BioEd Online

Tears shed by women contain chemical signals that decrease sexual arousal and testosterone levels in men, according to a study. The result, discovered by Noam Sobel, a cognitive neuroscientist at the Weizmann Institute of Science in Rehovot, Israel, and his colleagues, is published today in Science¹.

The existence of pheromones — secreted or excreted chemical signals that produce a social response — in humans has been debated, although research has shown that human sweat communicates information about individual identity, genetic relatedness, emotional states and health status, says Denise Chen, who studies human olfaction at Rice University in Houston, Texas. Mouse tears contain sex-specific pheromones², but scientists have not previously demonstrated that human crying is a form of chemical signalling. "This work provides really exciting evidence for another source of human chemosignals in tears," says Chen.

Weeping women

In the study, women watched sad films alone and captured their own tears with a vial that they held under their eye. Later, 24 men sniffed jars containing either the women's tears or saline that had been trickled down the women's cheeks, and then they wore a pad dipped in one of the fluids under their nostrils. Men who sniffed tears judged pictures of women's faces to be less sexually attractive than did men who sniffed saline, but their feelings of empathy were unchanged.

In a separate experiment, 50 men sniffed either tears or saline. Sniffing tears, but not saline, reduced their self-reported sexual arousal, levels of testosterone in their saliva and physiological measures of their arousal.

The researchers also exposed 16 men to tears or saline and measured their brain activity using functional magnetic resonance imaging. Men who sniffed tears rather than saline showed lower activation in brain regions implicated in sexual arousal, such as the hypothalamus.

Evolutionary enigma

"I think the study has used sound methodology and the results indeed are fascinating," says Ad Vingerhoets, who studies emotion at Tilburg University in the Netherlands. "But as far as I know, there is no clear logical, theoretical or empirical justification to design a study on the effects of tears on sex." However, because tears are thought to enhance social bonding and induce caring behaviour, and they act as sexual attractants in mice, Vingerhoets is puzzled as to why human crying causes no change in empathy, but decreases sexual arousal.

Robert Provine, who studies the evolution and neural basis of behaviour at the University of Maryland, Baltimore County in Catonsville, says that the results are consistent with previous suggestions that crying could reduce aggression. Testosterone may be linked to hostility, and lowering aggression could be evolutionarily adaptive, he explains.

Because the authors of the latest study did not compare the effects of emotional and non-emotional tears, they could not directly assess how the crying women's feelings influenced the signal. Chen says that to explore whether tears evoked by different emotions serve unique functions and have a distinct chemical makeup, the researchers should also perform experiments in which they elicit happy weeping, or neutral tears caused by allergens or irritants such as onions.

But first, says Chen, the authors should repeat their experiments while preventing the tears from rolling down the woman's cheek. The skin secretes chemical cues that vary with the emotional state of the individual, she explains, so the tears may have carried these signals into the vial. Despite her concerns, Chen says that "overall, as a first study, it's very exciting and it suddenly opens the door for a lot more studies in the future".