Scientists discover a chemical signal in human tears

January 6, 2011, Weizmann Institute of Science
This undated handout photo provided by the journal Science shows a donor woman watching sad films in isolation, using a mirror to capture tears into a vial. If a crying woman's red nose isn't a big enough turnoff to a man, a surprising experiment found another reason: Tears of sadness may temporarily lower his testosterone level. (AP Photo/Science)

Emotional crying is a universal, uniquely human behavior. When we cry, we clearly send all sorts of emotional signals. In a paper published online today in Science Express, scientists at the Weizmann Institute have demonstrated that some of these signals are chemically encoded in the tears themselves. Specifically, they found that merely sniffing a woman's tears – even when the crying woman is not present -- reduces sexual arousal in men.

Humans, like most animals, expel various compounds in body fluids that give off subtle messages to other members of the species. A number of studies in recent years, for instance, have found that substances in human sweat can carry a surprising range of emotional and other signals to those who smell them.

But are odorless. In fact, in a first experiment led by Shani Gelstein, Yaara Yeshurun and their colleagues in the lab of Prof. Noam Sobel in the Weizmann Institute's Neurobiology Department, the researchers first obtained emotional tears from female volunteers watching sad movies in a secluded room and then tested whether men could discriminate the smell of these tears from that of saline. The men could not.

In a second experiment, male volunteers sniffed either tears or a control saline solution, and then had these applied under their nostrils on a pad while they made various judgments regarding images of women's faces on a computer screen. The next day, the test was repeated -- the men who were previously exposed to tears getting saline and vice versa. The tests were double blinded, meaning neither the men nor the researchers performing the trials knew what was on the pads. The researchers found that sniffing tears did not influence the men's estimates of sadness or empathy expressed in the faces. To their surprise, however, sniffing tears negatively affected the sex appeal attributed to the faces.

To further explore the finding, male volunteers watched emotional movies after similarly sniffing tears or saline. Throughout the movies, participants were asked to provide self-ratings of mood as they were being monitored for such physiological measures of arousal as skin temperature, heart rate, etc. Self-ratings showed that the subjects' emotional responses to sad movies were no more negative when exposed to women's tears, and the men "smelling" tears showed no more empathy. They did, however, rate their sexual arousal a bit lower. The physiological measures, however, told a clearer story. These revealed a pronounced tear-induced drop in physiological measures of arousal, including a significant dip in testosterone – a hormone related to sexual arousal.

Finally, in a fourth trial, Sobel and his team repeated the previous experiment within an fMRI machine that allowed them to measure brain activity. The scans revealed a significant reduction in activity levels in brain areas associated with sexual arousal after the subjects had sniffed tears.

Sobel: "This study raises many interesting questions. What is the chemical involved? Do different kinds of emotional situations send different tear-encoded signals? Are women's tears different from, say, men's tears? Children's tears? This study reinforces the idea that human chemical signals – even ones we're not conscious of – affect the behavior of others."

Human emotional crying was especially puzzling to Charles Darwin, who identified functional antecedents to most emotional displays -- for example, the tightening of the mouth in disgust, which he thought originated as a response to tasting spoiled food. But the original purpose of emotional tears eluded him. The current study has offered an answer to this riddle: Tears may serve as a chemosignal. Sobel points out that some rodent tears are known to contain such chemical signals. "The uniquely of emotional tearing may not be so uniquely human after all," he says.

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8 comments

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t0mt0m
not rated yet Jan 06, 2011
Emotional crying is a universal, uniquely human behavior.

Because you've proved that negative, right? Scientifically double checked all mammals?
Did the paper actually say this? I'd imagine they'd use more nuanced wording. (Strange also that at the end of this article, you're pointing out Sobel who points rodent teards have chemical signals in...)

Are tears odorless to animals other than humans? We may not smell anything, but we might be affected by it (akin to the VNO, effects of sweat on humans for example).
kuntur2k
5 / 5 (3) Jan 06, 2011
A patch with female tears! Finally a contraceptive for men that will not neuter them.
ramanan
not rated yet Jan 06, 2011

Tears flow when in physical pain,eyes are irritated or when in mental stress.

Tears have anti-bacterial properties that ward off infection.When eyes are irritated they flow to expel the foreign object out.

Under Physical pain, the tears flow, triggered by the secretion of Adrenalin(or is it the other way?) as a Defense mechanism.It conveys the Universal message of ’I am under pain,please understand/help’.

It also helps to reduce mental strain.

Shedding tears , we have been told since time immemorial, is not manly.Wrong.It is natural for men as well. Nothing wrong in crying under strain.
Au-Pu
not rated yet Jan 06, 2011
Many women use tears as a control mechanism.
What is interesting is that they can now learn that whilst it might give them some social leverage at the time, it lowers their appeal to their target and is sexual turn-off.
Skepticus
not rated yet Jan 07, 2011
Now I know another reason why men are said to be disturbed or hate to see women cry. As for the odorlessness of tears, this is a non-issuse. A lot of chemicals are odorless, tasteless or colorless. Take carbon monoxide.
satyricon
not rated yet Jan 07, 2011
Now they should compare anger tears with happy tears with sadness tears to crodcodile tears .. the chemical properties of the tears might be very different and it might elicit different reactions in men.
TheWalrus
not rated yet Jan 07, 2011
A teardrop consists of electrolytes and salt /
The chemistry of crying is not concerned with blame or fault.

--Paul Simon
denijane
not rated yet Jan 19, 2011
Does the decrease of testosterone lead to decrease in aggression? If so, it will make sense.
However I find the study little bit one-sided. Not only women cry, men also do cry - both from happiness and from pain. Children also cry. How does tears affect women and children? Do they always lead to empathy and decrease in aggression?
And also is there difference between tears from sorrow and tears from happiness when it comes to that effect on men?

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