Debunked: Study shows link between menstrual cycle and the moon

[Rory]

An old and persistent myth is that the lunar cycle and the human menstrual cycle are (or should be) connected and synchronised. While many studies have shown this idea to be erroneous, others have appeared to support it. One of the most commonly cited is the study produced by Winnifred B. Cutler, PhD in 1980.

In the autumn of 1977, Cutler selected 312 college students (aged 19 to 22) who recorded their menstrual cycles over a 14-week period. Of these 312, 68 (21.8%) were reported as showing "a mean cycle length of 29.5 days, plus or minus 1 day", and these were the subjects Cutler focused on. She noted that "47 (69.1%) menstruated in the light half of the month" and 21 (30.9%) in the dark half. This was seen as statistically significant (z=3.16, p<0.001).

A pilot study carried out the previous year yielded similar results: 29 (22.8%) of 127 subjects showed a mean cycle of 29.5 days, ± 1 day, and of these 29, 22 (75.9%) menstruated in the light half of the month.

Cutler concluded that:

"Menstrual cycles similar in length to the lunar cycle in these selected populations [...] tended to occur during the light half-cycle of the lunar period. Thus, ovulation is occurring in the new-moon part of the cycle and is coincident with the greatest gravitational pull on earth."

She also stated that:

"Since the average lunar cycle is 29.5 days and the average menstrual cycle is also 29.5 days, the many conjectures of a lunar menstrual relationship [...] did make sense."

Source: https://www.athenainstitute.com/sciencelinks/lunarandmenst.html

Now, the first thing to note is that Cutler's results neither claimed nor showed any evidence of an explicit synchronisation with a particular day of the lunar cycle, but rather a tendency among her subjects to menstruate during the 16-day period between the first and third quarters. Results were presented as follows:

Really, the study falls apart for three main reasons:

1. Her sample size is tiny. Even allowing for the extremely selective use of her subjects' data, 68 participants is nowhere near enough to draw a conclusion

2. Data is cherry picked, the analysis is poorly executed, and it's apparently biased towards a predetermined outcome. There are quite a few examples of this, which I'll detail in the post below.

3. Most conclusively, larger scale analyses have shown no tendency towards any hint of a pattern. For example, in 2016 tracker app Clue analysed 7.5 million menstrual cycles submitted by 1.5 million users and concluded there was:

"no correlation between the lunar phases and the menstrual cycle or period start date. Period start dates fall randomly throughout the month, regardless of the lunar phase".

Source: https://helloclue.com/articles/cycle-a-z/myth-moon-phases-menstruation

Interestingly, they also note that "the global average menstrual cycle length is 29 days", as opposed to the commonly held belief of 28 days, and that "Statistically speaking, with a random distribution of period start dates, about 1 in 2 women will have their period start ±3 days from either the full or new moon" - which may go some way to explaining why some still feel there is validity in the notion of a connection between the menstrual cycle and the moon.

 

[Rory]

As mentioned above, I wanted to look more deeply at Cutler's study and put forward further reasons why this widely-cited source appears to be replete with 'bad science'. Here are some of the problems I see with the methodology:

