Menstrual Synchrony, the syncing of the onset of menstruation in women who spend time in close proximity, creates the classic “girl-bonding” scenario: snuggling with your roommate under blankets, while eating pizza, watching movies, and clutching onto a heating pad. A study found that a staggering 90% of women have experienced it and believe in it; but is it just a coincidence or is there some hormonal, pheromonal, or other factor at play?
In 1971, Harvard Professor Martha McClintock conducted one of the first studies on menstrual synchrony. The results indicated that as the school year went along, a significant number of female college student pairs found that proximity had a strong effect on synchronisation of menstruation. McClintock (1971) attributed this to pheromones and suggested that within a pair or group of close friends, an alpha uterus exerts a strong hormonal pull, causing the others to menstruate in unison. This led to menstrual synchrony being termed the ‘McClintock Effect.’ However, many subsequent studies have been unable to replicate these results and have even found gaps in the methodology and statistical analyses of McClintock’s original study. For example, a similar study conducted in 2005, using advanced statistical tests, did not find evidence for menstrual synchrony.
More recently, Clue, a period tracking app, in collaboration with Alexandra Alvergne, a Professor at the University of Oxford, used a data-driven approach to answer the question of menstrual synchronisation. Looking at one billion data points collected from their global community of users in 2017, they concluded that not only is there a lack of evidence for the McClintock effect, but also that cycles are equally likely to diverge, as opposed to converge, over time.
Even with the lack of scientific evidence, this phenomena is spoken about often - casually, as well as in popular media, making it a myth that many women swear is true. A possible reason for this is that there is something exciting, mystical, and comforting about menstrual synchrony. Beverly Strassman, a professor from the University of Michigan, who wrote a paper debunking the McClintock effect, calls it “an appealing narrative that overrides the science.”
A hurdle in most menstrual synchrony studies is that women usually have persistent menstrual cycles of varying lengths. Add to this, the fact that menstruation often lasts 5 days, it is not surprising that friends commonly experience overlapping menses, which is taken as personal confirmation of menstrual synchrony. Irrespective of whether women biologically influence each other’s periods, by pure statistical probability, there is bound to be some overlap. Although women don’t truly synchronize, but rather phase in and out of synchrony over the months, psychological biases and heuristics might trick our brains into believing this is a real biological phenomenon.
One such heuristic is confirmation bias, the tendency to search for, favor, or interpret information or phenomena that support one’s beliefs or feelings. And safe to say, there are many reasons why women might choose to believe in menstrual synchronisation. For one, though the stigma surrounding periods is slowly dissipating, menstrual pain and premenstrual syndrome (PMS) are not yet thought of as “real” struggles. A researcher summarises this by saying that “women are dismissed all the time for what they believe about their bodies. So, people want to believe in this.” Another explanation provided by Professor Alvergne is that, “as humans, we always like exciting stories. We want to explain what we observe with something that is meaningful. And the idea that what we observe is due to chance or randomness is just not as interesting.” Third, a very important social aspect of menstrual synchrony, is that misery loves company. It is validating, reassuring, and comforting to know that the women closest to you are allies in this monthly event — especially when it involves blood, cramps, bloating, and moodiness. Syncing with a close friend also adds a layer to one’s relationship, making it feel deeper.
Lastly, memory is fickle because it is so heavily mediated by emotions. This selective memory is why you are far more likely to remember the times you shared the discomfort of menstrual cramps with your roommate or friend than the times you did not. This makes confirmation bias and believing in menstrual synchrony very likely.
It is very important to remember, however, that just because menstrual synchrony isn’t biologically mediated, it doesn’t mean it never occurs. Use this ambiguity to your advantage because that time of month is not easy. If you are ‘out of sync’ with the people in your life, it definitely does not mean anything is wrong with your menstrual cycle or your relationships. On the other hand, if you do happen to sync with your friends, enjoy it! You are both going through the same biological event, and can draw strength and community from each other.
No matter what your period schedule is, community and connectedness around menstruation should be cultivated. Talk about your periods - share stories, advice, and support - when you are on or off your period. Women create a global community that is close to 4 billion strong; just by pure chance, the likelihood of finding people whose bodies are going through the same things as you will continue to increase as the shroud around the normal, monthly uterus bleed is lifted and more people begin talking about it. Find solace in, and join the movement to normalise menstruation, lack of menstruation, and everything in between.
Riya Sirdeshmukh
