Periods don’t sync, period

    5 May 2016

    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 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.