A psychological game in a lab and magnetic resonance imaging (MRI) scans of the players’ brains have bolstered scientific evidence for a longstanding belief — women’s tears can dampen male aggression.

Counterintuitively, however, the effect owes not so much to sympathy at the sight of the tears as to unconscious smelling of the tears.

A study by neurobiologists in Israel and the US has found that chemicals in women’s tears can lower aggression in men, corroborating in humans a chemo-signalling role of tears that had hitherto been experimentally documented in rodents.

Their findings, published in the journal PLOS Biology on Thursday, suggest that tears induce changes in the brain’s centres for smell and aggression and provide what the researchers have called “a chemical blanket protection against aggression”.

The role of tears other than their housekeeping functions in the eyes had once puzzled scientists. Charles Darwin had in his 1872 book, The Expression of Emotions in Man and Animals, his third major work on evolutionary theory, suggested that beyond eye maintenance, weeping is an “incidental result”.

But multiple studies have over the past two decades established that male aggression in rodents is blocked when they smell female tears.

In the new study, research scholar Shani Agron and her colleagues at the Weizzman Institute of Science, Israel, and the Duke University Medical Centre in the US looked at the effect of women’s tears in men playing a two-person game.

The researchers exposed a group of men to either women’s emotional tears or saline water as they played a game designed to elicit aggressive behaviour against the other player, causing a loss of money. The men did not know what they were sniffing.

The scientists found that sniffing emotional tears reduced the players’ aggression on average by 44 per cent. The scientists also found that smell-related receptors in the human brain reacted in a dose-dependent manner to tears — the greater the concentration of tears, the larger the receptors’ response.

Brain MRI scans of the players revealed that when the players were provoked during the game, two aggression-related regions in the brain became more active. But the regions did not become as active in the same situations of the game when the men sniffed tears.

The scientists found that sniffing tears increased the functional connectivity between the brain regions related to smell and aggression, reducing the overall levels of neural activity in the region associated with aggression.