Weizmann Institute of Science scientists discovered: tears contain chemical messages

Several studies done in recent years have shown, for example, that substances found in human sweat carry a variety of emotional and other signals, intended for those who smell them. Now it turns out that the tears are too.

Crying is a unique and universal human phenomenon, through which we send a variety of emotional signals to our environment. Scientists at the Weizmann Institute of Science discovered that the tears send out signals, some of which are encoded in chemical substances contained in the tear fluid itself. These findings are published today in the online version of the scientific journal Science, and they show that smelling women's tears - even if not in the presence of the woman herself - reduces the level of sexual arousal in men.

Humans, like most animals, release substances through body fluids, which carry messages to their own kind. Several studies done in recent years have shown, for example, that substances found in human sweat carry a variety of emotional and other signals, intended for those who smell them.

Unlike sweat, tears are odorless. In fact, in the first experiment, conducted by research students Shani Gelstein and Yaara Yeshuron, together with their colleagues from Prof. Noam Sobel's group, in the Department of Neurobiology at the Weizmann Institute of Science, male subjects were asked to distinguish between the smell of a jar containing tears - which were received from female volunteers who had watched sad movies - and an identical jar containing a salt solution. The subjects were unable to differentiate between the examples.

In a subsequent experiment, male volunteers smelled tears, or a saline solution, which served as a control. They were then asked to rate how sad women whose pictures were shown against a background, and how sexually arousing they were, when a sticker soaked in tears (or salt water) was placed under their nose. The following day, the researchers repeated the experiment, when this time the men who were exposed to tears received a saline solution, and vice versa. The experiment was done in a double-blind manner, that is, both the subjects and the examiners did not know which of the subjects received the tears, and which the control solution. The researchers discovered that smelling the tears did not affect the men's assessment of the degree of sadness or empathy expressed by the women in the photos. However, surprisingly, they found that after smelling tears, the men gave a lower rating of how sexually attractive they were to the women presented to them.

To better understand the surprising finding, another experiment was designed, in which men were asked to watch emotional films after smelling tears or saline solution. While watching the films, the subjects were asked to rate their emotional state. At the same time, physiological parameters indicating the degree of sexual arousal, such as skin temperature and heart rate, were measured. The data provided by the subjects showed that smelling the tears did not make the subjects feel sadder during the movie. However, the volunteers who were exposed to tears rated their level of sexual arousal lower. The physiological tests provided much more definitive evidence: the data showed that following the sniffing of tears there was a decrease in sexual arousal indicators, including a significant decrease in the level of testosterone - a hormone associated with sexual arousal.

Prof. Sobel and his partners repeated the experiment, while monitoring the brain activity of the men being tested, in real time, using an fMRI scanner. The scientists discovered that after smelling women's tears there was a significant decrease in activity in the areas of the brain known to be involved in sexual arousal.

Prof. Sobel: "The research raises a number of interesting questions. For example, what is the chemical responsible for this reaction? Do different types of emotional states send different signals to the environment? Are the components in women's tears different from those in men's or children's tears? This study reinforces the idea that chemical signals - even those that we are not aware of - that are sent off by humans, influence the behavior of other humans."

Human crying also occupied Charles Darwin, who sought evolutionary explanations for a series of emotional expressions. Thus, for example, he suggested that the ancient fear of spoiled food is what causes us today to twist our faces in disgust. However, he was unable to find a similar explanation for the crying. The current study offers a possible explanation for the phenomenon of human crying - as a chemical signal to the environment. Prof. Sobel points out that the tears of several rodents also contain chemical substances that transmit information. "It is possible that this special human phenomenon is not unique only to us", he says.

The research was carried out by research students Shani Gelstein, Yara Yeshuron, Liron Rosenkrantz, Dr. Sagit Shoshan, Idan Fromin and Prof. Noam Sobel from the Weizmann Institute of Science, in collaboration with Dr. Yehuda Roth from the Wolfson Medical Center in Holon.