The case against synchronized periods

Myths about menstrual cycles are a great example of why carefully collected data is important. Without that, it's extremely easy to see a pattern where none exists. For instance, there's no good evidence that menstrual cycles have anything to do with cycles of the Moon.

The evidence is also against there being such a thing as ladies synchronizing their menstrual cycles. I know. I know. It totally happened to you in college. I thought it happened to me, too. And there are few scientists who think the phenomenon is real. But the preponderance of evidence seems to be against them. Again (and this cannot be said enough) humans are really good at spotting patterns—even when patterns don't exist.

Kate Clancy, an anthropologist who studies the evolutionary medicine of women’s reproductive physiology, has a post on Scientific American blogs looking at the research that's poked holes in the synchronized periods hypothesis. A couple of the studies she talks about came out this year.

Maybe we should look to our primate relatives for evidence, then: in fact two papers have come out this year testing this hypothesis in primates! Setchell et al (2011) observed semi free-living mandrills, which is a kind of Old World monkey, a group to which the Great Apes belong. Out of ten observation-years of data, they found a single year that had significant synchrony… only to have that one year fail to be significant once they corrected for multiple testing. Multiple testing corrections are important because of the chance that if you test a hypothesis enough times you will get a spurious significant result (and for a brilliant take on this, see this xkcd comic).

The other equally interesting paper to come out this year on this topic is by Fürtbauer et al (2011), entitled “You Mate, I Mate: Macaque Females Synchronize Sex Not Cycles.” Their study population was wild Assamese macaques, also Old World monkeys. Fürtbauer et al (2011) observed behavioral receptivity and measured fecal ovarian hormones (yes, that means they measured hormones in poop) in order to assess behavioral and hormonal synchrony. They found long periods of behavioral receptivity that synchronized well across individuals, but that actual estrus cycles were randomly distributed within the receptive period. I thought this paper did a great job at providing an evolutionary framework for why mating might evolve to be synchronized, but not cycles, and because the paper was published in PLoS ONE, you can read it yourself for free.

This paper resolves a question I’ve had for a long time about menstrual synchrony, which is how in the world it could actually be beneficial to females, particularly those with covert ovulation. Why would you want all the females, or even a subset of them, to be fecund and receptive at the same time? And the answer is, you probably wouldn’t. Humans, other primates, even some cetaceans like dolphins have mating that is largely decoupled from reproductive cycling. That is, we don’t only mate at the time in our cycle when our chances are highest to conceive, though we might find ourselves slightly more proceptive or receptive at that time. Sex is not just about making babies, but is an affiliative behavior, promoting bonding but also plain old enjoyment. Instead, it may in some circumstances make sense to have extended periods of synchronous receptivity, as within a promiscuous species like the Assamese macaques (Fürtbauer et al. 2011). But this isn’t necessarily an adaptive feature of the entire primate lineage.

Image: A group of women from ILGWU Local 62 indicate their choice for president by pointing to a picture of Franklin Delano Roosevelt., a Creative Commons Attribution (2.0) image from kheelcenter's photostream



  1. I don’t know, I think it’s reasonable to associate menstruation with the moon.  I mean, it’s pretty much the same thing as being a werewolf, right?

  2. “Why would you want all the females, or even a subset of them, to be fecund and receptive at the same time?”

    I used to explain it to myself as being the temporal equivalent of a diamond district.

    1. Um, you don’t want all reproductive aged females to be receptive at the same time, but you do want them to be on a fertile cycle, if you are Mother Nature, that is. Remembering that most reproductive aged females are pregnant or nursing most of the time in M N’s plan (sorry… she didn’t plan exactly, but all she cares about is making more babies) there are only a few who are receptive at any given time in any given community… and the should all be, well, not fertile but pregnant or nursing… as far as M N is concerned.

  3. i have no strong opinion about whether menstrual cycle synchronization exists, but i find it weird that the article against it doesn’t mention martha mcclintock’s  experiment in the early 1970s that created the original consensus that synchronization was real. it’s weird for an article that casts doubt on the earlier scientific results to not even refer to those prior results.

