According to new research, human tears might just have the power to quell aggression.
A peer-reviewed study published in the scientific journal PLOS Biology, has found, that, despite being odorless, women’s tears activate certain human smell receptors and reduce aggression in men.
From the previous studies they concluded that, tears will reduce aggression in mice and mole rats, scientists at the Weizmann Institute of Science wanted to find out if they have the same aggression-blocking effect in people.
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 find the difference between the two, because both are odorless. To make sure that the women’s tears shed while watching sad film clips in isolation were odorless, all the donors were placed on combined hormonal contraceptives to eliminate the possible effects of ovulation on body odor and all were asked to avoid using cosmetics until after donation.
After they were exposed to the tears, the men played a two-person game designed to provoke aggressive behavior 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, even though they themselves gained nothing.
The scientists found that, among men who had sniffed women’s emotional tears, revenge-seeking aggressive behavior during the game reduced by about 44 per cent. In order to find out how the tears generated this effect, they looked to rodent physiology.
Many studies showed that mammalian tears contain chemicals which 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 which 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 behavior toward them.
On rodents, tears work this way 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 were odorless.
The tear exposure experiments were also repeated while examining the men’s brains in an MRI scanner. They found that two aggression-related brain regions — the anterior insula and the prefrontal cortex were less active when the men were sniffing the tears.
Furthermore, they also concluded 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.
Since 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 female tear donors and male participants, the scientists 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 restrict aggression against them and are therefore more likely to rely on chemo signals.
