girls, we're in this together

Most women know their monthly cycles coincide with those of their friends. But why does it happen? asks Annalisa Barbieri

Menstrual synchrony - when women living or working together start menstruating at the same time - is one of those subjects that sparks anecdotes. Tales abound of girls in boarding school, nuns in convents and girls living together whose periods suddenly go haywire and then settle into a pattern where they all come on at the same time. Even men have heard about it.

But despite all the anecdotal evidence, it was not until 1971 that it was proven to be a biological phenomenon, when Professor Martha McClintock conducted the first scientific research into it. McClintock studied 135 women living together at an American college. Over one academic year the students kept ''menstrual diaries''. She found that whereas at the beginning of the year they all started their periods at different times, by the end of the year they had all synchronised to within a few days of each other.

Since then, McClintock's studies have been replicated by various others. Today at the forefront of this research are father and son team Leonard Weller and Arran Weller from the Bar-Ilan University in Israel. In 1992 they looked at the cycles of 20 lesbian couples and found that their periods synchronised ''very frequently'' with half of them menstruating within two days of each other. In 1993 they studied mothers and daughters, and room-mates. The mothers and daughters especially ''displayed a significant degree of synchrony''. However two years later when they studied women working together - 72 women soldiers and 36 civilian office workers - no ''signficant synchrony'' was found. But they did find that friendship, mutual activity and length of time spent working together played a part in it.

Cynthia Graham, an adjunct assistant professor of psychology and psychiatry at Indiana University, researched synchrony ten years after McClintock's findings were published. She also believes that ''emotional attachment'' plays a large part. "Over the four months that I did my study, I found that menstrual synchrony was documented in women who were close friends," she says. "There is some suggestion that it is not just time spent together but the emotional attachment, the relationship between the women." So just living or working together isn't enough, but the closer you are with a woman the more likely you are to synchronise: women who bond together bleed together. Proven it may be, but despite all this research no one yet knows how it happens.

The most likely reason is that synchrony is mediated by some kind of olfactory communication via chemical messages. In an attempt to find answers, Professor McClintock studied rats (who do not menstruate; she studied their oestrus cycles). She fed air from one cage to another and discovered that rats could send airborne messages to each other and that in so doing, they could alter each other's cycles. The rats that had already ovulated could send ''slow down'' messages to the rats that hadn't and those rats that had ovulated could send ''hurry up'' messages to speed up ovulation in others. In a human experiment, volunteers had the sweat from the armpits of a woman with regular periods daubed on their upper lips. After four months the volunteers had started to synchronise with the woman whose underarm secretions they had been sniffing.

Whether one woman does actually ''lead'' in real life, is as yet unknown. "We don't know whether women converge together or one woman leads. There's been a lot of speculation about this, but no good studies,"explains Graham. Although it takes about four cycles to start synchronising, in any community about 30 per cent of the women will have ''independent'' cycles. Not being ''close'' to the women they live or work with is just one reason why they might not be influenced. Men, it seems, can also throw a biological spanner into the works. Sherri Matteo, a psychologist at Stanford University, California, found that women living with their male partners do not become synchronised, most probably due to male odour upsetting whatever it is that causes synchrony in the first place.

Why women should start bleeding together is a mystical question. But there is a theory. In his book Blood Relations: Menstruation and the Origins of Culture (Yale University Press, pounds 10.95) Chris Knight, lecturer in anthropology at the University of East London, discusses his theory on the origins of human culture and menstruation's effect on it. Put simply, the reason women started synchronising in the early days of homo sapiens, was in order to get the men to hunt meat because there was no need for the men to hang round as the women weren't fertile. Hence there was no point in copulation and because they weren't fertile, the women didn't need guarding from other predatory men looking to sow their seed. A sex-strike if you like.

Today there seems no obvious reason why women should synchronise. And much as some men might still leave the room at the mention of periods, there is more to most male/female sexual relationships than mere reproduction. It has also been suggested that women who synchronise have enhanced evolutionary fitness. So while we may not know fully why or how it happens, it's a little bit more complex than ''just women's things''.

Helen, 23: It's weird, I get pre-menstrual sometimes when I am not even due on. Then I find out my girlfriend is pre-menstrual as well. Then we both start on the same day. With my last girlfriend, we started coming on at exactly the same moment, and I thought that it proved we were meant for each other. Ha! Georgina, 25: I got used to having to wait around for Vicky, the girl I work closely with, to have her period so I could have mine. I came on so early one time that I had to go to the family planning clinic for advice and go straight on to another batch of pills without my seven-day break because my cycle was up the spout. The nurse was sure that I must have forgotten to take a pill and couldn't accredit it to menstrual synchrony, but I'm sure that's what did it. Suzanna, 30: Although I have lived away from home (Australia) on and off since I was 16, whenever I do spend any length of time - about two months - at home my mother and I have menstruated at the same time.

Emma, 35: A few weeks after my flatmate moved in we noticed that we were both in a PMT-induced bad temper at the same time, and both having our period at the same time. I had influenced her to fit in with me. It was quite a good thing: it was convenient to have the house like a war zone for just a few days each month, rather than both of us simmering away with PMT for twice as long. Also, we could both unite and shout twice as loud at our other [male] flatmate, if he happened to do anything sinful in the days when we were both pre-menstrual (eg fail to dry up immediately, etc).

