What is Menstrual Synchrony?
One of the most interesting myths about women’s reproductive functioning is that of menstrual synchrony. The romanticized notion that women have some kind of deep “intuitive” relationship with one another (and, possibly, with the moon), is hardly new, but there’s very little, if any, scientific evidence that it actually occurs.
Before getting into whether menstrual synchrony is “real” or not, we need to be clear on how we’re defining it. As we said in our paper, “Menstrual synchrony (MS) is widely understood to be a phenomenon whereby the duration of two or more women’s (or animals’) ovarian cycles shorten or lengthen so as to bring the timing of the onset of their menses into mutual alignment” (Harris and Vitzthum 2013). Confusion arises when the trigger for changes in menstrual cycling isn’t specified.
As my previous posts have mentioned, it is true that women’s cycle lengths may adapt to external environmental conditions, especially energetic, and possibly psychological, stressors. So it is possible that, within a population, changes in environmental conditions might induce certain changes in menstrual functioning in many women. For example, if a harsh winter moves in quickly, many women may experience amenorrhea for a time. However, this is not really an example of cycles becoming synchronized with each other—it’s just that each individual’s body is responding to similar environmental cues in a similar fashion.
The other, and perhaps more common, way to talk about menstrual synchrony suggests that individual women are capable of influencing the menstrual cycle onset dates of their friends, family members, or roommates (usually through pheromonal communication, which may or may not be possible). However, there’s really no evidence that such synchrony occurs.
Does menstrual synchrony occur?
Early studies, many of which were performed in university dormitories, (for example, see McClintock) reported that women who spent a lot of time together would eventually see their menstrual onset dates (the first date of the menstrual period) converge. These studies have been heavily criticized on the grounds that the methods used to establish how much time women spent together were seriously flawed and inconsistently applied, that the methods used for calculating menstrual onset dates were biased, that the statistical tests used were inappropriate for the data analyzed, and, furthermore, that the proportion of cycles that actually did converge (or, if you like, “synchronize”) could be reasonably explained by chance alone.
Studies designed to avoid these kinds of pitfalls have found no evidence that menstrual synchrony occurs in women. For example, Trevathan et al. (1993) recruited co-habitating lesbian couples and tracked their menstrual cycle onset dates. They found that, though menstrual onset dates did occasionally converge, they were even more likely to diverge over time. This study is especially interesting because early supporters of menstrual synchrony hypothesized that women who spend the most time together are the most likely to synchronize–and here we have clear evidence that even women in intimate relationships do not influence each others’ cycle onset dates. In a study of 186 Chinese university students living in dormitories, Yang and Shank (2006) found that natural cycle length variability could, in the short term, give the impression of synchrony. Over time, however, individuals who converged would eventually diverge, all of which could be explained by chance alone.
Well, you may say, perhaps this physiological phenomenon is weaker in women living in industrialized populations, where artificial light and other technologies alter our biological functioning. Maybe menstrual synchrony would be stronger in populations where women are living more like our ancestors did—no artificial light, no hormonal contraceptives to override the body’s “natural” inclinations, etc. Not so much. In a longitudinal study of menstrual patterns among the Dogon, a non-contracepting agricultural population in Mali, Strassman carefully categorized how much time women spent together and when their cycles started. She found no evidence that women’s menstrual cycles were synchronized.
Could menstrual synchrony occur?
When Strassman reported that she found no evidence of menstrual synchrony in the Dogon, she also pointed out that it would be almost impossible for synchronization to occur, given the natural variation in menstrual cycle lengths. Women’s cycle lengths vary for many reasons, including responding to nutritional availability, changes in physical workloads, early (and often undetected) pregnancy loss, etc. For synchronization to occur, these highly variable, even erratic, menstrual cycle onset dates would need to align and remain aligned over time—but the biology of reproductive functioning and the reality of menstrual cycle data makes this kind of regularity highly unlikely.
So, without any evidence that menstrual synchrony does (or even could!) occur, why does this myth have so many followers? Do we, as a culture, like to think that women have special “powers” over each others’ bodies? Perhaps we are unjustifiably committed to the idea that all women are linked, somehow, to each other, and to nature. Many cultures, including U.S. cultures, tend to associate women and their bodies with nature (see here, or here for more on this). So, perhaps some of us expect women’s menstrual patterns to be influenced by not only nature itself (for example, the moon) but also by their female companions, who are their own “forces of nature.” Ultimately, this mythologized menstrual sisterhood is just not supported by scientific evidence. What the scientific evidence actually shows is that women’s bodies and reproductive functioning is highly variable, adaptive, and individualized–which is really quite powerful itself!