Menstrual cycle sync

Why do women who live together, eventually have their menstrual cycles synchronize? What evolutionary benefit would this serve?

Does anyone have any ideas, or heard any theories?


That way, the menfolk only have to tiptoe for one week a month!

try for answers.

Some random columnist’s take on this.

This is farfetched and of almost no value, but I’m throwing it out there anyway.

Back in the hunter gatherer days food was probably pretty scarce in winter months (or the dry season, or some other seasonal thing). This could lead to amenorrhea (Lack of menstruation). When food became plentiful, menstruation would resume. Synchronized menstruation would lead to a bunch of babies being born at the same time. The time of those births would probably be while there was still plenty of food around, which would give the babies a month or so to grow strong before the lean times rolled around again. Also, I’m going to assume that women died in chilbirth pretty frequently back then. Having babies born at around the same time would all but guarantee that there would be a lactating woman or two hanging out even if the mother of a baby died in childbirth.

I didn’t really think all of this through, but my basic theory is that women synching cycles is so babies would be born at the same time instead of randomly throughout the year. Also, I’m pretty sure that the theory Cecil mentioned in Cecil’s column (not Cecil’s theory, but the theory of some other schlub) is a load of crap. When you’re around two women, and one of them is menstruating it isn’t too hard to figure out which one. Ok, maybe not all the time, but sometimes. However, the theory presented, assumed that the guy in question was sleeping with both of these women. Most guys could figure it out every time if this was the case.

Here is a weird, vaguely related story.
When I was in high school, my biology teacher was an extremely eccentric PhD Biochemist who worked in the development and testing of the birth control pill (what the hell was he doing teaching h.s. biology, I never could figure out). Those pills were used in a cycle of something like 24 days of hormones, 4 days of placebo (just to make sure people got used to taking the pill every day). The placebo pills without hormones would allow the women to menstruate normally. Or something like that, I forget the details.
Anyway, here’s the interesting part. They went to 3rd-world countries like Haiti and Costa Rica to test the pill, apparently their human testing protocols are much more lax. They had terrible trouble getting people to remember which day of the cycle they were on (none of those fancy date-specific dispensers were used yet, they just had bottles of pills). They tried giving people sets of beads and other counting devices, because the vast majority of their trial subjects were poor illiterate people. Nothing worked, the women could not follow the prescription accurately, which ruined the trials. Finally someone hit on the idea of telling the women to switch pills on the full moon. He said that just after the full moon, you couldn’t find tampons or pads in any store in the country, they were sold out. Just imagine thousands of women menstruating at the same time, every full moon. They actually considered using this system when the pill went on general sale, but mercifully, they didn’t.