Humans exhibit many questionable behaviours, but with a deeper look, most of our 'odd' behaviours may simply be left-over functions or responses from our ancestors that potentially played a fundamental role in the continuum of the human species.
For example, while it looks very different nowadays, without anxiety, we probably would have never gotten past the first few years of our species' development into today's existence. So what does crying have to do with our survival?
Why do we cry?
Crying releases oxytocin (the love hormone) and endogenous opioids, also known as endorphins. It is these chemical surges that help to soothe emotional and physical pain and crying appears to be one of the quickest ways to get them.
It is clear crying is necessary for the survival of humans. If babies didn't cry or we couldn't distinguish between their different cries (I'm hungry or I need to be held), they most likely wouldn't survive. Crying, especially for our infant ancestors, was an effective way to get immediate help - since their crying could attract dangerous predators, the adults in their vicinity would have had to spring into action and respond immediately. Because tribes lived and slept in such close proximity, someone was available to the infants' needs at all times. This means the 'cry-it-out method' didn't exist and babies were continuously held. Toddlers, who are not yet able to speak, cry to self-soothe and relieve pain and use other functions, such as pointing to where it hurts to get help or medical aid.
There is little research on the act of crying itself, but plenty to know that our emotions play a huge role in why we cry.
Humans have wondered where tears come from and why we cry them out since about 1,500 B.C. Philosophers and early scientists explored and debated whether tears came from the heart or the mind and debates continued for centuries, until 1662 when Niels Stenson discovered that the lacrimal gland is the proper origin point of tears - though, his theory was that tears are simply a way to keep our eyes moist.
But this doesn't answer why we would cry in response to emotional pain, and why, after we learn to talk and communicate, we would still cry into our adulthoods.
It looks as though, much like babies and toddlers, adults needed to develop a signal for help. Crying appears to be an extension of the human language, probably emerging around the time early humans began developing emotional awareness.
Tears are something that can only be noticed by those closest to us, and humans are observed to limit crying or emotionally disturbed behaviours to those we know, love and trust. When we see somebody that is emotionally aroused, the same neuronal areas of their brains that are activated become activated in ours. This suggests that somewhere in our evolution, crying became an effective way to promptly receive empathy and compassion. It is literally ingrained into our DNA to seek support from those closest to us and it was in our interests to keep other members of our tribes supported and healthy, for every member was essential to continue the bloodline and ensure the species' survival.
When a person is crying, we feel inclined to help and listen. When they are angry, we feel as though they are a possible threat. I wonder if crying is a way to signal to the person we need help from that they are safe, and that we trust them with our emotional disturbance.
Though, like most things, it's not always positive. We learn from the very beginning of our lives that crying elicits a response from those around us. It can have an influence on their mood, encourage empathy and patience and it can also neutralise anger in others. All of us are capable of using this subconscious knowledge to our advantage to manipulate somebody else.
With this said, the likelihood of an adult using tears to manipulate you is low. It's much easier and less draining to manipulate others by lying, keeping secrets and guilt-tripping. Children that have not yet developed the skills required to lie or keep secrets rely on the safest, most successful option: crying.
Are we less empathetic these days?
With human evolution and a rise in self-awareness and an increase in safety in our surroundings, crying changed its meaning. For our ancestors, we probably cried more about physical pain than emotional - but as our brains grew, so did our capacity for feelings. For whatever reason, emotions are strong enough to make us cry sometimes.
Emotions are rarely a direct threat to our lives, so perhaps, over time, they became less important for others to notice or respond to. As the world changed and humans became capable of more things - like having jobs, driving cars, creating other relationships and cultivating independence - we became somewhat aware that our survival doesn't directly depend on our tribes anymore - but rather, on a whole collection of other external resources (money and groceries for example).
Put simply, the things that used to collectively matter to our tribes, only seem to matter individually these days. We put less importance on others because our own, individual survival is paramount. Today, we can survive without our families. Our ancestors needed their tribe members (or at least the majority of them) to survive because that meant that they too, were more likely to survive. Providing nurture, support and care when one of our tribal members needed it improved their likelihood of survival and probably, had little to do with altruism and love and more to do with knowing that this kind of support would be returned should we need it.