The shedding of emotional tears is unique to humans, but our evolutionary, psychological and biological reasons for "crying it out" remain a mystery.
Although crying in babies serves an obvious purpose, many scientists from different fields have each put in their two cents about why adults do so. Charles Darwin considered tears to be a mere side-effect of facial muscle contractions, and a notable exception to the rule that useless processes would not be conserved through evolution.
A more modern theory from biochemist William Frey proposes that weeping expels toxins and stress hormones that explain the healing effects of "a good cry". However, other researchers have disputed his theory, arguing that the amounts of those expelled substances would be too small to make any noticeable difference.
In scientific terms, crying can be defined as the secretion of liquid containing proteins, enzymes, lipids and other substances from the lacrimal apparatus, a group of small organs around the eye.
"There are other animals that grieve, but they don't cry emotional tears," said Jay Efran, emeritus professor of psychology at Temple University. "They do tear when there's an irritation in the eye or to keep the eyes moist, but not what we would consider emotional tears as a result of a psychological event."
Efran had come across a two-stage theory of laughter, which inspired him to think more closely about how crying might obey a similar pattern.
Take the famous joke in the introduction of Annie Hall: "Two elderly women are at a Catskill mountain resort, and one of them says, 'Boy, the food at this place is really terrible'. The other one says, 'Yeah, I know, and such small portions'. All jokes of that sort first raise your tension level - you're listening intently because you're going to have to figure out what the joke is," he said. "Then when you get to the punchline, your level of tension drops, you get it, and you laugh."
Certainly early in life, crying indicates that the organism is tense and overwhelmed - it's a signal to the caretaker that they need some help.
Similarly, according to his two-stage theory of tears, people experience a crying fit when something happens to first spark high anxiety or distress followed by a moment of recalibration or release. For instance, a child who loses his mother at the grocery store begins by frantically searching for her, getting more and more worried as he scans the aisles. Suddenly, he hears her call his name from behind, sees her comforting face, and promptly bursts into tears.
Even tears of joy can be interpreted with a two-stage theory, said Efran.
A mother may cry at her daughter's wedding because of the built-up pressure of planning and fretting about the ceremony. Finally, the tension-breaking moment comes when her child utters "I do" - and commence waterworks.
"She's relieved that everything has gone well, and in a larger sense, her job as parent has reached another epic point by marrying off her daughter," he said.
Both laughing and crying seem to be reactions dictated by a rapid change in our autonomic system, the part of our nervous system that controls involuntary actions such as heart beat and pupil dilation. They emerge when the body shifts from a fight-or-flight state stimulated by the sympathetic nervous system, to the parasympathetic nervous system's relax-and-restore mode.
Efran's theory may explain why humans cry, but what would be the purpose of tears themselves? It seems odd to have excess liquid produced by glands in the eye when we experience sadness, relief or joy.
To hunt for possible clues, scientists analysed the tears of our furry laboratory stand-ins, mice, finding multiple compounds that serve as signalling chemicals for their peers. In 2005, researchers discovered exocrine gland-secreting peptide 1, or ESP1 - a pheromone found in the tears of male mice that heightens the chance that females will mate with them. Another pheromone only present in juvenile mice tears, called ESP22, protects them against unwanted mating by adults.
Building on this previous work, a 2011 experiment had 24 men sniff the tears of crying women - who had collected their specimens earlier while watching a sad movie - while rating the sexual attractiveness of various female faces. Even though the men never saw the criers, and the tears were odourless, the faces appeared less attractive after smelling women's tears than a control saline sample.
Their levels of salivary testosterone dropped, and functional magnetic imaging showed less activity in areas of the brain associated with sexual arousal.
Although some media outlets interpreted this as "tears are a turn-off", more likely the findings demonstrate a type of chemical signalling related to the need for comfort. As ESP22 protects pre-pubescent mice from the sexual advances of adults, a pheromone in human tears could serve to tell our partners that we need help and consolation at the moment rather than a romp in a bedroom.
"Certainly early in life, crying indicates that the organism is tense and overwhelmed - it's a signal to the caretaker that they need some help," said Efran.
"So in evolutionary terms, it's sort of efficient because it signals that help is needed and also indicates a system rehabilitation or recovery."
- Washington Post, Bloomberg