The Science of Sex: Blood Sisters

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Blood Sisters

Do women who live together get their periods at the same time? Anecdotal accounts have long suggested that they do, but scientific evidence was lacking until 1970. In that year Martha McClintock, then a graduate student at Wellesley College, decided to investigate the matter. Her results ignited a scientific controversy that still rages today, and could have significant implications for the future of birth control and women’s convenience.


McClintock kept records of the menstrual periods of the students in her dorm, and found that, over the course of a semester, the periods of women who spent a lot of time together occurred closer and closer in time. Her analysis, published in the prestigious journal Nature, appeared to give “menstrual synchrony” scientific grounding.


What’s more, her findings resonated with the spirit of 1970s feminism. Here was a biological expression of solidarity among women — a kinship that men knew nothing about and could never join. McClintock’s work was mentioned in many books about women’s lives. Before long, menstrual synchrony became common knowledge — something that most people had heard about and probably believed.


In 1998, McClintock, now a psychologist at the University of Chicago, published another paper in Nature. She and colleague Kathleen Stern did a set of experiments that purported to show how menstrual synchrony comes about, namely by means of pheromones. McClintock and Stern put cotton pads under the armpits of women at varying stages of their menstrual cycles. Some hours later, they removed the pads from these “donor” women and swabbed them under the noses of a second group of “recipient” women. The next menstrual period in these women occurred either earlier or later than it would have normally, depending on what stage of the menstrual cycle the “donor” women had been in when their armpits were sampled. McClintock and Stern concluded that women secrete at least two different pheromones at different stages of the menstrual cycle, and that these pheromones have the power to lengthen or shorten the cycles of other women who smell them, thus leading to synchrony.


Yet the existence of menstrual synchrony, as well as the belief that pheromones are its mechanism, remains highly controversial. Although some studies seem to support McClintock’s claims, other researchers have failed to detect it, even in circumstances very similar to her original study. For example, Wenda Trevathan and colleagues at New Mexico and Arizona State Universities, in one study, and Leonard and Aron Weller of Bar-Ilan University in Israel, in another, failed to observe synchrony in cohabiting lesbian couples, who one might imagine would be the most likely to synchronize. Furthermore, anthropologist Beverly Strassmann of the University of Michigan — the scientist who debunked the notion that the timing of menstruation is affected by the phases of the moon — is one of McClintock’s strongest critics. Strassmann has studied the Dogon, a cliff-dwelling people in West Africa. When Dogon women get their periods, they move into a “menstrual hut,” a practice which made it easy for Strassmann to keep track of their menstrual cycles. Yet she never observed synchronization of cycles, even between women who were sisters or close friends.


In 1999, Strassmann published a blistering critique of McClintock’s work. She pointed out (as have others) a variety of potential errors in the studies that have claimed to see menstrual synchrony. As to the pheromone study, Strassmann suggested that inappropriate statistical tests were used, that the claimed statistical significance of the results was marginal and that McClintock and Stern may have excluded women whose data went against their hypothesis. “Skepticism is warranted,” Strassmann concluded, which is academic lingo for “It ain’t so!”


Another critic of the synchronization theory is psychologist Madelynne Arden of Britain’s Leeds University. Arden re-analyzed a 1997 Israeli study that claimed to find “definitive” evidence of menstrual synchrony among Bedouin women. She concluded that the Israelis’ statistical methods would have found synchrony in any circumstances — even if the timing of the women’s periods had been completely random. Yet Arden still thinks that menstrual synchrony may sometimes occur, perhaps among women who have lived together for a long time.


McClintock herself remains adamant that the phenomenon exists, but she is willing to admit that it may be a lot more complicated than she originally thought. Sometimes women synchronize, she says, sometimes they desynchronize and sometimes they just remain random.


Like Strassmann, I’m skeptical that the phenomenon of menstrual synchrony exists at all. I know that quite a few women believe that they have experienced menstrual synchrony, but I wonder whether they have done the arithmetic to see how frequently menstrual periods overlap on a purely random basis. If two randomly chosen women have twenty-eight-day cycles and periods lasting five days, there is a one in three chance that their periods will overlap for at least one day. Such an overlap may be interpreted by the women as evidence of synchrony, but careful statistical analysis is required to distinguish chance coincidences from a real biological process.


In spite of my skepticism, I do believe that the topic deserves further study. After all, if menstrual synchrony and its pheromonal mechanism turn out to be real, there could be a big payoff for women. One can envisage bottled pheromones that would allow women to schedule their periods at will, or that might even act as contraceptives. And the added level of control would certainly help make monthly menstruation a more easily regulated aspect of being a woman.