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February 4, 2010 at 10:12 AMComments: 0 Faves: 1

Weird Science: Why Humans Cry (And No Other Animal Does)

By Erin Froehlich More Blogs by This Author

We understand crying as a behavior that is intrinsically human, but why? What is it that makes us cry, and what evolutionary benefits might our tears offer us? Scientists have been puzzled by these very questions and are only now really beginning to find the answers.

Different Types of Tears

Though tears viewed side by side are nearly impossible to differentiate between, there are actually three different types of tears. The main differences between tear types are their benefit to the body and their varying levels of tear chemicals. All types of tears contain three different types of stress chemicals: leucineenkephalin, an endorphin used by the body to moderate pain; ACTH - the hormone most commonly associated with stress; and prolactin - a hormone used for milk production in mammals.

Basal Tears: We may not think of these as tears, because unlike the other tear types, basal tears don't ball up. Instead, they are used to keep our eyes moisturized, prevent damage from debris in the air, and protect against bacteria that may enter the eye. These tears are made with levels of salt and water comparable to the rest of the body.

Irritant Tears: As the name suggests, these are the types of tears our eyes produce to remove irritants from the eye. Their goal of protecting the eye is similar to that of basal tears, but unlike basal tears, irritant tears contain greater amounts of antibodies and enzymes to break down any micro-organisms that might have invaded the eye. Irritant tears are also produced in greater abundance in order to wash away irritating substances.

Emotional Tears: These are tears produced at times of intense emotion - sometimes joy, but more often in sadness or anger. Unlike the other two tear types, emotional tears are not meant to protect the eyes. Instead, they are meant to rid the body of excess chemicals. In fact, emotional tears contain 25% more protein than any other type. The proteins are the stress chemicals of the body, which, if kept in the body, would weaken the immune system and other necessary processes. When we cry or sweat, we release these toxins and prevent their buildup.

How Crying Evolved

Evolutionary scientists point to the behavior of birds to explain the reason behind human crying. When a bird is newly hatched it will use certain behaviors to communicate its helplessness to its mother bird. This is extremely beneficial to the baby bird, because without these signals, the mother bird may not provide it with the food it needs to grow and survive. However, even after the little bird has grown its adult feathers and is able to fly, it will repeat its infant behavior, such as helpless wing shaking, in order to beg its mother for food. While humans are certainly not birds, the concept is similar.

Human crying behavior was favored in natural selection, because a crying person received food, while a noncrying person would be less likely when food sources were scarce. This is because crying behavior resembles that of a helpless newborn infant, spurring a sympathetic, nurturing response from others. Dr. Oren Hasson, a contributor for Evolutionary Psychology elaborates,

"Crying is a highly evolved behavior. Tears give clues and reliable information about submission, needs and social attachments between one another. My research is trying to answer what the evolutionary reasons are for having emotional tears. My analysis suggests that by blurring vision, tears lower defences and reliably function as signals of submission, a cry for help, and even in a mutual display of attachment and as a group display of cohesion"

Dr. Hasson believes these tears are used as a tool to elicit mercy and sympathy from the aggressor - or from those who may come to our assistance, such as friends and family. According to Hasson, crying enhances attachments. However, there are certainly social situations in which crying does more harm than good. While submission and helplessness can garner help and support, they are not signals you would want to display when you need to be strong, such as when at work.

Men Vs. Women

Though the gap is closing, when it comes to crying, expectations differ between men and women. Stephanie Shields, professor of psychology and women's studies, has been conducting studies on these differences. Through her research she found that both men and women are viewed more positively when they appear in control of their emotions, as indicated by moist - but not crying - eyes. Of the two however, men who controlled their tears were viewed more positively, an opinion that follows traditional views on displays of emotion. However, there may be more to the prospective than stereotyping. Remember the three stress chemicals in tears? Leucineenkephalin, ACTH, and prolactin? On average, women cry four times as much as men, which scientists explain by a higher concentration of prolactin in women's bodies. Shield's research seems to support this as participants in her study ranked women as crying more intensely and with more control over their tear flow. Looking at the findings of another tear study, this may very well be another evolutionary adaptation. It turns out there are chemicals in women's tears that cause a man's testosterone level to go down when he smells them. As testosterone promotes both libido and aggressive behavior, it stands to reason that this may have provide some additional protection for women.

Laugh Until We Cry

Though scientists now have a pretty firm grip on the reasons for crying when sad or angry, more confusing is the behavior of crying with joy, or laughing until you cry. What they do know is that crying is used to release strong emotions, and has actually been shown in studies to have a calming effect on participants. In one study, 60% of people reported an improvement in their mood after crying, while only 10% reported feeling worse.

Researchers even found that in most cases, unpleasant stress and arousal while crying, which includes an increased heart rate and sweating, actually lasts less time than the calming effects which occur once a person is done crying. This is actually quite similar to the effect laughing has on the body, which may explain its connection. In fact, a medical condition called Pathological Laughing and Crying (PLC) is neurological illness that causes uncontrollable laughter or crying. Robert Provine, PhD, psychologist and author of Laughter: A Scientific Investigation explains, "Both occur during states of high emotional arousal, involve lingering effects, and don't cleanly turn on and off."

It's Your Party

We cry in order to release stress and garner help and sympathy from others. In most cases, crying actually results in calmer feelings. Crying is a useful communication tool in humans, but it should be repressed in inappropriate situations, like at work. When you suppress tears too often however, toxins associated with stress can build up in the body and even cause immune problems. So, as long as you're in a safe place, follow Lesly Gore's advice and "cry if you want to."

Sources:

http://www.thenakedscientists.com/forum/index.php?topic=19782

http://www.froes.dds.nl/Tears.htm

http://www.sciencedaily.com/releases/2009/08/090824141045.htm

http://serendip.brynmawr.edu/exchange/node/1825

http://www.sciencedaily.com/releases/2007/10/071028130126.htm

http://www.sciencedaily.com/releases/2008/12/081217123831.htm

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