What’s the evolutionary use of crying?

loosing cry by Brittany Randolph, on FlickrCrying is one of those human behaviours that few of us ever bother to question. But when you stop to think about it, weeping is very strange. Why, indeed, do we excrete salty liquid from our eyes when we feel sad or – in some cases – really happy? Well, it turns out there isn’t a definite answer just yet, but many clever theories.

First, let’s start with where teary eyes definitely do make sense: when your eyes are under attack by unwelcome invaders (anything from bacteria to flies), toxins (think onions) or smoke. Tear fluid –a natural disinfectant – kills and flushes out intruders and keeps the eye hydrated.

In fact, we are constantly producing tears (aka lacrimal fluid); it’s only when there’s so much that our eyes’ drainage capacity is overwhelmed that we notice them. This works the same for most animals. The difference being that other animals don’t tend to shed a tear when they’re down in the dumps or witnessing a particularly beautiful sunset.

One school of thought suggests that emotional tears allow us to eliminate excess stress. This idea is based on research in Minnesota showing that emotional tears (unlike the onion-kind) have a different chemical composition – having higher levels of proteins and manganese. Emotional tears (‘psychic tears’) also include a natural pain killer called leucine enkephalin. This could explain why crying is said to make you feel better… but why does this have to happen in the eyes? Wouldn’t it make more sense to sweat, rather than cry out these pain killers?

Crying to manipulate others?

A completely different take on crying is that it has a more socially driven purpose. By blurring your own vision you’re said to be communicating to your peers that you’re not much of a threat. In fact, you probably could do with some help – crying as a white flag so to speak. Thanks to another trait we share with few other species – empathy – this show of weakness can generate pity.

Robert R. Provine (author of Curious Behavior: Yawning, Laughing, Hiccupping, and Beyond) suggests empathy came first: your peers would feel inclined to help you when your healing tears revealed your eyes were hurt in some way. Then later down the evolutionary line, you shed tears even though you haven’t been cutting onions, hijacking the ancient pity response from others. Now that’s a clever theory – but a tricky one to prove.

It seems tears might even help you understand your own feelings. Some people – diagnosed with Sjörgren’s syndrome – have trouble producing tears and, oddly enough, also have trouble recognizing and dealing with their own emotions.

So whatever the reason for tears – be it to relieve stress or identify you as a friend in need – they seem useful enough to deserve being let out.

Answer by Isabel Hutchison

Article by Isabel Hutchison

August 11, 2014

Isabel is currently working on a Ph.D. in the Cognitive Neuroscience of Sleep (which, ironically, involves not getting very much sleep at all). Besides doing science and writing about it (check out her sleep science blog here), Isabel loves music, dancing , travelling and art.


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