Tears in humans are not just for lubrication and the cleaning of the eyes. We shed tears also for emotional reasons. Sadness, joy, anger, or just being moved might lead to crying, but it is not obvious why humans have evolved such a capability. Or if animals could shed emotional tears.
Scientists have in the past shown that tears in several mammals contain chemicals serving as social signals that can be emitted on demand. And this might include humans as well. A series of experiments were conducted on male aggression something that was seen working in mice.
A group of men were made to sniff women’s tears or a saline solution (both of them are odorless) and then they were made to play a two-player game designed to elicit aggression They were made to believe that the other person was cheating, and they could get revenge (without gaining nothing). Players who smell the tears and not the control have a drop in aggressive behavior by 44 percent.
The team then looked at the olfactory receptors and discovered that out of 62 humans, four were activated by the tears. MRI scans of men sniffing the tears showed two aggression-related brain regions—the prefrontal cortex and anterior insula—that did not become as active as the men were provoked during the game.
“We found that just like in mice, human tears contain a chemical signal that blocks conspecific male aggression. This goes against the notion that emotional tears are uniquely human,” the authors wrote in a statement.
The protection against aggression might not be the only effect, but it is the one being recognized here and in rodents, and it might extend to other animals. Recent studies have found that dogs too might be shedding emotional tears.
So is there something about women’s tears that makes men less aggressive? Not really, the team expected that tears from people of any gender would affect testosterone levels. But culturally women are socialized to be allowed to cry, so it was easy to get volunteers. Lowering testosterone has a bigger effect on aggression in men than aggression in women.
Despite the gendered division here, the team thinks that the effects of tears are related to babies. And they will work to expand the understanding of the effects of tears beyond male aggression.
“We note that crying often occurs in very close-range interactions, to the extent that “kissing teary cheeks” is a recurring theme across cultures. Thus, chemosensing of tears is a viable possibility in human behavior,” the authors wrote in the paper.
“Moreover, although we tested tears from women donors, we speculate that all tears would have a similar effect. 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.”
The paper was published in PLoS Biology.