Human tears carry a substance that dampens aggression, according to researchers, who believe the droplets evolved over time to protect crying babies from harm.
Smelling emotional tears from women reduced male aggression by more than 40% in computerized tests, and caused corresponding changes in the brain, although the scientists behind the study think that all human tears would have a similar effect.
“The reduction in aggression was impressive to us, it seems real,” said Noam Sobel, a professor of neurobiology at the Weizmann Institute of Science in Israel. “Whatever is in tears actually lowers aggression.”
Charles Darwin surprised at the point of crying. Writing in The Expression of Emotions in Man and Animals in 1872, the great naturalist declared sobbing as “purposeless as the discharge of tears from a blow outside the eye”.
But in the 150 years since, researchers have proposed all kinds of roles, from signaling vulnerability and helplessness to clearing bacteria from the eyes.
Previous work at Sobel’s lab found that women’s tears are sniffed decreased male testosterone but it was unclear whether this affected behaviour. In animals, the picture is clearer: subordinate mole rats, for example, cover themselves in tears to protect themselves from attackers.
For the latest study, Dr. Shani Agron and others in Sobel’s lab collected tears that rolled down women’s faces while they watched sad movies. The researchers did not specifically advertise for female tear donors, but almost all who came forward were women, six of whom were chosen because they produced tears in such quantities.
The experiments involved 31 men who sniffed either saline or women’s tears before dabbing the swabs with the drops stuck to their upper lips. The men then participated in a computerized game used in psychology to provoke aggressive behavior by unfairly deducting players’ points.
Aggressive behavior, in the form of retaliation, was 43.7% lower when men sniffed women’s tears versus salt, the scientists write Plos Biology. Further tests in a brain scanner revealed that tear sniffers had more functional connectivity between regions dealing with scents and aggression, while activity in brain networks for aggression was lower. “This chemical seems to orchestrate the brain’s response to aggression,” Sobel said.
One puzzle the scientists faced was that while rodents have a sensory system that can detect such substances, there was no known way for humans to do so. In laboratory tests, researchers from the Duke University team found that four types of receptors on olfactory neurons were activated by human tears, suggesting that they may respond to the aggression-suppressing substance.
The chemicals in tears are unlikely to have much impact on the social interactions of adults, Sobel concedes, but he speculates that the composition of tears may have evolved to protect vulnerable infants.
“Babies can’t say, ‘Stop being aggressive with me.’ They are very limited in their ability to communicate, and they are also helpless. They have a vested interest in reducing aggression and this reflects the sad reality of aggression towards babies,” he said.
Ad Vingerhoets, an emeritus professor of emotions and well-being at Tilburg University, who was not involved in the study, said: “It makes sense if tears somehow inhibit aggression, because it is common knowledge that babies who cry a lot are at risk for physical abuse. It might help them survive.”
Dr Minna Lyons, a psychologist at Liverpool John Moores University, said the reduction in aggression was “remarkable”, but warned that it was important for studies on human behavior to be repeated before strong conclusions were drawn.
“In real life, things can turn out differently. The tears of the target of domestic violence can do little to reduce aggression from the perpetrator. Why doesn’t the chemosignaling work in these circumstances?” she said. “The social context of crying is highly complex, and I suspect that reducing aggression is just one of the many potential functions of tears.”
Sobel’s lab hopes to identify the active ingredient in tears. This would open the door to manufacturing the substance and potentially using it to reduce aggressive tendencies.
