A Subtle Sense

As an undergraduate at Wellesley College, Martha McClintock’s observation that many of the 135 women in her dorm were menstruating at about the same time led her to publish a groundbreaking paper in Nature documenting the phenomenon.

Now, 27 years later, she has published on the same topic in the same journal. But this time, McClintock, a professor of psychology and chair of the Committee on Biopsychology, has taken a few steps further, establishing the first scientific proof of pheromones in humans. Pheromones had already been shown to play a role in the physiological responses of animals. McClintock showed that some pheromones—chemical compounds undetectable by odor—play a specific role in the timing of female humans’ ovulation.

The 51-year-old researcher, along with coauthor Kathleen Stern, PhD’92, published in the March 12 issue of Nature a paper titled “Regulation of Ovulation by Human Pheromones,” concluding that “humans have the potential to communicate pheromonally, either by using an unidentified part of the main olfactory system, or perhaps with a sixth sense, with its own unique pathway.”

McClintock’s latest results build on her initial menstrual synchrony research. In 1988 McClintock began a 10-year study that involved 29 women between the ages of 20 and 35—at the start of the study—who had a history of regular, spontaneous ovulation. The researchers gathered samples of chemical releases from nine of the women at certain points in their menstrual cycles by placing wads of cotton under their arms; the women had bathed without perfumed products and wore the cotton for at least eight hours.

Next, each sample of cotton was cut into four sections, treated with alcohol, and frozen. The sections—smelling only of alcohol—were then wiped daily under the noses of the 20 other women, who then did not wash their faces for six hours.

The researchers discovered that the compounds obtained from the women in the early phases of their cycles shortened the cycles of the second group of women by speeding up their preovulatory surge of luteinizing hormone; conversely, compounds collected later, during ovulation, lengthened the menstrual cycles by delaying the LH surge.

After reviewing the data, McClintock and Stern concluded that 68 percent of the women responded to follicular pheromones and 68 percent responded to ovulatory pheromones present in the samples, noting that “the range of response magnitude was considerably more than variation in cycle length typical for the age group: cycles were shortened from one to 14 days and lengthened from one to 12 days.”

McClintock’s research team has yet to show how the pheromones trigger the menstrual-cycle changes. Because the samples were put on the subjects’ upper lips, McClintock admits that “we know absolutely nothing about where the chemical formula is acting, whether it’s through the skin, the mucus membranes in the nose, or a pair of tiny pits in the nose.”

McClintock, who received a Ph.D. in 1974 from the University of Pennsylvania and joined the U of C faculty in 1978, says her work could lead, on the more practical side, to a contraceptive in the form of a drug that constantly delays ovulation, or perhaps to a drug that helps cure some types of infertility by stimulating ovulation. Of the future, she says, “The main goals will be to identify the compounds, how they act, what their natural route is, and whether we could develop a highly efficient ovarian modulator.”—Betsy Rossen Elliot