Smelling the tears of a woman can quell a man's sexual desire, according to a study that determined female crying can have a direct chemical impact on male libido.

Scientists have found evidence to suggest that tears from a weeping woman contain a chemical signal that can have a subconscious effect on a man's sexual desire, even if he is not a witness to the crying.

The findings suggest a functional role for crying in humans, who are unique in the animal kingdom by expressing emotion with weeping eyes. Crying among women may be a way of controlling male desire and sexual aggression, the researchers suggested.

Biologists have never been able to come up with a satisfactory reason for the emotional tears produced during crying, as opposed to the protective tears produced to keep the eyes moist and free of dust and debris.

Mice, however, can produce a sort of tear that contains a pheromone, an odourless molecule that triggers basic instincts in many animals.

Although tears were obviously being used as an emotional signal, there was no obvious function attached to them, said the scientists, from the Weissmann Institute of Science in Rehovot, Israel.

"Despite psychological theories on the meaning of tears and biological theories describing tears as an adaptation related to their eye-protective nature, or a mechanism for expelling toxic substances, the functional significance of emotional tears remains unknown," the scientists say in a report to be published in the journal Science.

However, following a series of experiments involving men who were asked to sniff tissues soaked in the tears of weeping women, the researchers now believe that they have found convincing evidence to support the idea that female emotional tears contain some kind of chemical signal, or pheromone, that can directly affect a man's emotional state.

One of the studies involved asking men to rate a series of photographs of women's faces according to their sadness or sexual attractiveness. Sometimes the men were exposed to the tears of weeping women, and sometimes they were given tissues soaked in saline solution that had been carefully collected after being dribbled down the women's cheeks - to simulate any body odour that may be picked up.

None of the men said they were able to detect any difference in smell between the tissues soaked in tears and those soaked in the saline solution, and none knew that what they were given to sniff contained a woman's tears.

Professor Noam Sobel, who led the research team, said that a significant decline in the men's estimation of the women's sexual attractiveness only occurred after they had been exposed to the tears. Further studies showed that tears also resulted in a decline of testosterone in the men's saliva, as well as their own judgement about their state of sexual arousal.

A final part of the study investigated the brain activity of the men using functional magnetic resonance - a type of brain scanner. Again, the scientists found that men had lower activity in parts of the brain associated with sexual arousal after sniffing a woman's tears.

Professor Sobel said: "These effects materialised despite that subjects did not see a woman cry, nor were they aware of the compound source. Moreover, in Western culture, exposure to tears is usually in close proximity. We hug a crying loved one, often placing our nose near teary cheeks, typically generating a pronounced Nasal inhalation as we embrace."

Dr Esen Akpek of Johns Hopkins University's Wilmer Eye Institute, who wasn't involved with the new study, said: "We have never looked at tears in this way before."

The findings make sense, she said, because the glands that secrete tears bear receptors, or docking ports, for sex hormones - a connection most clearly seen with dry eye, which is most common in postmenopausal women.

Stay tuned: Sobel is now testing male tears, "as we finally have one good man crier".