Humans are the only animals who shed tears during times of emotional distress. The reasons why have long been a mystery to science, but there are some fascinating ideas about how crying could be a survival technique for social creatures like ourselves.
When adults cry, it's usually very different from the crying of babies. Babies cry loudly, and involuntarily. Adults usually cry quietly, and can control the spigots much better than children can. We can keep our tears in until we race to the bathroom, or find a secluded place. What could be the possible evolutionary reason for adults to cry silently, in semi-privacy?
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 adult crying may have its roots in the body's natural healing systems. But, Provine suggests, it evolved into a social signal that allowed people nearby to see that a person was in distress.
Vassar college psychologist Randy Cornelius suggests that emotional crying probably evolved to be silent because it was a social signal of vulnerability. "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," he told NPR. A crying person could get help from other humans without enemies or attackers realizing that one member of the group was suffering.
What's interesting about these speculations is that they both suggest that emotional crying is something that humans may have evolved relatively recently, as their social groups become more complex. Jesse Bering, a researcher at Belfast University's Institute of Cognition and Culture, suggests that adult crying only works because humans have a "theory of mind," or an ability to figure out what other people are thinking and feeling based on context and social signals. Silent crying plays to the strengths of animals like humans with a theory of mind, because we can see the tears and figure out that it's a distress signal.
Bering suggests that criers may have been good survivors in early human communities because they could solidify bonds with others and get help when it was needed. Not only that, but crying can cement emotional bonds. Bering told NPR, "Crying seems to elicit compassion and guilt and that itself may be an evolved mechanism to save relationships in distress."
When Crying Does Not Move Us
Given that crying is a social signal, it's no surprise that people who observe criers have a wide range of reactions — most of them conditioned by their social group. Psychology researchers Stephanie Shields and Leah Warner conducted a study in 2007 that showed a shift in how people perceived men's tears. Before the 1980s, men's crying was often viewed as inappropriate. But in recent years, men who cry are judged positively — especially if they are responding to an extreme situation like the death of a loved one. As gender roles change, our reactions to crying change too.
Shields and Warner found that people's responses to crying also changed based on the situation. Some crying was viewed as manipulative, while other crying was seen as genuine. And thus the social signal for help was sometimes ignored, and even viewed with scorn. Shields told Science Daily:
An adult's tears can be powerful elicitors of concern and sympathy. But we found that tears can also invite scorn or suspicion regarding the crying person's motives . . . The way tears are judged by others is affected by many factors, including the gender of both the crier and observer, whether the tears are perceived as angry or sad, and perhaps most importantly among adults, whether the observer 'reads' the crier's tears as genuine or manipulative.
The kind of crying that was most often judged genuine, and deserving of help and sympathy, was the silent type that may have helped our ancestors as they evolved into social animals with a theory of mind. Shields and Warner found that when people's eyes welled up with tears, in a quiet display, it was always judged positively — especially if it was connected to a stressful situation that was out of human control.