Sniffing women’s tears can reduce aggressive behaviour in men, according to new research – and could explain why crying evolved in humans.
Although tears are odourless, the study showed they emit a chemical signal that blocks aggressions in men, and that smelling these tears leads to reduced brain activity related to aggression.
However, the effect could be the same across all genders.
Male aggression in rodents has long been known to be blocked when they smell female tears, which is an example of social chemosignalling, a process that is common in animals but less common in humans.
To determine whether tears have the same effect in people, the researchers exposed a group of men to either women’s emotional tears or saline while they played a two-person game.
The game was designed to elicit aggressive behaviour against the other player, whom the men were led to believe was cheating.
When given the opportunity, the men could get revenge on the other player by causing them to lose money.
The men did not know what they were sniffing, and could not distinguish between the tears or the saline.
The results, published in PLOS Biology, showed that revenge-seeking aggressive behaviour during the game dropped more than 40% after the men sniffed women’s tears.
An MRI of their brains revealed that, when sniffing the tears, men’s two aggression-related brain regions – the prefrontal cortex and anterior insula – were not as active.
Individually, the greater the difference in this brain activity, the less often the player took revenge during the game.
However, the authors noted that the effect is likely the same in women. The team only studied the effect of women’s tears on men because the range of participants – most of those who replied to advert to ‘donate’ tears were women.
‘This becomes particularly ecologically relevant with infant tears, as infants lack verbal tools to curb aggression against them and are therefore more likely to rely on chemosignals,’ they wrote.
Lead author Shani Agron, a PHD student from the Weizmann Institute of Science, Israel, added: ‘We found that just like in mice, human tears contain a chemical signal that blocks conspecific male aggression.
‘This goes against the notion that emotional tears are uniquely human.
‘Finding this link between tears, brain activity, and aggressive behaviour implies that social chemosignalling is a factor in human aggression, not simply an animal curiosity.’
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