Although many animals cry, emotional tears — triggered by strong feelings such as joy and sadness — are considered unique to humans. Charles Darwin himself was perplexed by this feature, declaring them “purposeless.” Nearly 150 years later, emotional crying remains one of the human body’s more confounding mysteries.
Beyond the physiological (tears lubricate the eyes), some have proposed that crying may improve social bonding and human connection. More recently, scientists at the Weizmann Institute of Science in Israel have made a fascinating case. They reveal another potential hidden purpose for emotional crying.
Although tears are often seen as a universal symbol of vulnerability and sadness, the findings by these researchers flip this script. They suggest tears may hold a hidden power: to disarm aggression in men.
Researchers found that sniffing emotional tears from women led to a significant decrease in aggressive behavior and brain activity linked to aggression in male participants. This effect, caused by chemical signals in the tears, mirrors similar findings in rodents.
Tears to quell aggression
Previously, researchers found that the tears of female mice release chemicals that dampen intermale aggression. Scientists have also observed subordinate male mole rats cover themselves in tears, which reduces the aggression of dominant males towards them.
Prof. Noam Sobel, who has his lab in Weizmann’s Brain Sciences Department studying olfaction (sense of smell), was fascinated by these findings. In 2011, his team did their own experiments and remarkably found that smelling women’s emotional tears reduced testosterone levels in men and diminished their sexual arousal.
In their new study, Sobel and colleagues wanted to see whether tears also block aggression in people like they do in rodents. The researchers collected tears from six female donors who cried while watching sad films. They then asked twenty-five men to participate in a monetary two-person game designed to provoke aggression. Participants were tricked into thinking they were playing against another human who was cheating. It was, in fact, a computer. They were offered the opportunity to get payback by causing the other player to lose money (although gaining nothing themselves).
Before playing, the men sniffed either female tears or a saline solution. Both mixtures are odorless so you can’t tell the difference. The results showed a 43.7% reduction in revenge-seeking aggression after exposure to tears. To confirm the robustness of these findings, a statistical analysis showed only a 2.9% chance of obtaining this result by random chance.
The subtle chemical effect of tears on the human brain
These findings suggest that human tears have a real chemosignaling effect, particularly on aggression. Intriguingly, rodents have a special structure in their noses called the vomeronasal organ to pick up social chemical signals. But humans lack this organ and perceive tears as odorless. However, when the researchers applied emotional tears to 62 human olfactory receptors in a laboratory dish, they found four of these receptors activated in response to the tears.
The researchers then examined brain activity using fMRI scans. They found reduced activity in two brain structures linked to aggression after smelling tears. These were the left anterior insula cortex (AIC) and bilateral prefrontal cortex (PFC). Additionally, there was increased connectivity between the left AIC and the right amygdala and piriform cortex, areas involved in smell and aggression.
“We’ve shown that tears activate olfactory receptors and that they alter aggression-related brain circuits, significantly reducing aggressive behavior,” Sobel said in a statement.
“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.”
Next, the researchers plan to extend their research to women. Do the findings apply to women in response to emotional tears from both men and other women?
“When we looked for volunteers who could donate tears, we found mostly women, because for them it’s much more socially acceptable to cry,” Agron says.
“We knew that sniffing tears lowers testosterone, and that lowering testosterone has a greater effect on aggression in men than in women, so we began by studying the impact of tears on men because this gave us higher chances of seeing an effect. Now, however, we must extend this research to include women, to obtain a fuller picture of this impact.”
“Infants can’t talk, so for them relying on chemical signals to protect themselves against aggression can be critical.”
The findings appeared in the journal PLOS Biology.
