Rick Sampson | FreeImages

I recently came across a news item about a tearless onion developed by scientists in New Zealand. I, for one, shed many tears over the chopping board, and all because of a substance called the lachrymatory factor. Now, using molecular engineering techniques, the Kiwi scientists silenced the gene that codes for this factor, and voilá—a tearless onion.

This got me thinking: This kind of crying is really all reflex, a direct reaction to irritation. It is the same type of reflex that causes our eyes to well up when we are poked in the eye. Another type of tears is the so-called basal tearing, which bathes our eyes every time we blink. Now, these two types of tearing are common to many animals, and their function is straightforward: Housekeeping, or more specifically, preservation of the integrity of the outer structures of the eye.


What about crying?

Believe it or not, this is unique to us humans. Not even our closest relatives, the chimpanzees, are capable of shedding emotional tears. Yes, animals can howl in pain or in anguish; my dog yawls and howls to high heaven whenever I leave the house without getting his approval, but he doesn’t shed tears. And contrary to popular belief, crocodiles don’t shed tears, but real men do. So how did it come about?


There must have been a “mistake”

The fact that chimps cannot shed emotional tears means that a mutation occurred after our genus Homo split off the evolutionary branch common to us and the chimps, namely less than 30 million years ago. Obviously, we don’t know exactly when that occurred. For instance, if we knew whether Lucy the Australopithecus could cry, we could narrow it down to 300,000 years ago. She most likely had tear ducts like ours, but so do the chimps and the dogs.

At some point in our human history, a mutation occurred that connected our tear glands to the limbic system, which is responsible for feelings and expression of deep emotions. This system contains such organs as the amygdala, which is the seat of intense emotions as anger and pain. Who hasn’t experienced the connection between these emotions and tears? Or the hippocampus, the seat of memories? Have your eyes ever welled up when old memories floated up from a deep, long forgotten sad life story? The reward centers are part of the limbic system as well; just watch the glistening eyes of proud parents when their child graduates from college.

All this crying is totally involuntary. The autonomic nerves cause the mentalis muscle, the one in the chin, to quiver. They cause the lump in your throat, and the corners of the lips to depress (by activating the depressor anguli oris muscle). It is almost impossible to voluntarily control these actions.


What’s the use?

If we accept the premise that this mutation, like all other mutations, are random events, why did it persist? What advantage did it bestow on the newly crying-capable humans?

baby crying
Human Male White Newborn Baby Crying | Evan-Amos | Public Domain via Commons

Crying is the first mode of communication of babies. I always marvel at the capacity of parents to “read” their baby’s cry. Some are cries of hunger. Others are cries of fatigue. Yet others are cries of pain. And the ever discriminating parental ear can tell one from the other.

We can expand on this concept. Crying is not restricted to baby-parent communication. We, adults, communicate in a variety of ways: body language, speech, song, dance, laughter, and, yes, crying.

The reason for this rich palette of communication modes is that it has an important survival value. Being vulnerable creatures in the Savannah, our only hope for survival was through cooperation. Not only could we better defend ourselves better against faster and more powerful predators, but we could even turn the table on them and make them our dinner.

The Tragedy | Pablo Picasso
The Tragedy | Pablo Picasso

The key to cooperation is socialization, and one of the traits that characterize our capacity to socialize is empathy, the capacity to feel what the other person feels. Have you ever experienced sadness when talking to a person who had just experienced a terrible loss? Or looking at Picasso’s heartrending painting of “The Tragedy”? Or joy when your friend is beaming with happiness? This is your empathy in action.

Empathy is not purely a psychological phenomenon; it has a neurological basis. There are special neurons in the brain that mirror the emotional state of the other person. These neurons, called appropriately enough mirror neurons, are extra large and most numerous in the human brain. They are distributed through many areas in the brain, all having to do with emotions. The amygdala has them, and so do the reward centers. So that when my reward centers fire when I tell you about my fantastic trip, the mirror neurons in your reward centers would fire as well, I hope.

And back to crying. When a person dear to us is crying, it “breaks our heart” and we feel like crying as well. This is a form of bonding; those misty eyes and the outpouring of salt water are part of the glue that kept us together since we got off the trees and made our first uncertain steps in the hostile Savannah.

Dov Michaeli, MD, PhD
Dov Michaeli, MD, PhD loves to write about the brain and human behavior as well as translate complicated basic science concepts into entertainment for the rest of us. He was a professor at the University of California San Francisco before leaving to enter the world of biotech. He served as the Chief Medical Officer of biotech companies, including Aphton Corporation. He also founded and served as the CEO of Madah Medica, an early stage biotech company developing products to improve post-surgical pain control. He is now retired and enjoys working out, following the stock market, travelling the world, and, of course, writing for TDWI.


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