December 21, 2023
4 min read
For the first time, researchers have used behavioral measures, brain imaging and molecular biology to test how odorless chemicals from human tears affect people
When someone starts to cry, other people oftentimes feel empathy and concern. But the biological reasons for shedding tears can extend beyond merely prompting a sense of compassion. The tears themselves appear to take on a role as a chemical peacemaker between men and women, new research has discovered.
Women’s tears act to reduce aggression in men who are close enough to catch a whiff of the waterworks, according to findings published in PLOS Biology and confirmed through human behavioral studies, brain imaging and molecular biology. The researchers speculate that tears from people who aren’t women likely have a similar effect, but this has yet to be tested.
The new paper suggests that a fundamental reason why women shed emotional tears is “to convey a chemical signal that lowers aggression,” says Shani Agron, co-lead author of the study, which she conducted for her doctoral degree in neurobiology at the Weizmann Institute of Science in Israel. (Agron has since graduated.) “We believe this is a shared mechanism of many mammals.”
Producing tears has long been considered a uniquely human behavior, but that’s a flawed notion, says co-senior author Noam Sobel, a neurobiologist at the Weizmann Institute of Science, who was Agron’s adviser. Dogs, for example, shed tears when they reunite with their owners after a period of separation. Baby mouse tears contain molecular cues that sway female mice to reject male advances, while pheromones in female mouse tears encourage males to stop fighting each other and mate with the female instead. Mole rat subordinates, moreover, go so far as to cover themselves in their own tears to chemically deflect aggression from dominant members in their group.
Humans, like other mammals, communicate information with body odors. But it wasn’t obvious that tears would have any olfactory effect on human behavior because people cannot perceive a smell from them. Additionally, while most mammals have a second olfactory organ that is responsible for detecting pheromones, in humans, this organ is thought to be vestigial.
The first evidence that tears can chemically influence human behavior emerged in 2011, when Sobel and his colleagues published a study in Science that showed that women’s tears reduced levels of testosterone and self-reported sexual arousal in men. This earlier work raised more questions than it provided answers for. It took years of effort to conduct a more thorough follow-up, in part because of the difficulty of collecting tears from donors. The researchers need at least one milliliter of tears to use on each experimental participant, which is “a lot of tears,” Sobel says. Using an onion or other irritant to force someone’s eyes to water is not an option, Agron adds, because “these are a completely different type of tears.”
To collect the tears used in the new study, the team put out a call for volunteers who cry easily. Only a few men showed up, and none was able to produce enough tears to qualify. Of the 100 or so women who volunteered, only six were able to supply sufficient amounts of tears to warrant collection. Participants could use any means they wanted to elicit the tears—from listening to sad music to reading a sad letter—but most turned to the lab’s expansive “library of sad movies,” Agron says. In addition to tears, the researchers also collected drops of saline solution that they trickled down women’s face for use in control experiments.
The researchers next gathered data from 25 male volunteers who played a game in the lab that is often used in studies of aggression. During the competitive game, participants were led to believe that their opponent was another person. It was in fact a computer algorithm, however. Occasionally, the opponent stole money from the participants, who could then either choose to take revenge, with no monetary gain for themselves, or let their opponent get away with it but continue collecting more money for themselves. The researchers calculated aggressiveness by the number of times a participant chose revenge divided by the number of times they were provoked. The team also repeated this experiment in a second group of 26 male volunteers who played the game while inside a magnetic resonance imaging scanner, allowing their brain activity data to be collected.
All participants played the game twice, and before each session, they were asked to inhale from a “sniff jar” that they were told contained “assorted odors” but that in fact held either tears or saline. The researchers found that when the men sniffed the women’s tears, they were nearly 44 percent less aggressive in the game than when they sniffed the saline solution. The behavioral reduction in aggression was also accompanied by neuronal changes. The researchers observed that the men’s brain post-tear sniff exhibited less activity in the prefrontal cortex and the left anterior insula, regions that are associated with aggression and decision-making. Connectivity between the anterior insula and amygdala, a region that is responsible for emotional processing and also part of the olfactory network, increased as well.
Working with colleagues at Duke University, the team used molecular biology methods to test the effects of tears and saline on 62 human olfactory receptors in a lab dish. They identified four receptors that responded to tears but not saline. This finding helps to answer “a major question” about how pheromonelike signals are processed in humans, Agron says.
Agron, Sobel and their colleagues are now interested in running future experiments to test the effect of women’s tears on other women and that of babies’ tears on adults. Sobel hypothesizes that baby tears, especially, will likely have an aggressiveness-lowering effect on adults. “Babies cannot communicate with you in language,” he says. “But evolution may have provided babies with this tool to lower aggression.”
The fact that people continue to be able to produce tears throughout adulthood, Agron adds, indicates that crying is probably “a behavior that serves us throughout life.”