  4. I have often thought that the moon/menses hypothesis could be tested with a simple query of feminine hygiene product purchases, if you had access to a grocery database. I have run a similar query against 50K birthdates after hearing my co-workers content that babies are more likely to be born on the full moon. (results, negative.)
    I can post the T-SQL function I wrote for “days after new moon” if anyone is interested.

    1. Doesn’t necessarily work, though. Because people don’t just buy menstrual products when their period starts. It’s more convenient to buy them when you’re not bleeding, so they’re already around when you need them. 

      1. Sure, it is only a second level approximation, but I would expect some sort of correspondence if it were a real effect.

  5. “really good at spotting patterns”

    * * *
    My mother, an RN, says there is an urban myth among nurses that hospitals see a noticeable increase in Crazy Person admissions during the full moon. Likely more pattern spotting.

  6. My girlfriend forgot when she was supposed to start by a few days, so she asked her roomate.. they are all on cycle down to the day. The other roomate told her 2 days.. and they were right on.

    Could be cooincidence, but they have a 3rd girl on the same track too… and were all different when they started. So… go figure.

  7. My girlfriend and I cycle together, but it’s a bit unusual – we’re not at all regular, it’s just that whenever she starts, I start one to two days later.  This has happened to me with other women as well – basically any woman I was physically really close to, as in sleeping in the same bed.  They start bleeding, then I start bleeding – a trigger effect.

    I’m going to go with “there’s a mechanism here we don’t understand.” 

  8. Honestly? If menstrual synchronization isn’t real? I am stunned. I worked as a janitor in a day-care center and a large department store for a total of some ten years, and as the dude who disposed of the used tampons, it seemed as if all the women I worked with were on absolutely the same schedule. If a new employee came in, all of a sudden you’d have used tampons showing up in an off-week, and then once they synched up, it would be three weeks of no used tapons, one week with in the employee restrooms.

    I like having my preconceptions exploded, but coming up with an alternate explanation has me boggled.

  9. My mother has owned a retail store in a small town for over 25 years, employing women ranging from the ages of 16 to over 60 and can confirm, without a doubt, that nearly all of her employees after several mensus cycles will synchronize to within a day or two of each other.  Interestingly, in multiple instances, when a new female employee has been hired, the synchronized groups cycles will align with hers after a few months. 

    I’ve always considered a baseline “paleo” reasoning for this theory was that women impregnated at the same time would be lactating at the same time after childbirth and could thus support a larger number of infants than just their own if needed.


      I’ve always considered a baseline “paleo” reasoning for this theory was that women impregnated at the same time would be lactating at the same time after childbirth and could thus support a larger number of infants than just their own if needed

      Surely a couple of weeks either way makes no odds compared to the many months (or even years) of lactation?

      Fans of confirmation bias will notice that if two women have cycles of different length, then adjacent cycles can always be proved to be increasingly synchronous, as long as you choose to measure in the right direction.

    2. “My mother has owned a retail store in a small town for over 25 years, employing women ranging from the ages of 16 to over 60 and can confirm, without a doubt”

      Jesus Christ on a cracker!  What kind of employer can confirm without a doubt that her employees are menstruating?

      1. Is this that surprising?  Older women working along side younger women generated a bit of a matriarchal support structure of friends and colleagues.  I imagine they talked about it, like people do.

  10. Wouldn’t a simple reason for synchronizing births be the safety of numbers?  Eggs and babies having a high ratio of tastiness to mobility.

    Isn’t that why salmon spawn all at once instead of one at a time?  Isn’t that why mayflies hatch on the same day?

  11. I always guessed that the primary reason for syncing was because, if you’re in a hunter gatherer group, it makes sense for all the women to be bleeding at once.  It would allow for better coordination of group moves, etc.

    It could also provide a disincentive for the men to sleep around while their mate is bleeding.

    I’m not at all trying to be offensive, so you know.  These were just my hypotheses.  I think it’s interesting that the whole syncing thing may not even be true.

