Do menstrual cycles synchronise?

18 November 2012

Question

Hi naked scientists. Thank you for your awesome podcast.

I would like to know if there is a scientific or evolutionary reason that women's menstrual cycles synch up when living together.

Michele & Jesse Padberg Dixon, New Mexico

Answer

We put this question to Dr Jo Setchell, biological anthopologist at Durham University.

Jo - Menstrual synchrony is a fascinating topic. Lots of women will tell you that they have experienced it, that when they were living with other women, for example as students, they started to have their menstrual period at the same time. However, menstrual synchrony is also a controversial topic. In fact, although there are some very well-known studies that claim to show that it happens in humans, and in laboratory rats, and even some that claim to demonstrate that the synchrony is caused by "pheromones", these studies have all been strongly criticised for the methods used. In fact, if you use correct statistical methods then you find that there is no evidence for synchrony.

I've been studying mandrills - large forest-dwelling primates - for years, and we have records of all the female cycles. A big advantage of these primates is that the females have big swellings around their fertile period, so it's easy to track their cycles (unlike with humans!). When we looked to see whether female cycles are more synchronous than expected from a chance distribution, using some neat statistical modelling, we found that they're not. That's even though if I look at the data, I think you can see patterns!

So, the evidence suggests that, although it's a very appealing idea, cycle synchrony actually has more to do with our evolved ability to detect patterns where there are none than it is to do with real evolutionary forces. We're more likely to notice incidents when we cycle at the same time as a friend than we are to notice when we don't. So you can think of it like seeing faces in the clouds. And why do we do that? Well, because it's always better to spot a predator that isn't there than not to spot one that is!

Hannah - That was Jo Setchell debunking the myth that menstrual cycles synchronise and offering up an evolutionary explanation for why humans identify patterns when they don't actually exist.

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