2023 Dec 21
A fascinating study has found that sniffing female tears significantly reduced male aggression and decreased activity in aggression-related brain networks. It’s suggested that the effect, which is caused by chemical signals in tears and is also seen in rodents, serves a protective function.
Charles Darwin was perplexed by emotional tears; he thought they served no useful purpose other than lubricating the eye. Although shedding emotional tears is thought to be a solely human trait, since Darwin, researchers have found that mammalian tears contain chemicals that act as social signals, one of which is to reduce aggression.
For example, the tears of female mice contain signals that turn off intermale aggression by curbing activity in the males’ aggression brain networks. And, subordinate male blind mole rats cover themselves in tears to reduce dominant male aggression toward them.
Now, researchers from the Weizmann Institute of Science in Israel have conducted a series of experiments to investigate whether, like in rodents, sniffing human female tears reduces aggression in men and what functional effect it has on their brains.
“We knew that sniffing tears lowers testosterone and that lowering testosterone has a greater effect on aggression in men than in women, so we began by studying the impact of tears on men because this gave us higher chances of seeing an effect,” said Shani Agron, the lead and co-corresponding author of the study.
There’s limited evidence of human tear chemosignaling, but a previous study by some of the researchers involved in the current study found that women’s tears contain an odorless chemical signal that, when sniffed by males, reduced self-rated sexual arousal, physiological measures of arousal, and testosterone levels.
First, the researchers tested whether sniffing female tears reduced aggression in men. ‘Emotional’ tears were collected from six human donors aged 22 to 25 who watched sad film clips in isolation to induce crying. Twenty-five men were asked to play a two-person monetary game with an opponent they were told was human but was, in fact, a computer algorithm. The game was designed to elicit an aggressive response by the male toward their opponent, whom they were led to believe was cheating. When given the opportunity, the male could get revenge on their opponent by causing them to lose money with no personal gain to them.
Before playing the game, the participants sniffed either female tears or a saline solution – both are odorless – but were not told what they were sniffing. The researchers observed a 43.7% reduction in aggression following exposure to tears. To evaluate the robustness of their results, they ran a bootstrap analysis, a statistical procedure that resamples a single data set to create many simulated samples. The analysis found that the probability of obtaining this outcome by chance was 2.9%, suggesting that, like in rodents, chemosignals in human emotional tears have a primary aggression-blocking function.
Next, the researchers analyzed the effect of sniffing tears on the participants’ brains. After exposure to tears or saline, 26 male participants played the monetary game while undergoing a functional magnetic resonance imaging (fMRI) scan. In two brain structures implicated in aggression – the left anterior insula cortex (AIC) and bilateral prefrontal cortex (PFC) – the researchers noted reduced activity following exposure to tears. There was a significant correlation between the experimental condition (tears versus saline) and activity in these regions.
Investigating the brain’s functional connectivity, they found that tears only impacted the left AIC, which displayed significantly increased connectivity with the right amygdala and piriform cortex. As well as sharing structural connectivity, these regions are part of a functional network implicated in olfaction (smelling) and aggression.
“We’ve shown that tears activate olfactory receptors and that they alter aggression-related brain circuits, significantly reducing aggressive behavior,” said Noam Sobel, another of the study’s corresponding authors. “These findings suggest that tears are a chemical blanket, offering protection against aggression – and that this effect is common to rodents and humans, and perhaps to other mammals as well.”
Indeed, a 2022 study found that dogs’ tear volume increased significantly when they were reunited with their owner but not with a familiar non-owner, suggesting they could cry emotional – happy – tears. However, further research is needed to determine whether these tears contain chemosignals that can be picked up by other dogs or by humans.
Having confirmed the effect of sniffing tears on men’s behavior, the researchers are keen to expand their research.
“When we looked for volunteers who could donate tears, we found mostly women because, for them, it’s much more socially acceptable to cry,” Agron said. “Now, however, we must extend this research to include women to obtain a fuller picture of this impact.”
The study was published in the journal PLOS Biology.
Source: Weizmann Institute of Science
