A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Do the menstrual cycles of women living together tend to synchronize?

May 9, 1986

Dear Cecil:

Are the workings of the female half of our species always to remain a mystery? One of my lady acquaintances recently relayed the following piece of physiological trivia, which I won't believe unless I get confirmation from a higher source. She said that when a group of females live together, their menstrual cycles begin to coincide. Even harder to believe, she tells me this is because of a scent women react to. Is this true? Is this phenomenon observed in women's prisons and schools? How come I never heard about this before? What else don't I know about women's bodies?

Cecil replies:

What you don't know about women's bodies is something you'd better take up with your girlfriend, Marko. But you heard right about "synchronous menstruation," as it's called.  This amazing phenomenon was first described in 1971 by researcher Martha McClintock, now with the University of Chicago. Having asked around a bit, I'd say it's common knowledge among women, but I'll bet not one male in 50 has ever heard of it. Women do have their little secrets.

Synchronous menstruation has been observed among mothers, sisters, and daughters who live together, and sometimes among women who simply work together. McClintock tells of seven female lifeguards who started out one summer with widely scattered periods. Three months later they were all menstruating within four days of one another.

A study of 135 residents of a women's college dorm confirmed the effect. Most of the cycle shifting occurred within the first four months and was usually complete after seven months. Fortunately for the dorm's plumbing, the whole building didn't synchronize, just roommates and close friends. As often as not, the women were unaware of what had happened.  Later research has suggested that synchrony is caused by some sort of scent cue, or pheromone.

Scientists at the Sonoma State Hospital Brain Behavior Research Center in California identified several women who were believed to be menstrual pacesetters--they made other women conform to their cycles. The scientists placed cotton pads under the dominant women's arms for a day, and then wiped the pads on the upper lips of five female subjects three times a week. (One wonders how much the subjects got paid for this.) Within five months, four of the recipients were menstruating at the same time as their donors.

Interestingly, men also have an effect on women's menstrual cycles — and not just because they make women pregnant. Women who associate with males frequently find that their periods become shorter and more regular. One woman told McClintock that she had a six-month cycle length until she began hanging out with guys, at which point her periods began occurring every 4.5 weeks. When she resumed her solitary ways, her cycle lengthened again. Another round of cotton pad experiments, this time using males as donors, confirmed this. Having sex with a man at least once a week will also do the trick.

Why synchrony occurs is pretty much a total mystery. The only published theory I've seen treats it as an evolutionary holdover from prehistoric times, when it was common for men to take multiple mates and efficient reproduction was essential to the survival of the species. The author of this theory assumes that women in their brief monthly phase of peak fertility give off some pheromonic signal that drives men wild. (The author, I'd be willing to bet, was either the owner of a dog or the parent of a teenage girl.)

If his wives are on different cycles, hubby has a good chance of picking the wrong one when his nose tells him it's time to go into action, thus wasting precious bodily fluids. But if the women are all menstrually synchronized, he CAN'T pick wrong — they're all fertile. In short, Marko — and you can probably relate to this — synchronous menstruation compensates for congenital male cluelessness about women's bodies.

One little problem with this theory. Surveys show sex occurs most frequently around the time of menstruation, when women are LEAST fertile. So either the theory is BS, or male cluelessness is more deep-seated than we thought.

Menstrual synchronicity in action

Dear Cecil:

Your informational bursts usually don't blow me away, but the piece you did on "synchronous menstruation" sure did. Quite by coincidence it meshed with a business conversation I had the prior week with a computer company middle management functionary. I manage technology transfer for a university in Illinois. One of our most successful recent inventions is an innovation in computer monitors known as plasma display. Their high resolution, soft orange color, and versatility are well suited for those who must stare at computers on a daily basis, i.e., engineers, designers, bank functionaries, etc.

A company (which shall remain nameless other than the designation "Big Blue") has been utilizing this invention in their product line for the past eight years or so. Not long after production geared up, Big Blue management noticed that the junk bins were filling up with rejected screens at an inordinately high rate at a certain time of the month.

Plasma displays involve the chemical bonding of electronic circuitry to the inside of the screen's glass face. Prior to the recent introduction of automated production equipment, the bonding process was done by hand by female assembly workers.

Yes, Cecil, you guessed it. When Big Blue's efficiency experts got done number-crunching, they found that the workers' menstrual periods were collectively peaking at around the same time the junk bins were running over. Why? It seems that the perspiration on the fingertips of the assemblers became more acidic at their collective "time," which quite effectively screwed up the chemical bonding process. When people tell me the world runs on sex, I now understand what they mean.

Cecil replies:

On the assumption that the Big Blue you're talking about is more commonly known by the initials I-B-M, I called up to check on your story, Bob. Nobody at IBM can recall anything about it, but they admit it might have happened. Makes a good yarn, in any case.

[For an update on this column, click here.]

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