Almost all land mammals have tear glands around their eyes, and it is thought that these are primarily used to keep eyes most and clean.

However, humans are relatively unique in that we are one of only a couple of mammals that cry when overcome with emotion as well as when we have something stuck in our eye.

Now, new research published in the journal PLOS biology thinks it might know why we cry when we get emotional, and it’s all to do with keeping us safe when things get tense.

The researchers started by looking at previous research that found the tears of female mice contained chemicals which affected aggression networks in the brain of male mice. When exposed to these tear chemicals, the level of fighting between male mice reduced. The other study they looked at found that when human males smelled women’s emotional tears, their levels of testosterone were reduced.

Knowing that testosterone levels and aggression are related, the scientists then ran a series of experiments where human males were asked to play a video game designed to provoke revenge-seeking aggression from one player to another. During their game, the men were asked to smell an odourless liquid – for some this was a control liquid, and for others this was a liquid made up of ‘emotional tears’ from 6 female donors.

The study also scanned brain activity of the men while they were sniffing the ‘emotional tears’ using an MRI scanner.

The researchers found that in the game experiment, male aggression reduced by 44 percent for the players that smelled the women’s tears when compared to those who smelled the control solution. The brain scans also showed changes in the brain activity around regions of the brain that connect olfaction and aggression in the males who were sniffing the tears.

The researchers go on to suggest that this might be why babies cry so much - to help reduce aggression around when they are at an age where they are defenceless.

While this study only looked at the effect of female tears on male aggression, there is no reason to think that the effect might be the same for the tears of all genders.

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