by
Rick Weiss, Washington Post Staff Writer
March
12, 1998
Scientists have found long-sought proof
that people release potent chemical signals that can have profound
effects on other people.
The research settles a 40-year debate
about whether humans produce and can respond to
"pheromones," molecules that are usually airborne and
odorless and which, in other species, influence such physiological
processes and behaviors as mate choice, the recognition of one's own
family members, and the ability to "smell" the difference
between friend and foe.
Specifically, the new research shows
that women's underarm odors can alter the timing of other women's
reproductive cycles. It explains why women who live together often
develop synchronous menstrual periods, and could spur development of
"natural" fertility drugs or contraceptives.
The finding may also lead to the
discovery of compounds in sweat that could be incorporated into
fragrances to alter body chemistry or mood.
"This is definitely going to make
people sit up and take notice," said Charles J. Wysocki of the
Monell Chemical Senses Center in Philadelphia. Previous studies by
scientists at Monell and elsewhere showed similar results but later
were recognized as flawed. The new work, Wysocki said, seems to answer
the question for good.
"The evidence has now become quite
strong that humans produce and detect pheromones," agreed Edward
W. Johnson of Idaho State University in Pocatello.
The discovery was especially gratifying
to Martha K. McClintock, the University of Chicago researcher who,
with colleague Kathleen Stern, describes the work in today's issue of
the journal Nature. As an undergraduate almost 30 years ago,
McClintock observed that many women in her dormitory menstruated in
synchrony.
For decades McClintock immersed herself
in the task of identifying the timing mechanism. She and others
suspected pheromones, but proof was hard to come by.
Pheromones have been documented in many
species, ranging from insects to elephants, as sex attractants,
kinship identifiers or alarm signals. In many species they are
detected by a specialized organ inside the nose or mouth called the
vomeronasal organ, or VNO.
There was ample evidence that human
pheromones exist; babies show a clear preference for pieces of
clothing that have been worn by their own mothers, for example, and
research suggests that men and women choose their mates in part by
sniffing out partners with compatible immune systems. Several years
ago, researchers in Utah even said they had identified the first human
pheromones -- and turned their discovery into a line of perfumes that
today boasts revenue of $40 million a year.
But the Utah work has been criticized
by many experts. And the Monell work, on menstrual cycles, did not
take into consideration the fact that many out-of-phase cycles will
naturally converge over time.
Moreover, scientists have remained
uncertain whether the human VNO, a pair of tiny pits in the nose, is a
functional organ or an evolutionary vestige.
To find out if human pheromones exist
and can affect menstrual timing, McClintock and Stern asked nine women
to wear gauze pads under their armpits all day. (Sweat is a common
source of pheromones in mammals.) The pads, changed daily, were cut
into pieces and frozen, and a daily tally was kept of each woman's
menstrual phase.
Then, every day for four months, the
researchers rubbed thawed gauze pads above the upper lips of 20
volunteer women who had agreed to have any of 30 different
"natural essences" rubbed under their noses.
"Sweat" was among the 30. "We buried it in the
list," McClintock said.
For two months, 10 women sniffed sweat
from women in the early phase of their menstrual cycle, while the
other 10 sniffed sweat from women in a later phase of their cycle.
Then the groups switched and spent two months getting the opposite
scent.
The women smelled nothing, but the
results were striking: Those exposed to "early phase" sweat
saw their own cycles shortened by an average of 1.7 days per month,
and as much as 14 days a month. Those who sniffed "later
phase" sweat saw their cycles lengthened by an average of 1.4
days a month, and up to 12 days a month.
Computer models indicated there must be
two substances in the sweat -- one that lengthens cycles and one that
shortens them -- and that together they can quickly lead to groups of
women having synchronous periods.
"This carefully controlled study
clearly shows, for the first time, that the potential for chemical
communication involving sexual function has been preserved in humans
during evolution," wrote Aron Weller, of Israel's Bar-Ilan
University, in a commentary in Nature.
McClintock emphasized the word
"potential," since the experiment does not prove these
signals work under normal conditions, such as across a room.
"We put it on the upper lip,"
McClintock said, "so really we know absolutely nothing about
where it is acting, whether it's through the skin, the mucus membranes
in the nose or the VNO." Nonetheless, practical uses could
follow.
"The whole point would be to see
what the compounds are and how do they act and what is their natural
route and see whether we could develop a highly efficient ovarian
modulator," McClintock said. A drug that constantly delays
ovulation could serve as a contraceptive, while one that prompts
ovulation might cure some kinds of infertility.
Linda Buck of Harvard University, who
studies the molecular genetics of smell, said she has been unable to
find functioning VNO genes in people. But some animals detect
pheromones with their normal nasal cells, she said, and humans may
too.
If pheromones have a big effect on
human physiology, people may want to rethink their heavy use of soaps
and perfumes: It may be, Buck speculated, that the constant washing
away or covering up of these sweaty social signals account for some of
the loneliness or depression in modern society.
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Pheromones Story
