Why do humans weep during times of emotional distress? The reasons have long been a mystery to science, until now.

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Crying when you're feeling blue could have evolved as a survival technique for humans given that we're mostly social creatures. As neuroscientist Robert Provine explains in his latest book titled 'Curious Behavior':
Several lines of evidence suggest that the NGF [nerve growth factor] in tears has medicinal functions. The NGF concentration in tears, cornea, and lacrimal glands increases after corneal wounding, suggesting that NGF plays a part in healing. More directly, the topical application of NGF promotes the healing of corneal ulcers and may increase tear production in dry eye . . . Although more of a scientific long shot, I suggest that tears bearing NGF have an anti-depressive effect that may modulate as well as signal mood.

Non-emotional, healing tears may have originally signaled trauma to the eyes, eliciting caregiving by tribe members or inhibiting physical aggression by adversaries. This primal signal may have later evolved through ritualization to become a sign of emotional as well as physical distress. In this evolutionary scenario, the visual and possibly chemical signals of emotional tears may be secondary consequences of lacrimal secretions that originally evolved in the service of ocular maintenance and healing.
So while crying may have its roots in the body's natural healing systems, it has also evolved into a social signal that allowed people nearby to see that a person was in distress.

But what about folks who can only cry around people they completely trust? One Vassar college psychologist named Randy Cornelius suggests that emotional crying probably evolved to be silent because it was a social signal of vulnerability. He tells NPR:
"You can imagine there'd be a selection pressure to develop a signaling system that wouldn't let predators in on the fact that you're vulnerable."
Another researcher named Jesse Bering suggests that adult crying only works because humans have a "theory of mind," which is an ability to figure out what other people are thinking and feeling based on context and social signals.

While crying might be a social signal, people who observe criers have a wide range of reactions depending on how they are conditioned by their social group. For instance, a 2007 study showed that before the 80s, men's crying was often viewed as inappropriate. It was only in recent years when gender roles started to change that men who cry were judged more positively.

For more interesting theories on why humans cry, read the full article here