The scent of a woman
Seated at a table in her Institute for Mind and
Biology office, red marker in hand, Martha McClintock graphs a line
that rises like a swelling wave. The line, she says, is the monthly
ebb and flow of an average woman’s sexual desire. The surge
peaks days before a woman ovulates, only to dive after her eggs
are released. It’s this dip that McClintock, the David Lee
Shillinglaw distinguished service professor in psychology, and her
colleagues want to understand in hopes of combatting low libidos.
In their latest study, “Social Chemosignals
from Breastfeeding Women Increase Sexual Motivation,” reported
in the September Hormones and Behavior, the researchers
discovered a substance that kept the wave at its pre-ovulation high.
Produced by nursing mothers and their infants, this social chemosignal,
they found, increased other women’s sexual desire—and,
McClintock notes, “maintained it at naturally high levels.”
Absorbed through the nose, social chemosignals
affect bodily cycles, including mood and menstruation. Human pheromones—first
documented by McClintock’s team in 1998—are a type of
social chemosignal that influence hormone function in other individuals.
The newly discovered chemosignal meets some of the pheromone criteria,
such as altering behavior, but further investigation is needed before
categorizing it. Already the substance, likely a medley of compounds,
holds promise as a drug-free solution for low sex drives. “If
we could identify this compound or exactly how it works,”
she suggests, “we might have a treatment.”
For the study, researchers had 26 breastfeeding
mothers in Philadelphia place pads in their nursing bras and under
their arms. The collected pads were then cut into pieces, frozen,
and given to 47 Chicago women, ages 18 to 35. For two months the
participants sniffed the Philadelphia pads—without knowing
what substance they contained—at least twice a day. They recorded
their sexual activity and moods, including how often they had sexual
desires or fantasies.
Having earlier established baseline libido levels,
McClintock’s team compared the Chicago responses with a control
group’s. Women exposed to the chemosignal who had regular
partners experienced a 24 percent increase in sexual desire. Those
without partners experienced a 17 percent increase in fantasies.
The variation in results between women with partners and those without,
she explains, suggests that the effect depends on sexual context
and partner availability.
Chemosignals “are often neglected or not
studied as much as they should be, and this is evidence that they
have an effect on physiology and psychological state,” says
Suma Jacob, AB’91, PhD’98, MD’01, one of McClintock’s
coauthors. Learning about such substances, Jacob notes, will allow
women in the long term to have more reproductive options.
The idea for examining breastfeeding compounds
arose from earlier studies involving lab rats, McClintock says.
Pregnant and lactating rats, researchers found, produced pheromones
that changed the ovulation cycles in their female peers. The newest
findings also build upon two previous reports on those human cycles.
Her team pinpointed ovulation within about 12 hours by collecting
urine samples and measuring the luteinizing hormone, which triggers
the egg’s release from the follicle. Before this February
study, she says, a woman had to endure a series of blood samples
for such accuracy.
To establish the connection between ovulation
cycles and libido levels, the researchers took daily psychological
measures. “With that, we were able to show women’s sexual
motivations increase a few days before ovulation,” McClintock
says, referencing the dramatic peak on her hand-drawn graph. As
desire goes up, the luteinizing hormone rises. That timing makes
evolutionary sense, she notes, because the desire to have sex anticipates
ovulation, and the egg is released like clockwork to cross paths
with the sperm.
The team had already demonstrated that breastfeeding
compounds can hasten the onset of ovulation. Nursing mothers and
babies, they found in another February study, suggest to neighboring
women that the conditions are favorable for childbearing. Biologically
speaking, McClintock explains, potential moms want to know that
the food and disease environments are prime for pregnancy: “What
better predictor than a woman in your community who has had a baby
and is lactating?”
Despite the media interest—several October
news reports touted the newly discovered chemosignal as the future
female Viagra—the results, she argues, represent a beginning,
not an end. They open “the door,” she says, for work
by other biopsychologists, natural chemists, and anthropologists.
One question worth asking, McClintock suggests, is whether the compounds
cause a similar response in other breastfeeding women, “especially
because libido is typically lower in this reproductive state.”
The findings also merit determining if the chemosignal is indeed
a pheromone, the study concludes, and its effect on other behaviors.
She hopes that such research, like the graph line, will continue
to rise.—Sara Michael