Tears are more than just tiny droplets that roll down our cheeks when we're feeling blue or stricken by grief. A landmark study suggests they also contain a mysterious compound that can influence the feelings of those around us.
"We have found there is a chemosignal in human tears," said the study's senior author, Noam Sobel, a neurobiologist at the Weizmann Institute of Science in Israel.
Scientists have long known the tears of mice contain specific chemicals that convey signals to other mice apparently through their sense of smell. The new research, being published Friday in the journal Science, indicates a similar messaging system is at work in people, too.
For the study, the researchers collected tears from a group of female volunteers. They also trickled saline down the faces of the same women and collected the droplets for comparison purposes.
Then, in a series of experiments, male volunteers were exposed to the tears as well as the saline.
When asked to sniff both liquids, the men reported that neither had a discernible odour.
But, at a subconscious level, the real tears had a profound effect, causing a sharp drop in testosterone levels and a marked reduction in sexual arousal. High-tech scans confirmed less activity in the part of the brain normally associated with sexual arousal when the men sniffed the tears. That didn't happen when they were exposed to just the saline.
So, tears are essentially sending out a chemical signal that the woman is not sexually receptive at that moment - and the men in the study seem to have received the message.
The tears may also dampen aggression, Dr. Sobel speculated in a telephone interview. "Of course, a signal that would reduce aggression has very obvious advantages."
Dr. Sobel noted that humans developed language eons ago, but he is convinced non-verbal means of communication continue to operate.
"I believe that chemosignalling plays a much larger role in human behaviour than we've assumed," he said. "We are much more like rats and mice than we would like to think."
Although the study investigated tears collect only from women, the researchers think it's highly likely men's tears contain similar or separate chemosignals. What's more, children's tears may also contain distinct chemosignals.
"We are currently involved in a major effort to isolate the active components and explore potential applications," Sagit Shushan, one of the co-authors of the study, said in an e-mail.