Human tears carry a substance that dampens down aggression, according to researchers, who believe the drops may have evolved over time to protect wailing babies from harm.
Sniffing emotional tears from women reduced male aggression by more than 40% in computerised tests, and prompted corresponding changes in the brain, though the scientists behind the study think 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 puzzled over the point of weeping. Writing in The Expression of Emotions in Man and Animals in 1872, the great naturalist declared sobbing as “purposeless as the secretion of tears from a blow outside the eye”.
But in the 150 years since, researchers have proposed all manner of roles, from signalling vulnerability and helplessness to clearing bacteria from the eyes.
Previous work at Sobel’s lab found that sniffing women’s tears reduced 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 aggressors.
For the latest study, Dr Shani Agron and others in Sobel’s lab collected tears rolling down women’s faces as they watched sad movies. The researchers did not specifically advertise for female tear donors but nearly all who came forward were women, of whom six were selected because they produced tears in such quantities.
The experiments involved 31 men who sniffed either saline or women’s tears before having swabs dabbed with the droplets stuck to their upper lip. The men then took part in a computerised game used in psychology to provoke aggressive behaviour by unfairly deducting players’ points.
Aggressive behaviour, in the form of retribution, was 43.7% lower when men were sniffing women’s tears versus saline, the scientists write in Plos Biology. Further tests in a brain scanner revealed that tear-sniffers had more functional connectivity between regions that handle scents and aggression, while activity in brain networks for aggression was lower. “This chemical appears to be orchestrating the brain 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 lab tests, researchers on the team from Duke University found that four types of receptors on smell-sensing neurons were activated by human tears, suggesting they may respond to the aggression-dampening 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 babies.
“Babies can’t say: ‘Stop being aggressive towards me’. They are very limited in their ability to communicate, and they are helpless as well. They have a vested interest in lowering aggression and that reflects the sad reality of aggression towards babies,” he said.
Ad Vingerhoets, an emeritus professor of emotions and wellbeing at the University of Tilburg, who was not involved in the study, said: “It makes sense if tears in some way inhibit aggression because it’s common knowledge that infants that cry a lot are at risk for physical abuse. It might help them to survive.”
Dr Minna Lyons, a psychologist at Liverpool John Moores University, said the reduction in aggression was “remarkable” but cautioned that it was important for studies on human behaviour to be replicated before drawing strong conclusions.
“In real life, things may play out differently. The tears of the target of domestic violence may do little in reducing aggression of the perpetrator. Why does the chemosignaling not work in these circumstances?” she said. “The social context of crying is massively complicated, and I suspect the reduction of aggression is just one of the many potential functions of tears.”
Sobel’s lab hopes to identify the active ingredient in tears. That would open the door to manufacturing the substance and potentially using it to reduce aggressive tendencies.