female, menstrual-cycleA few of my friends and I were discussing menstrual-cycles. One friend stated that women living together will have their periods synchronized, or at least that they will tend to converge. Is this statement true, and if so what evidence supports it?
Verdict: Probably not.
From Scientific American:
Last year, [Jeffrey Schank at the University of California, Davis] co-authored a study in Human Nature following 186 female Chinese students living in dorms for an entire year, the longest menstrual synchrony study yet. He saw no evidence for the phenomenon, but plenty of random overlaps that could be seen as synchrony if viewed through a shorter time window.
So it appears to be mostly coincidence and experimental deficiency(after all, there are only roughly four weeks in a month, so people are bound to overlap, especially if their menstrual cycle is irregular).
However, here's an interesting study:
McClintock points to her 1998 Nature paper, which found that women exposed to cotton pads soaked with underarm secretions collected from donors undergoing the first and second (follicular and luteal) phases of their cycles resulted in significantly altered menstrual cycle lengths in the test women. The results, however, rested on a knife-edge of statistical significance, Schank says, and could have been due to chance.
But a team of Japanese researchers at Yokohama City University, led by Kazuyuki Shinohara, also found in a series of papers that donor women undergoing these two phases of the menstrual cycle release compounds that when inhaled by other women can significantly impact the frequency in the latter of pulses of luteinizing hormone (LH), which helps control the timing of ovulation and cycle length.
So there's no hard evidence that periods syncronize under normal living circumstances. Maybe they should try sniffing each other's armpits?
The current consensus seems to be that this does not, in general, occur. There’s actually a decent overview of the literature on Wikipedia:
https://en.wikipedia.org/wiki/Menstrual_synchrony
One thing to keep in mind when thinking about this is that any two regular periodic functions with different periods will have times when they converge for a while, and times when they diverge. Imagining two women, one with a regular menstrual cycle lasting 27 days, and another of 29 days, there will be times when the cycles appear to converge. This can be modeled with a simple pair of sine waves; see, e.g., http://www.google.com/search?q=sin(27x),sin(29x)
Add that to our tendency to notice the things that do synchronize (or more generally, that seem noteworthy) more intensely than the the things that don’t (or seem uninteresting), and it’s easy to imagine why this might continue to get reported, even if it’s only a relatively rare and temporary effect.
Of course, if two women were to start taking birth control pills or using the ring on the same day, they might well then have a regular 28 day cycle as long as they’re both using such methods, which would be an artificial way to create such synchronization. This happens because the hormones being put into the body are on a cycle, and the cycle is deliberately controlled. Birth control pills are typically administered in a way that provides hormones for 21 days, and then none for 7 days (either by not taking pills, or by taking pills that don’t have hormone in them), and the ring is typically replaced every 28 days.
Either of these could in theory vary, but the effect is normally to put someone onto a cycle that’s fairly exactly 28 days long. If two or more women were to start this on the same day, then they would always synchronize. If they started it 14 days apart, they would never synchronize, always being 14 days apart. If either or both women then stopped using hormonal birth control, and reverted to their natural cycle, and if their natural cycle was some length other than exactly 28 days (and not exactly the same as each other), then the cycles would again diverge.
No, almost certainly not.
For menstrual synchrony to work, women would not only have to get their periods in phase with each other, they would also have to converge on a common period length, and that simply doesn't happen.
The funny thing about confirmation bias is that a group of women will report that they experienced convergence, yet each woman will typically give you a different length for her normal period. I've talked to college roommates who insisted they were "synched by Thanksgiving" even though one had a reliable 27-day period and the other had one that was always 31-33 days. While it's true that their menstrual days will come closer and closer together for several months, it's also true that they will then slide further apart, since one of them is going to have roughly six periods for every five that the other one has.
You also find women who will insist that synchronization occurs even when some or all of them are on hormonal birth control, in spite of the fact that the artificially-supplied hormones disrupt the normal chemistry of ovulation. For example, in Attractiveness of women's body odors over the menstrual cycle: the role of oral contraceptives and receiver sex Kuukasjärvi et al. found that taking the pill completely eliminated the cyclical nature of female odor over the monthly period.
Another criticism of the so-called "McClintock effect" is that religious orders have been housing women together in crowded conditions for centuries and supplying their menstrual needs under much more primitive circumstances, which would have made synchronization glaringly obvious:
In fact, one of the most telling criticisms of that 1971 paper came from the historian and archivist of a Catholic nunnery, who pointed out that demand for menstrual supplies followed a flat line even when novices were crowded together, 6 to 8 to a room, for long periods. If there had been any synchronization, nunneries would have noticed it centuries ago.
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