Women's tears contain a chemical signal that lowers testosterone levels and dampens sexual desire in men, according to a team of scientists in Israel.

They studied the effect on men of sniffing fresh tears, which the researchers collected in vials as they rolled down the faces of women watching weepy movies.

The surprise discovery may help scientists solve the mystery of why humans shed tears when they're distressed or sad.

The researchers measured a small but persistent fall in men's sexual feelings when they sniffed women's tears, but the chemical or chemicals responsible may work primarily by suppressing aggression, which then has the effect of reducing sexual arousal.

"It is easy to see the advantage of having such a chemical signal in tears," said Noam Sobel at the Weizmann Institute of Science in Rehovot, who led the research. "Basically, you are protecting yourself when you are vulnerable." The study is reported in the US journal Science.

Tears are produced by the lachrymal and meibomian glands and contain all manner of chemicals, including proteins, enzymes and metabolic waste products. But the chemical make-up of emotional tears is very different from that of tears shed as a protective reflex, for example when grit gets into the eye.

In some animals, tears are known to contain chemicals that influence how other individuals behave towards them. The blind mole rat rubs its tears into its fur, which appears to make other mole rats less aggressive, said Sobel.

The scientists enrolled people for the study by posting adverts around the institute's campus asking for volunteers who were easily moved to tears. They didn't specify whether they were looking for men or women, but out of more than 60 people who responded, only one was a man. After an initial test that involved watching a sad film of their choosing, Sobel's team picked the most tearful, all of whom were women.

In one experiment, a group of 24 male volunteers sniffed either fresh tears, or drops of saline as a control, before being asked to rate the sadness or sexual appeal of women's faces that appeared on a computer screen. The men's sadness ratings were the same regardless of whether they sniffed tears or saline, but 17 men ranked women as less sexually appealing after sniffing tears.

At no time did the men or the experimenters know what was in the vials being sniffed.

A second test found that levels of testosterone in men's saliva were on average 13% lower after they sniffed fresh tears compared with saline, and their scores for physiological arousal were also reduced.

In a final experiment, a brain-scanning technique called functional magnetic resonance imaging (fMRI) revealed that sniffing women's tears reduced activity in parts of the brain that are involved with sexual urges, such as the hypothalamus and left fusiform gyrus.

Sobel said his lab is now trying to identify what chemical in tears might be responsible for the effect and whether it is also present in men's and children's tears.

"We don't think there is something special about women's tears," he said. "We definitely predict chemical signals in men's tears and children's tears too."