Some time ago, women in solitude cried into vials. Their tears were special. They held a chemical whisper that could rob desire from men….
Though this sounds like some kind of fairy tale, it’s in fact the description of a fascinating and important experiment by Noam Sobel’s lab at the Weizmann Institute of Science in Israel. Using a combination of brain scanning and other testing, these researchers have shown that women’s tears contain a compound that covertly inhibits sexual desire in men.
Unlike many other major findings, the significance of this one can be grasped immediately: humans secrete pheromones that can affect another’s thoughts, brain, and biochemistry.
The term ‘pheromone’ was coined in 1959 by Peter Karlson and Martin Luscher to help organize their thinking about certain intriguing biomolecules reminiscent of hormones. Although studies of the potent, specific, and sometimes evocative effects of blood-borne hormones were well underway, these investigators were chasing a slightly different question. In a foundational paper, the two scientists speculated that hormone-like compounds might travel from one animal to another via bodily secretions. With such a signaling mechanism, they reasoned, one animal could “release a specific behavior” in another. A mere whiff or lick of another’s secreted message could prompt defense, pursuit, reproduction, or many other possible behaviors.
It was a far-reaching idea, and one that’s now backed by a wonderful variety of colorful examples. Nest-building ants, suckling rabbit pups, and mating elephants are all impelled by specific chemicals that trigger and modify innate behaviors.
But what about us? Are there any behaviorally meaningful signals in the bodily secretions we usually try to mask or scrub away? This has been a lively (if contentious) research question, and studying it has resulted in some intriguing reports of candidate human pheromones. Smelling a male sweat component, for example, can raise levels of the hormone cortisol in women, and other sweat-derived compounds may synchronize the menstrual cycles of women living in close proximity.
Examples like these reinforce the idea that we might converse via molecules in a way that’s qualitatively different from our more familiar interactions. Whereas spoken conversations are abstract and nuanced, chemical conversations are physical and largely hard coded. This opens the door for some of the more imaginative hypotheticals surrounding the field of pheromone research. Could pheromones override out better judgment? Be distilled and weaponized to evoke mass panic? Crafted into a perfume with will-bending allure? Perhaps all of these things are already happening naturally, and we don’t even know it?
Probably not. Yet the newfinding by Dr. Sobel’s group is almost as exciting as scientists (and perfume makers) could have imagined. It turns out that we do indeed send a chemical message that others can’t resist. But that message is “hold off.”
In their first experiment, Sobel and his colleagues tested 24 experimental subjects, all men, to discover the answer to a simple question. Do women’s tears smell like anything? The men were presented with two samples of liquid. One contained saline (basically saltwater) that had been dribbled down women’s cheeks as a control, while the other had real tears collected from women watching movies with certified tear-jerking potential.
The result of this experiment was negative - tears were indistinguishable from odorless saline - but that’s actually one of the most exciting parts of the study. It means that however tears affect our thoughts, it’s not by bringing to mind a chain of conscious associations. Rather, they worm their way into perception by burrowing under it.
So what perceptions are changed by sniffing tears? One natural guess is that the olfactory signal in tears is consistent with the familiar visual signal. That is, perhaps a tear-related chemical amplifies the empathic sadness we feel when we see someone crying. While interesting, this possibility was ruled out by experiments that had tear and saline-sniffing men rate how sad women appeared. When shown repeated pictures of women with emotionally ambiguous facial expressions, sadness ratings were the same when sniffing tears or saline.
However, when men were asked how sexually arousing they found the women’s faces, there was a small but clear effect for tears. In addition to reducing baseline levels of self-reported sexual arousal, tears made men less likely to rate a woman’s face as arousing.
What causes this deadening of desire? If tears had a discernable smell, it would be easy to come up with some kind of cognitive explanation. A rancid smell would be uninviting, for example, or a floral smell might point one’s thoughts in a purer direction. Instead though, tears seem to bypass the cognitive high road and work more directly on our viscera - the parts of us that sweat and sustain a pulse.
To test the physiological effects of tears, Sobel’s group measured men’s galvanic skin responses (GSR’s, basically a fancy measure of sweatiness) as they watched either neutral or sad movie clips. GSR’s recorded while sniffing tears were markedly different from the control, saline-sniffing condition. The tears enhanced the emotional responses to neutral clips, while diminishing the response to sad clips.
Although the specific mechanisms of these emotional and perceptual effects remain to be discovered, it’s likely that they’re the result of hormonal changes coordinated by the brain. Indeed, testosterone levels were lower in saliva samples taken from men sniffing tears than men sniffing saline, and arousal-associated brain areas were muffled by tears as well.
It’s tempting to armchair-theorize on how these results relate to everyday life and our evolutionary past. What does it mean that the secretion of a desire-defusing chemical is coupled to the emotion of sadness? Which came first -- tears of sadness, or a strategic dribble of molecular stop signals? Though there are no simple answers here, Sobel and colleagues offer the simple vignette of hugging a crying lover -- a situation for which attentive nurturing is probably more appropriate than a sexual advance.
Like many remarkable findings, this one calls for an adjustment of perspective. Specifically, we need to think more carefully about what it means for humans to interact. Naturally, the communicative roads traveling through sight, sound, and speech still do the vast majority of the work. However, we now know that specific suggestions -- perhaps quite potent ones -- can leap from the body of one person to the nose of another.
Amazingly, this leaping sneaks past conscious perception. For all our deliberate parsing of our fellow humans’ speech, our careful guesswork in decoding intents from expressions, we may be more bound (or repelled) by chemistry than we appreciated. Without a doubt, this line of investigation will take off, as we work out the roles of our noses in our relationships.
Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters co-editor Gareth Cook, a Pulitzer prize-winning journalist at the Boston Globe, where he edits the Sunday Ideas section. He can be reached at garethideas AT gmail.com