Periods don’t sync, period

Study after study has shown that the original research was deeply flawed… but it fits a stereotype of female empathy

So, um. Ladies. I think we need to talk about. Er. You know. Those things? Your monthly… things. Don’t worry. This is a health-science column, not a meeting of the 1922 Committee, and I am entirely comfortable with using the words ‘period’, and ‘tampon’, and even ‘menstrual blood’ if called upon to do so. But we do need to talk about them.

It’s a truism, pretty much, that women’s periods line up if they live closely together for long periods. And most people who’ve shared a house or flat with two or more women will have experienced this apparent fact: women doubling over with stomach cramps and nausea at the same time; digs becoming a tricky place for a few days out of every 28, male housemates finding excuses to be at the student union bar or elsewhere.

We all know that story. But, regular readers will not be surprised to learn, there’s not much evidence that it actually happens. The existence of ‘menstrual synchrony’, to give it its medical term, was first mooted in a study by Martha McClintock, a psychologist at Harvard University. She noted that, in several species including mice, the oestrous cycles of females living in groups were affected by each other’s pheromones, and decided to have a look to see if that was the case in humans, too.

She looked at 135 female undergraduates, aged 17–22, living in a suburban college dormitory. Over an academic year she asked them the dates of the period onset. Crucially, she also asked the women they were close friends with, and who they were room-mates with. In October, near the start of the school year, close friends and room-mates’ cycles started an average eight or nine days apart. In March, near the end, she found they were five days apart. A control group of randomly chosen pairs of women who didn’t spend time together started with cycles ten days apart and that didn’t change.

McClintock’s hypothesis was that ovulating women release pheromones, and that these trigger oestrus in other women. A few other studies found similar results and the idea that women living closely together end up having periods at the same time became, essentially, common knowledge. One of those scientific facts everyone knows, without always knowing exactly how they know it.

But problems started to arise. For one thing, no one ever found a pheromone that could account for it. ‘Not a single report has appeared in which the chemical identity of the alleged menstrual cycle synchrony pheromone has been made,’ grumbled Richard Doty in his book The Great Pheromone Myth 35 years later.

Of course, the fact that you don’t know why something happens doesn’t mean that it isn’t happening. It’s perfectly possible that something we don’t understand is going on.

But let’s remember the lesson of homeopathy. If you Google ‘how does homeopathy work’, you’ll find a thousand articles about the memory of water or quantum woo magic, or something. But the one you need is howdoeshomeopathywork.com. It takes you through to a page which simply says, in 50-point type: ‘It doesn’t.’

And in the years after McClintock’s apparent discovery, it became increasingly clear that the lack of a mechanism to explain menstrual synchrony was very much not the main problem with the hypothesis. In 1991, two studies published in the journal Psychoneuroendocrinology both failed to find any hint of synchrony. Another in the same journal in 1993 also failed.

It got worse. Scientists returned to the work of McClintock and the others who followed her and noticed a few things that were wrong. Nothing ill-intentioned, but statistical mistakes — technical things like assuming that ‘close friends’ and ‘room-mates’ were mutually exclusive groups, which exaggerated the size of the effect. She’d also miscalculated the onset dates for periods, making them look further apart at the start of the year, which made it look like they’d changed more at the end. Several follow-up studies that replicated her results had similar problems.

Later studies, notably Yang and Schank’s work, published in Human Nature in 2006, took account of these problems, and failed to find any evidence of synchrony. Yang and Schank also pointed out that women’s menstrual cycles vary an awful lot — some are as short as 21 days, some as long as 35. So two women with different length cycles would find themselves having periods at the same time every so often in the same way that a fast clock will eventually catch up with a slow one.

And that’s basically that. There is so little reliable evidence that a 2013 review of the literature sighed: ‘One is left wondering why so much attention has been given to searching for elusive mechanisms and constructing convoluted evolutionary scenarios’ to explain it. The authors suggested that it fitted the stereotype of women as ‘empathic’, linked to one another by a near-psychic understanding, and to nature by the rhythm of the lunar cycle.

Whatever the reason, the myth — I think we can reasonably call it a myth — is persistent. A 1999 study found that 84 per cent of women were aware of the concept. But what’s really striking is that 70 per cent reported experiencing it themselves. It’s amazing how good we are at seeing patterns in randomness.


