Health Sense

Why cry?

Tears appear when we're happy, sad, stressed — and the cause of them is an emotional topic

By Judy Foreman
Globe Correspondent / March 8, 2010

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I cry.

At mushy Hallmark commercials when the son finally gets home on Christmas Eve. At weddings, because everybody’s so happy. At funerals, because everybody’s so sad. Even watching the Olympics, when I bond with the skaters who get teary because they’ve finally won.

But why, really, do I - do any of us - cry?

It turns out, say evolutionary biologists and neuroscientists, the main reason we cry is that we’re human.

As far as scientists can tell, no other critter on earth cries emotional tears, as opposed to tears that merely lubricate the eyes, the way we humans do - despite scattered reports of an elephant or gorilla not just vocalizing in distress but actually shedding tears. (Intrigued by one such report of an Indian elephant crying after being captured, Charles Darwin sent a colleague to check it out; he couldn’t confirm it.)

If emotional tears are indeed a uniquely human phenomenon, there must be an evolutionary advantage to crying, and possibly, a big one. But what? Does crying signal submission and thus disarm aggressors? Does it increase empathy and bonding, promoting community? Do tears promote health by relieving stress, giving a survival advantage to the weepy?

What is it about the human brain that creates this ability to cry?

Relatively little study has been done on the subject, though some researchers are plunging in, with fascinating results.

Last year, for instance, psychologist Robert R. Provine at the University of Maryland Baltimore County reported in the journal Evolutionary Psychology that tears may have evolved to enhance the power of facial expressions of emotions.

Provine’s team of researchers asked 80 undergraduates to rate the intensi ty of facial expressions of sadness in photos. Half of the images showed a person with tears streaming down the face, while the other half were the same images but with the tears digitally removed. These images were interspersed with “distractor’’ pictures of people with other facial expressions.

Regardless of age or gender, the students overwhelmingly ranked the pictures showing tears as conveying more sadness than the same faces without tears. In the images without tears, students often perceived the faces not just as less sad, but as expressing awe, concern, contemplation or puzzlement. In other words, says Provine, tears helped reduce ambiguity.

In a paper published last June in Evolutionary Psychology responding to Provine’s work, Israeli evolutionary biologist Oren Hasson at Tel Aviv University theorized that emotional tears may also be signals of appeasement. Because tears blur the vision of the person crying, they may be a biological signal to adversaries of non-aggressive intentions, he said in an e-mail interview. They also act as a call for help, and for bonding, conveying the message: “I can lower my defenses or attacking options, therefore you can trust me.’’

Emotional tears are almost always involuntary, says William H. Frey, a biochemist, neuroscientist, and author of the 1985 book “Crying: The Mystery of Tears.’’ Once a person feels a strong emotion and starts crying, he says, “it is difficult if not impossible to stop.’’ If a person can recognize early on that tears are coming, getting away from the situation - like walking out of the room - can sometimes stop the tears, “and men in fact will often do that,’’ Frey says.

But because tears are so involuntary, the idea that people can make themselves cry to manipulate others is, for most adults, “totally erroneous,’’ he says. “We have had actors come in and try to cry in front of us and they can’t do it - and we needed the tears to study!’’ The only way actors could induce crying, he adds, was to conjure a feeling of sadness - they had to be thinking sad thoughts to do it.

Emotional tears may have another use, says Frey. “I propose that humans evolved the ability to shed tears as a means to alleviate stress, and evolution favors this because it has survival value.’’

This may help explain why people don’t just cry when there’s an audience, but also cry alone, when there’s nobody around to receive the communication.

Emotional tears are chemically different from lubricant tears, Frey says. Among other things, tears brought on by emotions contain more protein - surprising, since the higher volume of emotional tears might be expected to dilute the concentration of proteins. It’s unknown whether there are more stress hormones, such as prolactin and ACTH (adrenocorticotropic hormone), in emotional tears; the increased presence of such hormones could indicate that tears reduce stress by flushing out the hormones.

So far, though, there are no known controlled, clinical studies to test whether emotional tears indeed reduce stress.

Still, Frey says his own research has found that 85 percent of women and 73 percent of men say they “feel better after crying.’’ Women cry more than men - 5.3 times a month, on average, versus 1.7 times, he says. Precisely why is not known.

Researchers do know that male and female tear glands are anatomically different. Female lacrimal glands - the organs that produce tears - have smaller cells, says Darlene Dartt, a senior scientist at the Schepens Eye Research Institute in Boston. Hormones may play a role, too, she says. But what smaller cell size means, if anything, in terms of quantity or quality of tear production is unknown.

Researchers are also trying to puzzle out exactly what thoughts or feelings, in which parts of the brain, cause the lacrimal glands to secrete tears.

To unravel this, Dr. Josef Parvizi, a neurologist at Stanford University, studies patients who have abnormally frequent and intense crying and laughing spells, that is, “pathological laughing and crying,’’ a condition that can occur in cases of multiple sclerosis, stroke, tumors, epilepsy, or traumatic brain injury. Such patients typically laugh and cry abnormally because of brain damage, not an underlying mood disorder.

Emotional tears are probably triggered, says Parvizi, by nerves running from the limbic system - the part of the brain that controls emotion - to the brain stem at the top of the spinal cord, and from there to the lacrimal glands. It makes sense that many brain systems are involved, he says, because both laughing and crying require understanding an emotional context. Also with both, a cluster of physiological responses occur: The muscles of the face are engaged, heart and respiration rates rise, even the voice changes tone.

But what sets off those triggers for some people more readily than others - why some people cry easily and others do not - remains, for now, unknown. At the very least, this elaborate brain system suggests that emotional tears are important. As Provine of Maryland puts it, “This is evolution occurring right before us. We’re getting a ringside seat about exciting things happening.’’