduced aggression.
The new study tried to tackle a long-standing mystery: why do we cry? Charles Darwin, for example, thought that this was a meaningless evolutionary mistake. However, since then many studies, especially in rodents, have shown that in addition to protecting the eye plate, mammalian tears also contain chemicals that convey social messages. For example, the tear fluid of mice contains substances that affect neural networks in the brains of mice and lead to less violent behavior. Among rats, males at the bottom of the social ladder smear themselves with tears to reduce aggression towards them from dominant males.
in the laboratory of Prof. Noam suffers The neuroscience department of the Weizmann Institute of Science hypothesized that, similar to sweat and other body odors, human tears also contain chemicals that serve as social signals. In 2011, in a study published inScience, his research group showed that exposure to women's tears reduces testosterone levels in men. In the new study, led by PhD student Shani Agron, the researchers tested whether, like rodents, human tears also protect against aggression. The researchers exposed a group of male subjects to women's tears or alternatively to a saline solution that was dripped onto the same women's cheeks; The subjects did not know what they were smelling and anyway could not distinguish between the tears and the salt water, since both are odorless. Immediately after the exposure, the subjects participated in a game designed to provoke aggression in them and based on a valid method for measuring aggressive behavior under laboratory conditions. As part of the game, the researchers made the subjects believe that they were playing against another player who occasionally teased them and made them lose money. The participants could choose whether to take revenge on that unknown participant, without receiving a monetary reward for it, or continue to earn money for themselves. The results were amazing: after exposure to tears, the most vengeful behavior decreased by about 44%.
These findings alone reveal a human behavioral effect similar to that known in rodents. However, rodents, as well as other animals, have a dedicated organ in the nose for receiving social chemical messages known as Jacobson's organ. Humans do not have such an organ, so how do they sense these chemical signals? To answer this, the researchers dripped tears onto human olfactory receptors grown in a laboratory dish. 62 out of about 350 different types of human smell receptors were tested in the experiment, and about four of them responded to tears even though they are odorless, but did not respond to the salt water solution.
Later, the researchers examined how the chemical signal in the tears affects brain activity. To this end, they repeated the first experiment while functionally scanning the brains of the participants in an MRI machine. The scans revealed that when exposed to tears there is a decrease in the activity of two brain areas associated with aggression: the prefrontal cortex and the anterior insula. In addition, exposure to tears increased neural communication between the anterior insula and the amygdala - a brain area associated with processing memories and emotions and also involved in processing smells. The researchers identified that there is a correlation between the brain activity and the degree of vengefulness of the subjects - findings that further strengthen the hypothesis that the chemical signal in tears reduces aggression.
"Through a unique combination of behavioral tests, molecular biology and brain imaging, we showed that tears activate human olfactory receptors, affect brain circuits related to aggression, and significantly reduce aggressive behavior," says Prof. Sobel. "This line of findings indicates that tears are a chemical defense umbrella against aggression - and that this effect is shared by rodents, humans, and possibly other mammals as well." In fact, studies from recent years have already shown that dogs, like humans, shed tears as an expression of emotion, but it has not yet been discovered whether these tears contain chemical messages that may affect other dogs or people.
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