We cry when we are in despair. We tear up when we yawn. Our eyes well up during moments of immense joy. But why do we cry in the first place? What biological functions does crying serve? While the scientific community struggles to provide concrete explanations for this teary condition, researchers have offered several plausible theories based on the chemical composition of tears and the apparent evolutionary advantages of shedding these salty droplets.
In a sense, we actually cry every second of our lives. The lacrimal gland under the upper eyelid constantly secretes protein-rich, antibacterial liquid at a rate up to two microliters per minute. The fluid flows down from the outer edge of the eyeball toward the cornea, and lubricates the entire eye surface every time we blink. Excess liquid then drains into the nasal canal via the tear duct on the bottom eyelid.
Of course, we do not normally call this process “crying.” Rather, we are more concerned with the times when the secretion of lacrimal fluid overwhelms the draining capacity of the tear duct—when tears overflow and roll down our cheeks.
One stimulant of crying is irritation. If sand gets into our eyes, the lacrimal gland secrets excess fluid to help us wash out the irritant. Similarly, we cry when an onion releases sulfuric fumes that acidify the eye surface. Secretion of more tears lowers the eye’s acidity. Even yawning is a form of irritation—the manipulation of facial muscles near the eye squeezes the lacrimal gland and forces out excess tears.
The type of crying that we are most familiar with is the shedding of tears as an emotional response. Why we release tears when we feel grief, hurt or joy still confounds the scientific community, but several observations have been made.
Emotionally charged tears contain several protein-based hormones produced at times of stress—specifically prolactin, adrenocorticotropic hormone and leucine encephalin. The altered chemical composition of these tears have led scientists to speculate that crying is the body’s method of ridding excess stress hormones. Otherwise, stress hormones would reach harmful levels within the body. Indeed, this explanation fits well with the anecdotal knowledge that crying allows us to feel better.
More recently, researchers observed that emotional tears could also serve an evolutionary purpose. Excess lacrimal fluid disables the fight-or-flight response when it blurs our vision. By revealing our vulnerability, crying could be seen as a signal for defeat or submission, thereby eliciting mercy or sympathy from an attacker. Similarly, such a show of weakness can evoke pity and help one gain support from a larger community.
So why do we cry? Observationally, we cry because we have something in our eyes, or because we are part of a species that, through evolution, recognizes certain social cues. But more importantly, crying is a form of human expression.
- K. Doheny, Why We Cry: The Truth About Tearing Up. Available at http://www.webmd.com/balance/features/why-we-cry-the-truth-about-tearing-up
- M. Helmenstine, Anne, Why Do Onions Make You Cry. Available at http://chemistry.about.com/od/chemistryfaqs/f/onionscry.htm
- J. Flintoff, Why Do We Cry. (2003) Available at http://www.theage.com.au/articles/2003/08/27/1061663846142.html
- A. Skorucak, The Science of Tears. Available at http://www.scienceiq.com/Facts/ScienceOfTears.cfm
- Q. Choi, Charles, New Theory for Why We Cry. (2009) Available at http://www.livescience.com/7854-theory-cry.html
- Cry Me A River: The Psychology of Crying. (2008) Available at http://www.sciencedaily.com/releases/2008/12/081217123831.htm