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STEVE INSKEEP, host: When a woman at the movies starts crying, it may be having an affect on her male companion. That may not surprise many guys out there, but this could. It may affect her male companion even if he can't see the tears. That's the conclusion of a team of Israeli scientists. NPR's Joe Palca has more.

JOE PALCA: You don't have to a psychologist to know that tears can have a powerful effect. You see someone cry, and anger tends to turn to compassion. But a group of Israeli scientists wondered if the effect was strictly visual. In other words, might there be some chemical in human tears that was responsible for the urge to care for someone. So they advertised for males or females willing to donate tears.

Mr. IDAN FRUMIN (Weizmann Institute for Science): We got, mainly, female donors.

PALCA: Idan Frumin is a biochemist at the Weizmann Institute for Science in Rehovot. He and his colleagues decided to see whether just smelling a woman's tears had a measurable effect on men. So they had their female tear donors watch sad movies, then they took a few drops of a woman's tears and had men sniff them to see if the tears had any recognizable smell.

Professor IDAN FRUMIN: And they don't.

PALCA: Even though they didn't smell anything, Frumin and his colleagues still asked their subjects to fill out a questionnaire.

Prof. FRUMIN: One of the questions in the questionnaire was: What is the state of sexual arousal of this specific subject in this specific moment? And to our amazement, we saw that - a drop in arousal.

PALCA: That prompted them to do a bunch more studies, and the findings were consistent: Testosterone（睾丸酮） levels went down and brain areas involved in sexual arousal were quiet after men sniffed a woman's tears. None of those changes occurred when men sniffed salty water that had been dribbled down a woman's cheek.

As they report in the journal Science, Frumin and his colleagues say there must be some chemical in human tears causing this effect.

Thomas Cleland is a psychologist at Cornell University.

Professor THOMAS CLELAND (Psychologist, Cornell University): It's probably the best study yet showing that chemical communication between humans is a reality.

PALCA: Cleland says it shouldn't be surprising that humans can communicate with chemical signals. Animals clearly do it. It's just been hard to prove for humans.

But psychologist James Cherry of Boston University says while the Israeli study convinces him that there is some chemical signal, you can't say it's only in tears from the Israeli experiments.

Professor JAMES CHERRY (Psychologist, Boston University): Certainly there are effects, but whatever substance or substances that may be there could be found in a lot of places. You just don't know that.

PALCA: Cherry says the chemical could be in sweat or saliva.

Psychologist Ad Vingerhoets of Tilburg University in the Netherlands has also been studying human tears. He says women's tears may reduce male testosterone and sexual arousal, but that's not the main effect.

Professor AD VINGERHOETS (Psychologist, Tilburg University): It's my hypothesis that tears has an effect not primarily on testosterone, but on oxytocin(催产素).

PALCA: Vingerhoets says oxytocin is a hormone that promotes social bonding and caregiving.

Joe Palca, NPR News, Washington.

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