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I found a Wikipedia article on Menstrual synchrony, which claims:

Menstrual synchrony, also known as the McClintock Effect, or the Wellesley Effect [1] is a phenomenon reported in 1971 wherein the menstrual cycles of women who lived together (such as in homes, prisons, convents, bordellos, dormitories, or barracks) reportedly became synchronized over time.

Is the claim verified?

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If I remember correctly, they all synchronize to the socially dominant one. –  Borror0 Oct 27 '12 at 3:13
Further along in the article, you'll find a section labeled criticism, which gives a pretty good account of some the scientific evidence against it. –  user9017 Oct 27 '12 at 23:14
Because personal experience doesn't have an online link, I won't put this as an answer. When I was over at my cousin's home (female), she and my sister (after my sister moved in), became synchronized. There were only three times they were out of sync, and that was from an unfinished pregnancy, a nasty breakup, and the start of birth control meds. –  Souta Oct 30 '12 at 0:14
@Soute birth control would invalidate the data as that has it's own influence on the cycle –  ratchet freak Oct 30 '12 at 10:04
@ratchetfreak That is why I left it as the last thing listed. I should have further iterated the fact that that was the final reason for why they were out of sync. I should have also stated that the events listed are in order and therefore, ratchet freak, your comment is inapplicable because the birth control was the last time they were out of sync. (I'm guessing it would be fair to also indulge and say that they are no longer living together and birth control meds aside, definitely not in sync.) –  Souta Oct 30 '12 at 20:05
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1 Answer

up vote 8 down vote accepted

No, there doesn't appear to be very good scientific support for this popular idea.

This 2011 article at ScienceBasedMedicine appears to cover all the important bases (the article is well-referenced, I'll reproduce a few key links here):

Synchrony Is difficult to define

Consider that the normal menstrual cycle can vary from 21 to 35 days and can last 2 to 7 days. Consider that some women are regular and consistent, while others have variable patterns, even “regularly irregular” patterns. Consider that anovulatory cycles and other conditions often lead to menstrual irregularities that fall outside the normal range. Consider that strenuous exercise and other life events can affect menstruation. Put all that together, and you can see that often cycles will overlap simply by chance, and that it is difficult to define synchrony.

Research is mixed at best

A Scientific American article did a good job of reviewing the literature as of 2007. Suffice it to say that about half the published papers support the synchronization hypothesis and half don’t; and the half that do have been harshly criticized for their poor design and poor statistical analyses. So we haven’t reached a consensus, but it’s looking more likely that synchronization is a myth.

Comparative studies on animals have been negative

It was originally thought that other primates and other mammals demonstrated menstrual synchrony due to pheromones, but recent studies have shown that it doesn’t occur in chimpanzees, hamsters, mandrills, or golden tamarins.

The proposed mechanism (human pheromones) has little to no scientific support

In humans, pheromones have been postulated and even sold as sexual attractants. But there is little or no peer-reviewed evidence to suggest that any pheromone influences human behavior. No human pheromones have been identified, and the vomeronasal organ that detects pheromones in other mammals is rudimentary and nonfunctional in humans.

The first comment on the article, by "Al Morrison", summarizes it nicely:

No agreed upon definition of menstrual synchronization + no plausible mechanism + poor research supporting the hypothesis + split research results + no support from comparative research + a tendency to over report synchronization = MYTH

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In this episode of Skeptics Guide to the Universe, Steven Novella says, "probably the best case for pheromones is the synchronization of female menstrual cycles through some sort of a remote hormonal effect." –  Articuno Oct 30 '13 at 23:01
Ha, that's ironic, @Articuno (since it is Steven's site that debunks the idea 2 years after that podcast). But based on the above evidence, it turns out he may be right, just in the opposite way he intended (if the case for cycle synchronization is weak, then there goes the strongest case for human pheromones). –  BradC Nov 1 '13 at 13:42
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