Scientists at the Weizmann Institute of Science in Israel have discovered that human tears contain social signals acting as a 'chemical blanket' that reduces aggression in others.
The research, published in PLOS Biology, may solve the longstanding mystery of why humans cry.
Unlike most animals, which use tears primarily for eye lubrication, humans and dogs uniquely shed tears in response to emotional states. This phenomenon, once thought to be pointless by scientists, including Charles Darwin, now appears to have a profound social function.
The Israeli scientists discovered that tears contain 'chemical signals' that act as a protective mechanism for the individual expressing them. In a series of controlled experiments, men unknowingly inhaled either women’s emotional tears or a saline solution, both of which are odorless.
Subsequently, these men participated in a game engineered to provoke revenge-driven aggression. Remarkably, exposure to women’s tears resulted in a 44 percent decrease in aggressive behavior compared to those who sniffed the saline solution.
Furthermore, brain imaging revealed diminished activity in aggression-related brain regions while men were exposed to tears. Laboratory tests on 62 human olfactory receptors confirmed that pheromones in tears activate some of them.
Professor Noam Sobel, leading the study, stressed its significance, saying: "We’ve shown that tears activate olfactory receptors and that they alter aggression-related brain circuits, significantly reducing aggressive behavior." He suggested that this 'chemical blanket' of protection may be a shared trait among mammals, including rodents and humans.
Recent research has broadened our understanding of tears beyond their basic function of eye lubrication in land mammals. Studies have shown that female mice's tears contain chemicals that influence male mice's aggression, and subordinate blind mole rats use tears to mitigate aggression from dominant males. This expansion in knowledge extends to discovering emotional tears in dogs, suggesting potential for interspecies emotional communication.
Sobel’s team alsdo found that when men inhale the scent of tears, it leads to a decrease in their levels of testosterone, the hormone associated with sexual arousal. That suggests a deeper biochemical impact of tears in human interactions.
Doctoral student Shani Agron, a member of Sobel’s team, highlighted the importance of expanding their research to include women for a more complete understanding of tears' effects beyond the initial focus on men due to their pronounced aggression reduction in response to lowered testosterone.
The study also offers valuable insights into infant behavior, raising the possibility that infants may use tears as a chemical signal to protect themselves against aggression — explaining why babies cry all the time.