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Hank: People that live together have their periods together. An average menstrual cycle lasts around 28 days: it starts with shedding built-up tissue in the womb, then prepping for pregnancy again. But one person might have a relatively long cycle, where someone else has a shorter cycle -- and they fluctuate month to month. So what’s the scientific basis for menstrual cycles syncing up... or is it just a myth?

The first publication about this phenomenon in humans was in 1971, in the scientific journal Nature. Psychologist Martha McClintock was supposedly inspired by what she noticed as an undergraduate at a women’s school: All of her close friends seemed to get their periods at the same time. So, she did a study, and surveyed 135 students in a dorm at her alma mater, Wellesley College. Three times during the academic year, she asked them for information: when their last two periods were, and who they were hanging out with.

Then, McClintock analyzed these data to see how menstrual cycles changed in close friend groups. In October, near the start of the school year, she found that groups of friends had cycles that were off by an average of 6.4 days.

By April, groups of friends had periods closer together -- only about 4. 6 days apart. So it seemed like their menstrual cycles were syncing up, and this phenomenon was dubbed the McClintock effect, though it’s more formally known as menstrual synchrony.

But the driving force was still a mystery. Some researchers, including McClintock, hypothesized that pheromones were involved. Pheromones are chemical substances that are released by an animal and detected by others in the same species, causing a behavioral or physiological reaction.

Some animals use them to communicate, and they’re common in lots of social insects, like ants or bees. A couple studies in the late 1970s and 80s, including experiments by McClintock, looked into menstruation and pheromones in some mammals, like, rats, hamsters, and two non-human primates. Most of these studies involved groups of around 2 to 5 animals.

Their menstrual cycles seemed to align if they were breathing the same air, so maybe airborne pheromones were involved. But researchers in the late 1990s and early 2000s tried to repeat some of the rat and hamster studies, and weren’t able to reproduce the results. Without consistent findings, we can’t really know if menstrual synchrony exists in mammals, or if it’s caused by pheromones.

Not to mention, those animals and humans aren’t the same. For one thing, we still don’t really even know if humans make pheromones. But some researchers have conducted human studies to try and replicate McClintock’s initial findings.

Throughout the 1990s, a team of psychologists surveyed close-knit groups of women, including college students in dorms, coworkers, mothers and daughters, and lesbian couples living together. Sometimes menstrual synchrony occurred, and sometimes it didn’t. Even as recent as 2006, scientists haven’t been able to show that the McClintock effect is a definite thing, or pinpoint potential causes.

Plus, several researchers have re-analyzed McClintock’s initial study, and found some problems with her methods. One researcher, for example, suggested that how the data were collected and analyzed could’ve exaggerated the lack of synchrony at the beginning of the study. Like, if someone had their period just before data collection started, their next cycle would be in 28-ish days.

So if someone had had their period right after data collection started, it would have seem'd like there was a whole month between those cycles, rather than just a couple of days. Another team redid the statistical analysis, and suggested that the shift from 6.4 to 4.6 days could’ve happened by chance. Any miscalculations or extra data points in a small study can make a huge difference when it comes to finding statistically significant data. It’s been years since the last published study on menstrual synchrony, and the inconclusive results seem to suggest that it’s just a coincidence.

So why do lots of people still swear that this thing exists? It probably comes down to our psychological perception. Since different people have different menstrual cycles, they might not exactly synchronize. But they can phase in and out of sync.

It’s like when you’re stopped at a red light, and you notice your turn signal starts blinking in sync with the car in front of you: If you wait for a little bit, you’ll phase out of sync. And if you wait even longer, then you’ll be blinking in unison again. So maybe with a relatively short study on menstrual cycles -- like seven or eight months in McClintock’s study -- it might seem like there’s synchrony.

Overall, it seems like the McClintock effect just doesn’t have enough conclusive research to back it up... But it does show us that a good science experiment isn’t just about proving or disproving something. It’s about checking your biases, designing well-controlled studies that are reproducible by other scientists, being careful with your statistics, and always asking new questions.

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