University of Chicago researchers reported today that they have proved the existence of human pheromones - powerful airborne chemicals that people may emit and receive to influence one another without even noticing.
The discovery - which, if confirmed, could relegate romance to simple body chemistry - suggests that human beings do indeed possess a sixth sense, as recently has been documented in other mammals and long has been known in insects. It could turn the sciences of psychology and physiology on their respective ears.
But the lead scientist behind the work published in the journal Nature, University of Chicago psychologist Martha C. McClintock, said she hopes for more practical benefits, including better natural methods of birth control and infertility treatments that do not require steroids to regulate ovulation.
“The implications are huge,” said McClintock, who has spent 30 years in pursuit of the elusive pheromones. “It’s the first study that shows human beings have the capacity to communicate with pheromones - please underline the word “capacity.” If you look at any perfume ad, you’ll see all kinds of claims about this. But they have just been claims, not science.
“Now that we know they exist in humans, we have to do the scientific work to determine how many there are, if we use them in everyday life and what effects they actually may have.”
McClintock gained scientific fame in 1971 when, as an undergraduate at Wellesley College, she published a paper in Nature documenting a phenomenon she had noticed: that many of the 135 women living in her dorm, no matter the individual timing of their menstrual cycles, gradually started to have their periods at about the same time.
How could such “menstrual synchrony” occur? Perhaps it was modulated by sights and sounds, through mutual activities or by similar daily schedules. Or perhaps there was something in the air, some chemical that reset the biological clock that controls ovulation, cycling, and hence, reproduction.
The paper represented one of the first examples of how social interactions could affect biology. It has since been learned that pheromones can block pregnancies in animals, influence mating preferences, affect the timing of puberty, establish dominance, permit the identification of territory and affect a wide range of other behaviors. McClintock has specialized in studying reproductive behavior in rodents, showing that lab rats can produce signals that accelerate or inhibit ovulation in other rats.
Mammals usually detect pheromones through receptors found in the nasal cavity. But unlike smells, which are wired directly into the brain, pheromones may rely on a different neural system, which may be why humans do not consciously detect them.
For her latest study, McClintock and a former student, Kathleen Stern, a 1992 doctoral graduate in psychology now working in private industry, recruited volunteers among the U. of C. student body and staff.
“No one knew what the study involved,” McClintock said. “We wanted 29 women between the ages of 20 and 35 whose periods were regular and spontaneous. That was all.”
As McClintock earlier had proved in animals, the researchers established the presence of two pheromones in women. One, produced prior to ovulation, shortens the ovarian cycle; the second, produced just at ovulation, lengthens the cycle.
From nine women, designated as “donors,” the team gathered samples of chemical compounds by placing pads under the armpits. The women bathed without perfumed products every day and wore the pads for at least eight hours.
Samples were obtained at distinct phases of the menstrual cycle. Each pad was cut into four sections, treated with alcohol to kill any bacteria that could contribute to body odors, and frozen in a glass vial.
The pad portions were then wiped under the noses of the 20 “recipient” women every day for four consecutive menstrual cycles. They women were asked not to wash their faces for the next six hours.
“Each recipient had a cycle where she was exposed to the placebo and two cycles in which she was exposed to the other chemicals,” McClintock explained. “Our control group never sniffed anything but alcohol pads. In fact, alcohol was the only thing actually smelled by any woman in the study.”
About 70 percent of the women responded: Their biological clocks were affected, changing the timing of their ovulation and menstruation.
The range of response was considerably greater than the normal variation in cycle length typical for this age group: cycles were shortened from 1 to 14 days and lengthened from 1 to 12 days, the researchers reported.
McClintock cautioned that humans probably are not as strongly affected by pheromones as other animals, particularly in mating behavior. Many other factors contribute to romance, she believes.
“It may be that this sense is merely a remnant of evolution that we don’t need anymore,” she said. “Perhaps it’s just vestigial. Or perhaps it’s really important.