After all, humans are the only species that does. (Despite the best intentions of the ASPCA and Sarah McLachlan, their popular TV commercial does not show dogs and cats in tears. It'd be impossible.)
But according to scientists who study evolution, crying has likely evolved to be a tool -- a leg up in natural selection -- to help the species persist.
For sure, tears themselves have a quotidian purpose: they keep our eyes lubricated. But scientists say tears -- the streaming-down-your-cheek, no-no-I'm-fine ones -- have evolved as a deceptive signal to other humans.
Deceptive, of course, because they immediately reveal our most intimate feelings to others nearby.
With consideration to evolution, that sounds a bit counter-intuitive: why would you want to reveal your vulnerability to another? But scientists say that communicative cue elicits similar feelings from others -- whether happy or sad tears -- and moves them to sympathize.
Scientists call it "theory of mind" -- the unique human ability to attempt to understand the psychological reasons that are causing another person's behavior. It's present in adults, kids and babies alike.
In a great report, NPR elaborates (emphasis mine):
This power of empathy is huge, and it's fundamental to pretty much everything we do, from forming close relationships to living in complex societies. [Belfast University Institute of Cognition and Culture director Jesse] Bering says those of our early ancestors who were most empathic probably thrived because it helped them build strong communities, which in turn gave them protection and support.
Within these communities, Bering says, tears could be powerful tools. They did more than just signal vulnerability — they were perhaps a way of keeping social and reproductive bonds strong. Maybe good criers were survivors.
"Crying seems to elicit compassion and guilt," Bering says, "and that itself may be an evolved mechanism to save relationships in distress."
Tearful crying: an emotional fast lane, effective where conversation may not be.
Fundamentally, crying is a way to get what you want -- that's why babies do it. (Sorry, mothers.) Or, that clever turn of phrase adults use: "The squeaky wheel gets the oil."
It also gets the person protection. It's easier to spare a teary-eyed person punishment.
Related on SmartPlanet: