When
biopsychologist Martha K. McClintock documented in 1971 that women
living in college dormitories often have synchronized menstrual
periods, scientists suspected that chemicals called pheromones were
responsible. Animals give off pheromones, which convey messages to
others of their species, but scientists have found only sketchy
evidence that people do.
Now, McClintock has uncovered clear evidence of at
least two human pheromones.
In a series of tests at the University of Chicago,
she and her colleague Kathleen Stern showed that most of the women
exposed to chemicals shed by other women found that their monthly
cycles sped up or slowed down, depending on when the samples were
taken from the donors. The scientists report their findings in the
March 12 Nature.
McClintock and Stern enlisted 29 women between the
ages of 20 and 35 for the test. Nine donated pheromones; the other 20
received them. The donors kept a gauze pad in each armpit for 8 hours
a day. The researchers then mixed perspiration from these pads with
isopropyl alcohol to mask odors and dabbed the mixture under the noses
of the recipients.
Women were influenced by the samples only during the
2 to 4 days before they ovulated. Samples taken from donors who were
in the pre-ovulation stage shortened a recipient's monthly cycle by
roughly 2 days. In contrast, samples taken from donors during
ovulation delayed the cycles of recipients by about a day and a half.
The donors, used as a control group, received an inert dab of the
alcohol; they showed no changes in cycle.
To ascertain that the changes in menstrual cycles
weren't random, the researchers tested recipients with one set of
samples for 2 months and then switched, testing them with the other
set for 2 more. The first set sped up the cycles two-thirds of the
time; the second set slowed them down just as often. Nasal congestion
in some participants apparently hampered the effect.
By using this crossover technique, the researchers
"have come out with some nice, crisp data," says David H.
Abbott, a behavioral endocrinologist at the University of
Wisconsin-Madison. This study indicates that there are at least two
human pheromones, McClintock says. "There are likely to be
others, but that has not yet been established."
Pheromones' role in animal life is well described.
For example, when two ants meet on a trail and pause to rub antennae,
they are passing pheromones back and forth to ascertain each other's
species and, often, their colony identity, says William C. Agosta, a
chemist at Rockefeller University in New York.
Higher animals, such as mammals, have individual
pheromones or special combinations of these chemicals that signal
their identity, enabling babies to recognize parents and vice-versa,
Agosta says.
Although research over the past 2 decades has hinted
at the existence of human pheromones, some scientists have remained
unconvinced that people harbor and react to them, says Charles J.
Wysocki, a neuroscientist at Monell Chemical Senses Center, a
nonprofit research facility in Philadelphia. This study "is the
final nail in the coffin" of those doubts, he says.
Several puzzles remain. The chemical structure of
these pheromones is unknown. Moreover, studies have failed to
determine whether men exude pheromones that affect fertility. As to
why women radiate these pheromones, one theory holds that simultaneous
ovulation in a group of women helped in prehistoric times to promote
genetic diversity, since one man couldn't impregnate everyone in the
group.
Aside from affecting the hormones that induce
ovulation, no one knows what reactions human pheromones trigger.
"What is the neural mechanism?" Agosta asks. In insects, the
answer to that question has proved complicated, he says. In humans,
the answer is still elusive.
Back to Pheromones Story
