Investigations

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