It is well-documented that our bodies give off coded chemical signals via sweat, excretions and pheromones that convey messages to other members of our species. Yet the significance of odorless human tears has continued to draw a blank since Charles Darwin first suggested that emotional displays were originally motivated by functional purposes. One hundred and fifty years later, new research from scientists at the Weizmann Institute’s Neurobiology Department suggests that in fact, tears may be a chemo-signal, as a chemical in women's tears seems to discourage sexual arousal in men.

The results were obtained through a series of probing experiments; the first sought to ascertain whether men could differentiate between the smell of saline and the smell of tears from women who had been watching sad movies, which they were unable to do. Next, male volunteers sniffed a pad soaked either in tears or in control saline solution, whilst judging the expression of women on a series of computer images. The experiment was repeated the following day, with either the tears or the control, and the tests were double-blinded, meaning neither the volunteers nor the researchers knew what was on the pads. Sniffing the tears did not affect the men's ability to assess sadness or empathy, but unexpectedly the results showed a significant lowering of sexual attraction attributed to women's faces after sniffing tears.

Intrigued by the results, researchers pursued the experiment by surveying male volunteers watching emotional movies after sniffing tears or saline. Self-ratings throughout the films showed again that emotional response and empathy was not affected by sniffing tears, but again that sexual arousal was lower. This response was backed up physiologically with a significant drop in testosterone, a hormone related to sexual arousal. This experiment was repeated using a Functional Magnetic Resonance Imaging (fMRI) machine, allowing researchers to measure brain activity. This also showed that activity in areas of the brain associated with sexual arousal was reduced.

“This study raises many interesting questions," said Professor Noam Sobel. "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.”

As rodent tears are also known to contain such chemical signals, Sobel added, "The uniquely human behavior of emotional tearing may not be so uniquely human after all.”

The study by Shani Gelstein, Yaara Yeshurun, Liron Rozenkrantz, Sagit Shushan, Idan Frumin, Yehudah Roth at the lab of Professor Noam Sobel was published online in Science Express, and publicized via The Weizmann Institute of Science in Rehovot, Israel.