Dr Hypothesis (Issue 7)
Dear Dr. Hypothesis,
I have heard that women who live together tend to menstruate together. I am a girl, and back when I used to live with four other girls I noticed this synchrony effect. Now, I live with just one female flatmate. We both menstruate regularly, but never at the same time. Could it be that a critical number of women must be present in order for the synchrony to start?
Out of Sync
Dear Sister Sync,
The idea that women who spend a lot of time together will menstruate together has carved out a firm place in the annals of dormitory lore, thanks to decades of anecdotal reports from cohabitating women (and their male observers). Still, believe it or not, after almost 40 years scientists have yet to agree on whether the phenomenon constitutes myth or reality.
The theory first hit the science scene in 1971, when psychologist Martha McClintock, now at the University of Chicago, published a paper in Nature. She reported that for female undergraduate students living together in dormitories, the start of their menstrual cycles became closer to that of their friends over time.
McClintock was the first to formally study menstrual synchrony, which has come to be called the ‘McClintock Effect’, but she has not been the only scientist to explore it. For instance, the largest study carried out to date was published in the journal Human Nature in 2006. Authors Zhengwei Yang and Jeffrey Schanck collected over a year’s worth of menstruation data from 186 Chinese women living in dorms and also reanalyzed McClintock’s original data. They titled their paper, “Women Do Not Synchronize Their Menstrual Cycles”.
Lest anyone worry that university students do not accurately represent the entire human population, a study published in 1997 focused on the Dogon, a population of cliff-dwelling people in West Africa. In this study, author Beverly Strassman, an anthropologist at the University of Michigan, also failed to find evidence of menstrual synchrony (believers in lunaception may be disappointed to learn that the menstrual cycles of Dogon women did not appear to follow the phases of the moon either).
As if the dispute over menstrual synchrony were not enough, McClintock’s work fans the flames of another controversy: the existence of mammalian pheromones, or social signals that we emit and detect subconsciously. Similar signals have been identified in insects such as ants, and some believe pheromones must be the force behind menstrual synchrony. McClintock published a paper in Nature in 1998 providing ‘definitive evidence of human pheromones’. The evidence was that exposure to underarm secretions affected the length of women’s menstrual cycles, but critics have complained that the study’s methodology had serious flaws.
As for the mammalian pheromone idea in general, after fifty years of discussion, it has yet to advance from speculation to accepted fact. Richard Doty, a zoologist at the University of Pennsylvania and author of The Great Pheromone Myth, staunchly doubts it ever will. In a recent article in New Scientist, he argues, “Not only have mammalian pheromones not been found, but the idea oversimplifies the nature of chemical communication among mammals.”
So if human pheromones might not exist and the McClintock effect is not real, where did all that anecdotal evidence come from? One explanation is that we might just be fooling ourselves. As Irrationality author and psychologist Stuart Sutherland explained, “People selectively remember items that are in line with their beliefs.” Synchrony will happen by chance; only when it does do we notice and remember it, skewing our observations. That is, like toast ‘always’ falling jam-side down, menstrual synchrony appears to occur more often than it really does.
Getting back to your question, Sister Sync, even if the McClintock effect is an illusion, random and pairwise synchrony is indeed more likely to occur when more women are present. Supposing all women menstruate for 28 days, the chance that two roommates would start their periods on precisely the same day is less than 4%. But for 5 women together, the chance that at least two start together jumps up to 31%. The probability increases above 50% if you allow overlap within a day or two to count as synchrony. It also increases if you take into account that women’s cycles vary around the 28 day average.
The bottom line? Debates about menstrual synchrony and mammalian pheromones are not over yet. However, current evidence suggests that, unlike swimmers and boy bands, menstrual cycles do not tend to be ‘in sync’.