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Pheromones: The Scent of Sex
August 22, 2006
Last reviewed on May 21, 2009
By Michael Craig Miller, M.D.
Harvard Medical School
Everyone would agree that the most important human sex organ is the brain because that's where sexual impulses, drives and reproductive behavior are programmed. We're also accustomed to thinking of sight, touch, and sound as the most important sensual cues for humans. And let's not forget the uniquely human capacity for creative imagination and fantasy.
But the second most important sensory organ may be . . . the nose. Decades of research demonstrate that in our mating behavior, we humans also follow our noses. That's because of substances called pheromones, which we now know influence sexual behavior not only in animals but also in humans.
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What are Pheromones?
Pheromones are odorless chemicals released by one animal that trigger either a biological or behavioral response in another animal. The animal might, for example, become aggressive or attempt to mate. Or, these chemical messengers may induce a gradual change in the other animal's hormone system.
Most scientists thought that humans were immune from this kind of influence. The preconceived notion was that such primitive mechanisms could never be at work in homo sapiens. But there was a scientific basis for the skepticism, too.
Most mammals, which include any warm-blooded animal that has fur or hair and gives birth to live young rather than laying eggs, have a special structure in the nose called the vomeronasal organ (VNO) that doesn't exist in a functional form in humans. The VNO has receptors for pheromones. When the receptors are triggered, signals are sent up to regions of the brain that trigger hormonal changes. This sequence of events makes the animal ready for sexual behavior. The VNO is distinct from the lining of the mammal nose that detects odors and triggers other behaviors such as a dog running into the kitchen at the smallest whiff of hamburger.
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Chemical Communication Among Humans
The cause for scoffing at human pheromones began to vanish a generation ago when University of Chicago researcher Martha McClintock, Ph.D. showed that women living together tend to synchronize their menstrual cycles. She's also demonstrated that when rats share only their air supply, one rat could speed up or slow down the ovulation in another rat. In 1998, McClintock and her colleague Kathleen Stern, Ph.D. showed that women exposed to the body odor of other women undergo changes in their menstrual cycle similar to what is seen in rats. This series of experiments established that, like lower animals, humans also have a system for communicating chemically about sexual function.
Scientists have known for a while that nasal information is important to us. A mother can identify her baby's T-shirt. Children can recognize pads that have been in contact with their mothers' underarm or breast. These observations, however, didn't show that pheromones could induce changes in another person's sexual behavior. And since humans didn't appear to have a specialized pheromone-sensing organ like the VNO, scientists still couldn't explain how we sense these odorless chemicals. Enter the work of Linda B. Buck, Ph.D.
About 15 years ago, Buck and fellow scientist Richard Axel, M.D. published work that eventually led to their receiving a Nobel Prize in 2004. They discovered a family of genes that codes for odor receptors of which there are more than 1,000 in the lining of the nose. Armed with this battalion of receptors, we are equipped to sense tens of thousands of odors. Until recently, this smelling system, which is similar in mammals, wasn't thought to overlap at all with the pheromone-sensing VNO system.
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The Conscious and Unconscious Nose
In the last year, Dr. Buck and her team at the University of Washington along with colleagues at Harvard and Princeton Universities have produced evidence showing that humans are likely to have an unconscious pheromone-sensing nose to match the conscious odor-detecting nose.
They first established that cells in the odor-detecting lining of the rodent nose are wired to brain circuits involved in reproduction. Then Buck and another co-investigator, Stephen Liberles, Ph.D., most recently found receptors in the lining of the nose (outside the VNO) that can be triggered by at least one known mouse pheromone. They found several mouse genes that code for these presumed pheromone receptors. And they determined that some of those genes also appear in humans.
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Sexual Chemistry May Be Real
Is it possible that our choice of a mate could be determined by the subliminal equivalent of body odor? Marketers of love potions would like you to think so and sell you a sure-fire way to attract a sexual partner. But dont get your credit card out just yet.
Even if scientists now believe human pheromones are active, these airborne communicators will never be the whole story behind human sexual behavior. But it's not too soon to anticipate some exciting new findings about the biological underpinnings of human sexual behavior.
For example, pheromones in lower species don't only influence reproductive behavior; they also modify appetites and drives related to pleasure and survival. Recent research has shown that smelling male and female steroids activates different parts of the brain in humans depending not just on the smeller's sex but also on the smeller's sexual orientation. The circuits involved in pheromone detection work along side circuits that control mood and motivation. Pheromones may also play a roll in the brain's intellectual functioning.
The next steps for researchers are to identify molecules in humans that may act as pheromones. Likely places to look are in immodest secretions like urine, sweat and vaginal fluid. Once identified, scientists will need to find ways of testing which receptors in the lining of the nose get triggered, which signals get passed to the brain, which hormone changes occur, and ultimately what, if any, behavioral changes emerge. We may be getting close to showing that the mysterious "chemistry" between men and women is real.
Michael Craig Miller, M.D., is Editor in Chief of the Harvard Mental Health Letter. He is also associate physician at Beth Israel Deaconess Medical Center and assistant professor at Harvard Medical School. He has been practicing psychiatry for more than 25 years and teaches in the Harvard Longwood Psychiatry Residency Program.