  • Synchronicity is the Non Local Event – and yet in time is everything relative to its ‘past’ or future’. As you define yourself in past and future terms – so will you set up an interference pattern in which to posit a focus of awareness that grows to operate as ‘consciousness of’ this or that, then and so anticipate when. Because you focus Existence (see) the ‘randomness’ will never be – although the idea of order must create it as its supporting polarity.

    I haven’t performed or written up a peer reviewed study on synchronicity of menstruation cycles in women working or living together – though I have often met its occurrence – so I wont engage on whether this is or isn’t so for you, for women or for glimpses of an implicate vibrational resonance to manifest existence.

    The belief in randomness is a mythic displacement to the belief in a sense of imposed or imprisoning meaning. Again see the polarities that give each other apparent autonomous ‘existence’ – but not outside and apart from the mind that embraces them.

    Meaning that is added on or imposed is a false god or self concept reflected back. So if you accept yourself separate from Life – you will perceive a separate world of separate things. If you accept yourself as conflicted purposes vying for order or unity amidst adversity or chaos then you perceive a world of conflict amidst conflicting forces and competing interests.

    Like attracts Like – and yet in terms of feeling recognition – there is a repulsion that guarantees the persistence of the separating principle – so as to lead us to fight our own unrecognised shadow or shout at our own unrecognised echo and hate our own unrecognised hate.

    Life Works – because nothing else is – but definitions allow the freedom to experience it working against itself – and identify in this or that ‘side’ as righteous or gain fuel for hate in hatred of powerlessness or a sense of illegitimate limitation.

    Myths are not the same as falsehoods – though science has largely been corrupted by hatred of the Mythic and equates myth with lie or imaginative delusion. Myths are archetypal symbols of meanings that embody our primary conditioned consciousness – and so are rich in meanings that can be used to persist such conditioned identity in conflict OR awaken and reintegrate to a wholeness of being that cannot be translated into terms of conflicted and separated verbal mental constructs. But synchronicity proves itself relative to whatever you choose to accept true for you – most of which is ‘unconsciously’ active as the mythic sense of conditioned self. So of course such a self ‘sees’ a mechanistic universe – without even a creative impulse from which patterns of harmonics and dissonances arise. It ‘sees’ that which is resonating or synchronous to the vibrational quality of its own thought – and thought devoid of creative impulse is as close to dead as to deny the true Mater in material embodiment – and so operate against Life within the fantasy of power. All such power is conditional upon denying it to others – then it is not power but limitation in support of fantasy – an archetype of censorship to shut down Communication.

    The patternings that we grow and embody through the purposes we accept are the Ideas through which Culture flowers or withers. Love operates human synchronicity – but the fear and denial of love operates a falsely imposed ‘meaning’ upon the chaotic and conflicted negative synchronicity.

    The ‘elitist’ power class think to use contrived myth or controlled narrative to leverage or trigger outcomes in their favour. The truth of it is not in the story – but in where you are truly coming from – the active purpose.

    You cannot communicate ‘data’ to any meaningful end without interpretation and translation into story definitions at some level. If you choose to be out of sync with the movement of your being – you can get a hit of power for a while before it fragments into a paralysis of complex frustration or meaninglessness in which the game is no longer worth the candle. But it’s your light – and your choice as to where to focus and receive reflection.

    I feel you have an axe to grind – and seeking the ammunition to wage vendetta. Perhaps the meaning of Life escapes you – and you feel cheated or denied something precious? Whoever ‘started it’ I see the dynamic of conflict as the cause of the oppositional reflection – and so I feel for ways of expanding perspective rather than reinforcing rejection and denial.

    I just ‘happened’ to read your article and to then write whatever ‘happened’ to arise as to the desire to share communication – and yet the synchronicity extends according to the quality of a like attention and curiosity – finding its ‘timing’ within the free willingness of acceptance – which is always a recognition of presence and not a mental construct to apply in some other moment than this.

  • Aside from the lack of evidence that menstrual cycle synchrony exists, what evolutionary advantage would it create? If evolution was to select for such a mechanism, there would have to be an advantage in it from a reproductive point of view. Thinking about it for a moment, I can’t imagine what it would be.