Save the Crocodile Tears…Or Don’t!
By: Kim Nguyen
Slow down, go outside, and stop to smell the…tears? Researchers have found that sniffing women’s tears may lower aggression and increase pro-social decision-making in men.
A recent study from the Weizmann Institute of Science suggests that women’s tears contain chemicals that block aggression in men, activating a process called social chemosignaling. Social chemosignaling refers to the chemical stimulations exchanged between members of a particular species, which are detected through olfaction, taste, or sweat.
Social chemosignaling mechanisms have been discovered in several animal species, but their role in humans has historically been under-researched. The phenomenon was first studied in rodents, where researchers found that females’ tears contain chemicals that block male aggression through olfactory reception. A 2011 study published by Shani Agron’s lab at the Weizmann Institute of Science found that human tears also contain a chemosignal that lowers male testosterone, but the role of tears in regulating behavior was unclear. Since lowering testosterone plays a greater role in reducing aggression in men than in women, Shani Agron and her team hypothesized that human tears block male aggression, much like rodent tears do. Through their study, they found that sniffing women’s tears led to reduced aggression-related brain activity in men, resulting in a 43.7 percent decrease in overall aggressive behavior.
Initially, Agron wanted to learn more about why humans cry. She was inspired by Charles Darwin, who studied the evolution of emotional expressions and their functions. Specifically, she was intrigued by The Expression of the Emotions in Man and Animals, a book by Darwin that contains a chapter entirely devoted to crying.
“In this chapter, Darwin argues that every emotional expression has a [particular] function,” Agron said. “For example, when an animal sinks its teeth into prey or an enemy, this [act] is a sign of aggression. This same evolutionary process applies [to] the face of disgust, since the physical function was to spit out bitter or spoiled food, but it has evolved to be a general expression of distaste, [even for non-food].”
In these examples, physical function evolved into emotional expressions. However, Darwin could not find a function for crying, an obvious emotional expression. For this reason, Darwin proposed that crying and tears simply evolved by chance as a result of an evolutionary mistake. The question of why humans cry remained largely unanswered before the Weizmann team took on their project. Using the existing body of literature on social chemosignaling, Agron’s team hypothesized that tears convey chemical signals similar to pheromones in animals.
To test whether chemicals in human tears can reduce aggression in others, the researchers conducted an experiment in which female participants were asked to watch sad movies, and their tears were collected for later use. Interestingly, according to Agron, collecting tears was the most challenging part of this study, as it took nearly six years to collect a sufficient amount. Before the tears were donated for research, the scientists trickled saline down the cheeks of the tear donors. This saline was used as a control to ensure that the findings resulted from the tears and not from any skin-bound signaling molecules, or compounds emitted by the skin. The tears and saline were then put into separate “sniff jars.”
In the main experiment, male participants were invited to the lab, where they played a game based on a model called the point subtraction aggression paradigm (PSAP), a validated measure of aggression in response to provocation.
“The experimental design was within-subject, meaning that each participant received saline on one day and tears on the other. It was counterbalanced for order, meaning that some received saline on the first day, tears on the second, and others [received them] the other way around,” Agron said.
The experiment was also double-blind for condition, meaning that neither the experimenter nor the participant knew if the “sniff jar” contained tears or saline. The participants were asked to play the game on two consecutive days, playing once after sniffing saline and once after sniffing tears.
In the PSAP game, the participants’ goal was to obtain as much money as possible. However, instead of playing against another human opponent, participants played against a fictitious opponent who they were led to believe was a real person. This fictitious opponent provoked participants during the game by stealing money from their game accounts. Participants were then given two choices: to disregard the opponent’s act and continue collecting money or to retaliate against the opponent for no personal gain.
Aggression levels were calculated by dividing the number of retaliation responses by the number of provocations, resulting in an aggression provocation ratio (APR). Higher APRs reflected higher aggression levels. The data showed that the APR for saline was 1.67, and the APR for tears was 0.94. This meant that men who sniffed tears had a 43.7 percent reduction in aggressive response during the PSAP game compared to those who sniffed saline. In other words, these male participants decided more often not to take revenge against the fictitious opponent after sniffing tears.
Functional magnetic resonance imaging (fMRI) scans were also used to observe the participants’ brain activity before and after sniffing tears or saline during the game. After sniffing tears, the fMRI revealed reduced activity in the prefrontal cortex and the left anterior insula, areas linked with the brain’s aggression network. Additionally, sniffing tears significantly affected the functional connectivity, or communication, between the left anterior insula and the amygdala, a brain region critical for emotional processing. These brain regions form a functional brain network associated with olfaction and aggression. Therefore, tears greatly increased functional connectivity within a network of brain regions associated with aggression and olfaction, and this augmentation was correlated with the individual behavioral impact of sniffing tears.
In another experiment, researchers tested the olfactory receptors’ response to tears. In animals, pheromones act through the vomeronasal organ of the accessory olfactory system, an organ that humans don’t have. Instead, humans possess the main olfactory system, so the researchers sought to test if the receptors in the main olfactory system could respond to odorless bodily secretions such as tears.
An assay was used to detect the real-time activation of receptor responses. In the initial screening, sixty-two human olfactory receptors were expressed in vitro — twenty-one of which were activated by tears, but not by a trickled saline solution. Another screening of these twenty-one olfactory receptors was repeated, activating four of the twenty-one receptors in response to tears but not to saline. This suggests that although human emotional tears are essentially odorless, they can activate specific human olfactory receptors in vitro. This conclusion may provide the molecular premise for social chemosignaling through tears in humans.
Currently, Agron’s team is collecting tears to identify which compounds found in tears are responsible for inducing less aggressive behavioral outcomes. Agron’s research may have extensive social significance, which has fueled her interest in these studies.
“When I come to think of it, children cry [the same way],” Agron said. “When it comes to babies, this mechanism is really important, [since] they can’t speak. They need to rely on other ways to signal their needs, and the ability to have a chemical signal [that] can lower aggression towards them is something that is critical for their survival.”
Due to the challenging process of collecting tears, the experiment could not be repeated to test if men’s tears lower aggression in women. As a result, there is still limited information about tear chemosignalling in human behavior. Based on existing information, though, tears are thought to promote caring behavior and enhance social bonding. Humans are inherently social beings, so it is important to study tears as a potential mechanism that protects individuals against aggression.
