Smelling women's tears reduces aggression in men (also happens in mice)

Charles Darwin was particularly disconcerted from human crying aroused by emotions and, given the lack of an apparent function other than that of keeping the eye healthy, he concluded that crying is “an accidental result” . However, a large body of data has since convincingly demonstrated that tears serve a function that goes beyond defending eye health as they also serve mammals as a means of social chemo-signalling that can be emitted on demand.

New research, published on December 21 in the open access journal PLOS Biology , shows that women's tears contain chemicals that block aggression in men. The study conducted by Shani Agron at the Weizmann Institute of Science, Israel, finds that smelling tears leads to reduced brain activity related to aggression, which results in less aggressive behavior. The researchers do not give any suggestions on the possible practical implications of this discovery. But some reflections deserve to be made.

Testosterone

It is known that, in male rodents, aggression is blocked when they smell female tears. This is an example of Social chemo-signaling, a process common in animals but less common, or less understood, in humans. Human tears also contain a chemical signal that lowers male testosterone, but its behavioral significance was unclear. So long as reduced testosterone is associated with reduced aggressionresearchers tested the hypothesis that human tears act like those of rodents to block male aggression.

Chemical signals and behavior

In particular, the authors write, «there are several cases of chemical signals that alter hormone-dependent behavior in humans. Examples include maternal behavior, dietary behavior, social behavior in general, and sociosexual behavior in particular. In other words, that a chemical signal can alter human behavior is not unusual. Furthermore, especially emotional behaviors are an excellent candidate for modulation by chemical signals, perhaps a reflection of their shared neural substrates in the amygdala complex and an associated extensive brain network spanning the ventral temporal cortex, frontal cortex , the anterior cortex cingulate cortex and insula striatum”.

«Given this neural connection and given that human aggression can be measured at a behavioral level using various standardized tasks, we decided to measure the aggressive behavioral and brain response after smelling tears produced by an emotional state”, they add.

Experimentation through a role-playing game

To determine whether tears have the same effect in people, the researchers “exposed” a group of 25 men (average age, 26 years) to the tears collected by a group of “donors” (6 women, aged between 22 and 25) or to a saline solution while they played a partner game. The game was designed to elicit aggressive behavior towards the other player, who the men were led to believe was cheating.

When given the opportunity, men could take revenge on the other player by making him lose money. The men didn't know what they were smelling and they could not distinguish between the tears and the saline solution, both of which were odorless. Aggressive behavior seeking revenge throughout the game decreased by more than 40% (43.7% to be precise) after the men smelled the tears produced by women's emotions.

In vitro and magnetic resonance imaging tests

To probe peripheral brain substrates of this effect, tears were applied to 62 human olfactory receptors in vitro. Four were identified that responded in a dose-dependent manner to this stimulus. Finally, to probe the central brain substrates of this effect, the experiment was repeated by subjecting the participants to functional magnetic resonance imaging (fMRI).

Imaging showed two brain regions linked to aggression – the prefrontal cortex and the anterior insula – becoming more active when men were provoked during the game, but they didn't become as active in the same situations in which the men were smelling the women's tears.

The implications

Individually, the greater the difference in this brain activity, the less often the player retaliates during the game. Finding this link between tears, brain activity and aggressive behavior implies that social chemosignaling is a factor in human aggression, not simply animal curiosity. The authors add: «We found that, just as in mice, human tears contain a chemical signal that blocks conspecific male aggression. This goes against the idea that tears produced by emotions are uniquely human».

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