Jan. 6, 2011 -- Men rarely jump at the chance to catch the latest tear-jerker with their wives or girlfriends, although some grudgingly oblige. And there might be a biological reason for their hesitation.
New research suggests that women's tears emit chemical signals that turn men off.
After sniffing odorless tears from women who cried during a sad movie, men in a study were less attracted to the opposite sex and less sexually aroused, as assessed by self-reports, physiological measurements and neuroimaging tests, than men who sniffed saltwater. Their testosterone levels also sank, according to the study, published in Science Express.
The authors, led by Noam Sobel, associate professor of neurobiology at the Weizmann Institute of Science in Rehovot, Israel, suggested that this tear-borne "chemosignal" represents a novel, functional role for crying.
Shedding tears has long been considered a form of human communication. Babies, who can't ask for food, cry when they're hungry. And even among speaking adults, tears can provoke empathy and sympathy. But whether emotional tears themselves serve a function has long been a mystery.
"Charles Darwin thought that tears were incidental and purposeless, but evolution doesn't favor the development of purposeless processes," said William Frey, director of the Alzheimer's Research Center at HealthPartners and Regions Hospital in St. Paul, Minn.
Darwin's book, "The Expression of the Emotions in Man and Animals," was published in 1872.
"Humans are the only animals that have the ability to shed tears in response to emotional stress. "It's unlikely that it's incidental," Frey said
They might taste like saltwater, but the tiny drops of liquid that spill from glands nestled in our eye sockets when emotions run high are much more complex. They contain proteins, enzymes and electrolytes; even drugs. And work done by Frey suggests they're chemically different from tears roused by cold temperatures, chopped onions and rogue eyelashes.
In mice, tears contain a pheromone that triggers a social response in other mice when sniffed. But the human equivalent of this process remains a topic of controversy, according to Steven Munger, professor of anatomy and neurobiology at the University of Maryland School of Medicine.
Humans are missing parts of the olfactory machinery used by other animals to detect pheromones. But that doesn't mean we can't detect them, Munger said.
"Women who are living together will have synchronized menstrual cycles, which can be manipulated by exposure to other women's odors," Munger said. "This study supports the idea that humans do use odor-mediated social communication."
Humans, compared to other animals, have excelled in the ability to communicate by developing language, lead author Sobel said.
"But in communication, the more words you have, the better off you are," he added. "We've identified the chemical word for 'no,' and having that word in your vocabulary has clear advantages."
If women are communicating a chemical message that they need comfort, not sex, it seems appropriate that a man's testosterone level would take a dive. But Sobel and his colleagues have an alternate theory they plan to investigate.
"It could be that the signal we've identified that lowers sexual arousal is primarily lowering aggression," he said. "You would think that with low testosterone comes low aggression. And a signal that lowers aggression in others is very valuable from an evolutionary perspective."
Although the study was relatively small, it provided a lead for further investigations. The researchers plan to explore whether women's tears trigger behaviors other than sexual disinterest, such as decreased aggression, and whether unemotional tears evoke a similar response.
They also intend to examine how women and children react to sniffed tears. Ultimately, they hope to identify the pheromone-like compound in tears responsible for the reaction.
"What they're publishing are really some very thought-provoking studies," said Frey, who coauthored the book, "Crying: The Mystery of Tears" in 1985.
Frey hypothesized that crying serves to release stress chemicals from the body, citing the Roman poet Ovid: "It is some relief to weep; grief is satisfied and carried off by tears."
But the two theories -- that crying sends signals and that it relieves stress -- need not be mutually exclusive, Frey added.
"Both theories support the existence of a biochemical function for human tears," he said.