Woman crying
Woman cryingiStock

All land mammals have tear glands in their eyes, but the human tearing experience was until recently considered unique. After all, we are the only animal to shed a tear while watching Beaches.

Now, a new Weizmann Institute of Science study reveals that human tears have much more in common with those of other animals than previously thought: They contain chemicals that reduce aggression in others, as do the tears of, for example, mice and blind mole rats. The study, which will be published December 21 in PLOS Biology, showed that sniffing women’s tears lowered brain activity related to aggression in men, reducing aggressive behavior.

The study addressed the long-standing mystery of why we cry. Charles Darwin was puzzled by emotional tearing, which appeared to have no useful function – beyond the role that tears play in lubricating the eye – so he concluded that such tearing must have evolved in humans by chance. Since then, however, numerous studies, particularly in rodents, have shown that mammalian tears contain chemicals serving as social signals that can be emitted on demand. One of their most common purposes is to reduce aggression. The tear liquid of female mice, for example, contains chemicals that affect aggression networks in the brain, thereby reducing fighting among male mice. Subordinate males of blind mole rats smear themselves in tears to reduce the dominant male’s aggressive behavior toward them.

Prof. Noam Sobel, whose lab in Weizmann’s Brain Sciences Department studies olfaction, the sense of smell, has hypothesized that human tears also contain chemicals that serve as social signals. Back in 2011, in research published in Science, his team had shown that sniffing women’s emotional tears reduced testosterone levels in men, resulting in somewhat diminished levels of sexual arousal.

In the new study, researchers led by PhD student Shani Agron from Sobel’s lab wanted to determine whether tears have the same aggression-blocking affect in people as they do in rodents. In a series of experiments, men were exposed to either women’s emotional tears or saline, without knowing what they were sniffing and without being able to distinguish between the two, since both are odorless. Next, they played a two-person game. The game was designed to elicit aggressive behavior in one player toward the other player, who the men were led to believe was cheating. When given the opportunity, the men could get revenge on the other players by causing them to lose money, though they themselves gained nothing.

After the men sniffed women’s emotional tears, their revenge-seeking aggressive behavior during the game dropped by about 44 percent – that is, nearly in half.

This seemed equivalent to the effect observed in rodents, but rodents have a structure in their noses called the vomeronasal organ, which picks up the social chemical signals. Humans don’t have this organ, so how do they sense the social chemicals? To find an answer, the researchers applied the tears to 62 human olfactory receptors in a laboratory dish and found that four of these receptors were activated by the tears, even though tears are odorless.

Furthermore, the researchers repeated the experiments while examining the men’s brains in an MRI scanner. Functional imaging showed that two aggression-related brain regions – the prefrontal cortex and the anterior insula – were less active when the men were sniffing the tears. The greater the difference in this brain activity between saline and tears, the less often the player took revenge during the game.

“We’ve shown that tears activate olfactory receptors and that they alter aggression-related brain circuits, significantly reducing aggressive behavior,” Sobel says. “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.”

In fact, recent studies have found that dogs also shed emotional tears. However, more research is needed to determine whether these tears contain chemical signals that can be picked up by other dogs or by humans.

As for social interactions among humans, future research will explore whether the new study’s findings apply to 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.”

Agron adds that this effect is likely to gain in importance when verbal communication is impossible, for example in interactions with babies: “Infants can’t talk, so for them relying on chemical signals to protect themselves against aggression can be critical.”

The study was conducted in collaboration with Prof. Hiroaki Matsunami of Duke University School of Medicine, whose former postdoctoral fellow, Dr. Claire A. de March, led the research together with Agron. Study participants also included Reut Weissgross, Dr. Eva Mishor, Lior Gorodisky and Dr. Tali Weiss of Weizmann’s Brain Sciences Department, and Dr. Edna Furman-Haran of Weizmann’s Life Sciences Core Facilities Department.