January 6th, 2011
02:00 PM ET

Why we cry: What's inside tears

Whether you just had a fight with a partner or just watched "Revolutionary Road," crying is a natural expression of sadness. But surprisingly little is known about why humans evolved to cry, and what purpose tears serve.

New insights come from the journal Science this week. Israeli researchers studied the question of why we have tears, and found that tears may actually have chemical signals that - at least in men - lower testosterone and reduce sexual arousal.

Tears shed in emotion are actually chemically different from tears created to keep the eye moist and clean, scientists say. And it's the tears of deep feelings that seem to have the effects on arousal and  testosterone .

"We’re smelling each other a lot more than what we’re aware of," said Noam Sobel of the Department of Neurobiology at the Weizmann Institute of Science in Rehovot, Israel. "We’re constantly collecting information, emitting signals, and these things influence our behavior."

It's not that you can spray tears, or even that you should try to cry more, in order to influence other people in certain ways, Sobel said. And these signals are just one part of communication between humans; they don't "take you over," he said.

Researchers gave the male participants samples of tears that women had shed while watching sad movie scenes on separate occasions, as well as saline solution. They did not see the women cry, and did not know that the compound they were sniffing was made of tears shed in emotion.

The men could not distinguish these two liquids in terms of smell, but the tears were having measurable effects on them that they were probably completely unaware of. One part of the study found that men who sniffed the women's tears reported lower sexual arousal, but not sadness, than when they sniffed the saline. A separate research component did brain imaging of male participants, and corroborated the men's self-reported lower sexual arousal when sniffing tears.

This is not, however, a large study. There were 24 men who attempted to distinguish between tears and saline. Then, 50 men watched a sad movie once after sniffing tears and once after smelling saline. To look at sexual arousal in the brain, 16 men participated. While a good start to look at these questions surrounding tears, the small number of volunteers means more research should be done to confirm these findings. Also, it was done only with female tears and male participants; a different pattern might be seen in females who smell emotional tears.

But the findings might, for example, explain why women tend to cry more during menstruation, a time of the month that - at least from standpoint of reproduction, as well as certain cultures - is inappropriate for sexual activity, he said. And there is evidence from animals that tears also lower aggression in others.

"If you have a way to lower testosterone at a time when lowering testosterone is in your interest, it's obviously an effective tool," Sobel said.

Now that you know more about tears, check out what's inside your spit.

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Get a behind-the-scenes look at the latest stories from CNN Chief Medical Correspondent, Dr. Sanjay Gupta, Senior Medical Correspondent Elizabeth Cohen and the CNN Medical Unit producers. They'll share news and views on health and medical trends - info that will help you take better care of yourself and the people you love.