  12. great apes are actually not old world monkeys. cercopithecidae are the old world monkeys, apes are in hominidae. we’re as much monkeys as any other ape is, and i’m not your little fucking monkey.

  13. The only time sisters, friends, roommates and I were in sync is when we were all on oral contraceptives and everyone was force  into a 28 day cycle and we would choose start dates that allowed us to enjoy certain upcoming calender events.

  14. Note that for two women’s cycles to synch up, both the onset AND the length would have to become equal.  There is considerable variance in cycle length across women (SD=2.4 days, Cole, Ladner, & Byrn, 2009; SD=7.5 days, Chiazze et al., 1968), and there can be variance across cycles for one woman.  If two women maintain different cycle lengths, then it is impossible for their cycles to synch up.  The onset of their periods may co-occur just due to chance sometimes, but across many months there would be more mismatches than matches.  (But as Adela Doiron points out, oral contraceptives could cause more similar cycle lengths.)  Oh, and given that menstruation can typically last around 2-8 days, that means there’s an even greater probability of co-occurrence of periods (not just onset) due to chance alone.

    The anecdotes are interesting, but they are simply not compelling evidence, because human memory is fallible and because unsystematic observation may be biased (e.g., we might just be more likely to notice the times when two periods happen to co-occur vs. not).  Compelling evidence would be records stored external to a human nervous system, and collected systematically over a long duration of time (e.g., years) for many women, before, during, and after co-habitation.

    If synchronization is convincingly documented in some situation, we’d have to think about (and scientifically investigate) possible causes that may not be universal (e.g., something environmental specific to the location).  The whole topic is really interesting in presenting methodological and statistical challenges. 

  15. I dunno, I’m pretty sure this is a well-known phenomenon. That doesn’t make it true – see lemmings – but still. It could easily happen through entrainment if there’s any kind of pheromonal communication between people. Not to be weird about it, but I can certainly smell a difference when ladies are menstruating. Their whole body odor changes character. Studies show that men can detect when a woman is fertile, and it may not even be scent-based.

    It also seems reasonable to me from an evolutionary perspective. There’s no advantage to women as a group to have synchronized fertility – but there is individual advantage. The man has to choose. It works against male promiscuity, literally breeding it out of the population… just part of the ongoing arms race between the sexes. All in all, I’d like to see some metastudies before I’ll accept this phenomenon isn’t real.

    Also – Wikipedia says a large study of more than 30,000 cycles from more than 2300 women showed that the mean cycle length was 29.1 days. The synodic period of the moon is about 29.5 days, a remarkable coincidence.

    EDIT: Just talked to my girlfriend, who’s a doula. Not only did she experience synchronization with her daughter when she started menstruating, three other women in the house did as well. Notably, the daughter had a 28-day cycle, the most fertile. This synchronization didn’t happen before, and the women living together had cycles of varying length. Her theory is that entrainment happens to the most fertile cycle nearby. Also, an excellent point – nobody was on the pill. Everyone was experiencing natural periods. Finally – whenever she was helping deliver a baby, she would have a period the next day, no matter what. Not just spotting, a genuine 3-4 day period, even if she had a regular one the week before. It was the final clue that her body was trying to synchronize with someone fertile.

  16. I thought it had to do with the women smelling each others phermones or whatever, and that triggering the cycle.  

    I wonder if that leads to some subconscious battling for dominance between women, on whose cycle is leading and whose following, like the supposed Alpha male monkey crap people keep extrapolating to every human male interaction. I wonder if the most attractive woman is the one everyone else syncs up to. or the most fertile or something. 

  17. Any possibility this post was inspired by the latest episode of the Walking Dead? A male character brought up the theory because of two women who weren’t acting as he expected or hoped they would, therefore must be on their periods. Another character shot down the theory as baseless.

  18. Artificial light. Destroys synchrony. Any woman can make herself ovulate with the full moon on a 28 day cycle with a little diligence and a set of black-out curtains.

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