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I cry. At mushy commercials when the son finally gets home on Christmas Eve. At weddings because everybody's so happy. At funerals because everybody's so sad. But why, really, do I - do any of us - cry?

The main reason, say evolutionary biologists and neuroscientists, is that we're human.

As far as scientists can tell, no critter cries emotional tears the way humans do, despite scattered reports of an elephant or gorilla 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. But what? Does crying signal submission and thus disarm aggressors? Does it increase empathy and bonding, thus promoting community? Do tears promote health by relieving stress, thereby giving a survival advantage to the weepy? What is it about our brain that creates this ability to cry?

The lack of research money for such questions is enough to make a science writer cry. But some researchers are plunging in anyway. Last year, for instance, Maryland psychologist Robert Provine reported in the journal Evolutionary Psychology that tears may have evolved to give more oomph to facial expressions of emotions.

In one intriguing experiment, 80 undergraduates were asked to rate the intensity of sadness. 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 pictures of people with other expressions.

Regardless of age or gender, the students overwhelmingly ranked the pictures showing tears as conveying more sadness than the same faces without tears. More surprising, 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 Dr. Provine, tears function to reduce ambiguity.

The fact that animals don't cry emotional tears - and that it takes newborns several months to add tears to their crying - buttresses researchers' belief that emotional crying is a recent evolutionary development, says Dr. Provine.

In a paper published in Evolutionary Psychology in response to Dr. Provine's work, Israeli evolutionary biologist Oren Hasson theorized that emotional tears may also function as signals of appeasement. Because tears blur the vision of the person crying, they may be a biological signal of non-aggressive intentions. They also act as a call for help, and for bonding, conveying the message: "I can lower my defences or attacking options, therefore you can trust me."

But there's another reason why emotional tears may have evolved, says William Frey, a biochemist, neuroscientist and author of the 1985 book Crying: The Mystery of Tears. "I propose that humans evolved the ability to shed tears as a means to alleviate stress, and evolution favours this because it has survival value."

Viewing tears not just as communication signals but also as stress relievers may help explain why people cry alone.

Emotional tears are chemically different from tears that simply lubricate the eye, Dr. Frey says. Among other things, they contain more protein.

The still-unanswered question, though, is whether there are more stress hormones such as prolactin and ACTH (adrenocorticotropic hormone) in emotional tears. So far, there've been no clinical studies to see if emotional tears reduce stress.

But there is a hint that they may improve immune function. A small Japanese study looked at 60 patients with latex allergies and measured their skin responses to latex before and after viewing two types of videos - one about weather, the other, Kramer vs. Kramer, a tear-jerker. Nobody cried at the weather video, but most cried at Kramer vs. Kramer; in the weepy patients, skin reactions to latex were reduced.

Moreover, 85 per cent of women and 73 per cent of men say they "feel better after crying," which supports the stress-reduction hypothesis, says Dr. Frey, adding that, on average, women cry more than men - 5.3 times a month versus 1.7 times.

Why women cry more is not known, but 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.

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

To unravel this, Josef Parvizi, a neurologist at Stanford University, studies patients who have abnormally frequent and intense crying and laughing spells, a condition that often occurs as a result of brain damage after multiple sclerosis, strokes, tumours, epilepsy or traumatic brain injury.

Emotional tears, says Dr. Parvizi, are probably triggered by nerves running from the limbic system, the part of the brain that controls emotion, to the brainstem (at the top of the spinal cord) and, from there, to the lacrimal glands, which produce tears. It makes sense, he says, that many brain systems are involved because laughing and crying require understanding the emotional context before inducing appropriate changes in the musculature of the face, the larynx (getting choked up), the eyes, the heart rate and respiration.

At the very least, this elaborate brain system suggests that emotional tears are important. As Dr. Provine puts it, "this is evolution occurring right before us."

Judy Foreman is a syndicated columnist and former Boston Globe writer.