Alex, 29: I remember trying to calm myself down one evening by having a relaxing bath and planning what to do with an unexpected baby because my period was so late (I'm normally regular). My body felt as if it was going to pop, I was suffering so badly from water retention. Then the next morning, sure enough, I came on. The next moment my flat-mate runs in saying she had just come on and it was far too early. Suddenly it all made sense. Cathy, 30: I was told about this phenomenon when I was 18 and joined the army. I thought 'what's she on about?' I didn't take any notice, but sure enough, after about five months of living with these other women I was on parade one day, two weeks away from my period, and suddenly I came on.

Have you tried new the Independent Digital Edition apps?
Life and Style
ebookNow available in paperback
ebookPart of The Independent’s new eBook series The Great Composers
Arts and Entertainment
Books should be for everyone, says Els, 8. Publisher Scholastic now agrees
booksAn eight-year-old saw a pirate book was ‘for boys’ and took on the publishers
Life and Style
Mary Beard received abuse after speaking positively on 'Question Time' about immigrant workers: 'When people say ridiculous, untrue and hurtful things, then I think you should call them out'
Life and Style
Most mail-order brides are thought to come from Thailand, the Philippines and Romania
Life and Style
Margaret Thatcher, with her director of publicity Sir Gordon Reece, who helped her and the Tory Party to victory in 1979
voicesThe subject is being celebrated by the V&A museum, triggering some happy memories for former PR man DJ Taylor
  • Get to the point
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating

By clicking 'Search' you
are agreeing to our
Terms of Use.

ES Rentals

    iJobs Job Widget
    iJobs General

    Recruitment Genius: Installation and Service / Security Engineer

    £22000 - £40000 per annum: Recruitment Genius: This company is part of a Group...

    Recruitment Genius: Service Charge Accounts Assistant

    £16000 - £18000 per annum: Recruitment Genius: Are you a a young, dynamic pers...

    Cancer Research UK: Corporate Partnerships Volunteer Events Coordinator – London

    Voluntary: Cancer Research UK: We’re looking for someone to support our award ...

    Ashdown Group: Head of IT - Hertfordshire - £90,000

    £70000 - £90000 per annum + bonus + car allowance + benefits: Ashdown Group: H...

    Day In a Page

    General Election 2015: Chuka Umunna on the benefits of immigration, humility – and his leader Ed Miliband

    Chuka Umunna: A virus of racism runs through Ukip

    The shadow business secretary on the benefits of immigration, humility – and his leader Ed Miliband
    Yemen crisis: This exotic war will soon become Europe's problem

    Yemen's exotic war will soon affect Europe

    Terrorism and boatloads of desperate migrants will be the outcome of the Saudi air campaign, says Patrick Cockburn
    Marginal Streets project aims to document voters in the run-up to the General Election

    Marginal Streets project documents voters

    Independent photographers Joseph Fox and Orlando Gili are uploading two portraits of constituents to their website for each day of the campaign
    Game of Thrones: Visit the real-life kingdom of Westeros to see where violent history ends and telly tourism begins

    The real-life kingdom of Westeros

    Is there something a little uncomfortable about Game of Thrones shooting in Northern Ireland?
    How to survive a social-media mauling, by the tough women of Twitter

    How to survive a Twitter mauling

    Mary Beard, Caroline Criado-Perez, Louise Mensch, Bunny La Roche and Courtney Barrasford reveal how to trounce the trolls
    Gallipoli centenary: At dawn, the young remember the young who perished in one of the First World War's bloodiest battles

    At dawn, the young remember the young

    A century ago, soldiers of the Empire – many no more than boys – spilt on to Gallipoli’s beaches. On this 100th Anzac Day, there are personal, poetic tributes to their sacrifice
    Dissent is slowly building against the billions spent on presidential campaigns – even among politicians themselves

    Follow the money as never before

    Dissent is slowly building against the billions spent on presidential campaigns – even among politicians themselves, reports Rupert Cornwell
    Samuel West interview: The actor and director on austerity, unionisation, and not mentioning his famous parents

    Samuel West interview

    The actor and director on austerity, unionisation, and not mentioning his famous parents
    General Election 2015: Imagine if the leading political parties were fashion labels

    Imagine if the leading political parties were fashion labels

    Fashion editor, Alexander Fury, on what the leaders' appearances tell us about them
    Phumzile Mlambo-Ngcuka: Home can be the unsafest place for women

    Phumzile Mlambo-Ngcuka: Home can be the unsafest place for women

    The architect of the HeForShe movement and head of UN Women on the world's failure to combat domestic violence
    Public relations as 'art'? Surely not

    Confessions of a former PR man

    The 'art' of public relations is being celebrated by the V&A museum, triggering some happy memories for DJ Taylor
    Bill Granger recipes: Our chef succumbs to his sugar cravings with super-luxurious sweet treats

    Bill Granger's luxurious sweet treats

    Our chef loves to stop for 30 minutes to catch up on the day's gossip, while nibbling on something sweet
    London Marathon 2015: Paula Radcliffe and the mother of all goodbyes

    The mother of all goodbyes

    Paula Radcliffe's farewell to the London Marathon will be a family affair
    Everton vs Manchester United: Steven Naismith demands 'better' if Toffees are to upset the odds against United

    Steven Naismith: 'We know we must do better'

    The Everton forward explains the reasons behind club's decline this season
    Arsenal vs Chelsea: Praise to Arsene Wenger for having the courage of his convictions

    Michael Calvin's Last Word

    Praise to Wenger for having the courage of his convictions