Sniffing women's tears makes men less aggressive: study

A woman is shown with tears running down her cheeks. New research has found that human tears might have the power to quell aggression. (Pexels)

Human tears might just have the power to quell aggression, according to new research.

A peer-reviewed study published in the scientific journal PLOS Biology last month has found that, despite being odourless, women's tears activate certain human smell receptors and reduce aggression in men.

Inspired by previous studies that found tears reduce aggression in mice and mole rats, researchers at the Weizmann Institute of Science wanted to know whether they have the same aggression-blocking effect in people.

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In a series of experiments, 25 male participants were exposed to either women’s emotional tears or salt water, without knowing what they were sniffing or being able to distinguish between the two, because both are odourless. To ensure the women's tears — shed while watching sad film clips in isolation — were odourless, all the donors were placed on combined hormonal contraceptives to eliminate the possible effects of ovulation on body odour, and all were told to avoid using cosmetics until after donation.

After they were exposed to the tears, the men played a two-person game designed to elicit aggressive behaviour in one player toward the other player, who they were told was cheating. When given the opportunity, the men could get revenge on their opponents by causing them to lose money, though they themselves gained nothing.

 "We observed a remarkable reduction in aggression following exposure to tears," the authors wrote.

The researchers found that, among men who had sniffed women’s emotional tears, revenge-seeking aggressive behaviour during the game dropped by about 44 per cent.

In order to figure out how the tears generated this effect, they looked to rodent physiology.

Multiple studies have shown that mammalian tears contain chemicals that work like social signals that can be emitted on demand. These signals are especially strong in rodents. The tears of female mice, for example, contain chemicals that affect aggression networks in the brain, reducing fighting among male mice. In another rodent species — the blind mole rat — subordinate males smear themselves in tears to reduce the dominant male’s aggressive behaviour toward them.

Tears work this way on rodents because they have a structure in their noses called the vomeronasal organ, which picks up the social chemical signals.

However, humans don’t have this organ, which begs the question, how do they sense the social chemicals in tears? To find an answer, the researchers applied women's tears to 62 human olfactory receptors in a laboratory dish and found that four of these receptors were activated by the tears, even though tears are odourless.

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They also repeated the tear exposure experiments while examining the men’s brains in an MRI scanner. They found that two aggression-related brain regions — the prefrontal cortex and the anterior insula — were less active when the men were sniffing the tears.

Moreover, they found that the greater the difference in this brain activity between saline and tears, the less often the player took revenge during the game.

"We’ve shown that tears activate olfactory receptors and that they alter aggression-related brain circuits, significantly reducing aggressive behavior," stated professor Noam Sobel, whose lab in Weizmann’s Brain Sciences Department studies olfaction, in a media release. "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."

Because their sample size was limited, the researchers took their results and ran a random sampling method called a bootstrap analysis in order to see what the results of their experiments would look like on a larger scale. The results of 10,000 randomly-selected analyses pointed to the same conclusion.

While their study involved male participants and female tear donors, the researchers say their findings could have implications beyond the effect women's tears have on men. In fact, they point to tears serving as a chemical survival mechanism in some of the most vulnerable humans: babies.

"We speculate that all tears would have a similar effect," the study reads. "This becomes particularly ecologically relevant with infant tears, as infants lack verbal tools to curb aggression against them and are therefore more likely to rely on chemosignals."