• The 78.2% of subjects whose menstrual cycles didn't show a mean cycle length of 29.5 days, plus or minus 1 day, were discarded from the data which informs the conclusion. Cutler writes that "data contamination with cyclers of other than approximately 29.5 days would hide any demonstration of a high incidence of menses onsets in one phase of the lunar cycle" - which seems an extraordinary statement of bias and evidence suppression. In a nutshell, it's akin to saying "menstrual cycles are the same length as the lunar cycle - if we ignore all the ones (78%) that aren't"
• When these subjects' data was charted - or rather, "a random sample of 248 [of the 312]" - Cutler noted that "[onset of menses] appeared to array themselves more evenly about the clock" - i.e., there was no evidence of a pattern or correlation
• The study length of 14 weeks allows for the recording of only three or four cycles (of the select group of those whose cycles averaged more or less 29.5 days), which is insufficient to draw conclusions
• This select group were not described as showing "regular 29.5 day cycles", but rather "average cycles of 28.5-30.5 days". Some subjects, therefore, could have recorded up to four cycles which fell outside this range, but whose average was inside
• I'm sceptical about the "29.5 days plus or minus one day". Does this mean she only counted those whose cycles started less than 24 hours either side of 29.5 days? Or would she have counted a subject, for example, who began a period at 7am on October 1st, and then again at 8pm on October 31st - i.e., 30 days later, but not between 28.5 and 30.5 days later?
• Some of the dates in the chart are a little bit skewed. Some segments contain only two dates, while others have four. December 5th and 6th are in the wrong order. And the new moon in October fell on the 12th, not the 13th
• While the 47/21 split between light and dark halves of the month is analysed statistically with regard to the null hypothesis, it seems too small a sample size to use as proof of correlation; and in a later study, of 97 subjects - still not enough - Cutler found the split as being 55/42
• The figures for the 1976 pilot study are related quite differently in different papers. For example, in her 1987 paper, which replicated the original study, Cutler states that, of the 127 subjects, "40 were lunar period cyclers, and 27 of these 40 menstruated in the light half of the month", while the 1980 paper - as quote above - puts these figures at 29 (lunar period cyclers) and 22 (who menstruate in the light half of the month)

Other relevant points of interest:

• While an average period length is around 29 days, normal length is cited as 21 to 38 days, with variations of up to 9 days. Also, cycle length changes with age, shortening towards middle age, and then lengthening as menopause approaches. This would appear to indicate that only a tiny percentage of women have regular cycles of 29-30 days
• There is no known mechanism that could explain any possible lunar effect. The strength of the moon's gravity on an individual has been described as being roughly equivalent to that of a bag of sugar placed in close proximity (I've often read mosquito, though that is apparently debunked), while comparisons with the ocean's tides and our bodies' water content (~60%) are based on a misunderstanding (Cutler proposes a natural rhythm of electromagnetic radiation which affects only the most fertile)
• Anecdotal evidence may provide support for the idea, but this can be explained by probability. There are bound to be some who occupy the intersection of: regular cycles; cyles of 29-30 days; and correlation of onset, cessation, or ovulation with either the full or new moon. Even if it's only 1 in 1,000, that still means there are around 3.7 million women in the world who sync perfectly with the moon
• Some propose that our modern way of life has thrown things out of whack - but after extensive studies of the Dogon in Mali, who were living in a traditional culture without electric lighting, Beverly Strassmann concluded that there was "no evidence for either lunar or menstrual synchrony" (that is, synchronising of individuals' cycles due to proximity)
• Humans are not the only mammals who menstruate, with varying lengths of average cycle found (as well as significant variables among individuals). Some of these species include: elephant shrew (~13 days); short-tailed fruit bats (~24 days); rhesus monkeys (~28 days); orangutans (~29 days); gorillas (~30 days); Leschenault's rousette (~33 days); and chimpanzees (~36 days). Proponents of the lunar synchrony idea, therefore, would also need to explain why humans, orangutans, and gorillas were particularly favoured
• Even a regular-as-clockwork cycle of 29 days would never actually be in sync with the moon

I suppose some may think this lack of a lunar connection takes something away from life; but I would argue it adds to it. Through personal interaction and reading online, I've seen that some experience a pressure to align their bodies with the moon, and a sense of disappointment that there may be something wrong with them if this isn't the case; as well as the feeling that there's something they can do to change this - which I imagine will usually be both fruitless and only add to a sense of disappointment. Knowing, however, that there is no "biological or cosmological standard" and that "normal" encompasses a huge range of cycle lengths and variations could, I feel, be quite liberating.

I do understand, though, why some may want to stick with the belief, and I think the penultimate paragraph of this well-balanced article by English Taylor sums it up quite nicely:

Given that the most recent evidence with a very large sample size points to no correlation between the menstrual and lunar cycle, it's unlikely there's a connection. However, if you're on a White Moon or Red Moon Cycle, that's cool - especially if you derive personal meaning from this experience. But if you're not on one of these cycles, there's no need to worry. "Your menstrual cycle might be regular - about the same length every month - or somewhat irregular," write experts at the Mayo Clinic. "Within a broad range, 'normal' is what's normal for you."

 

[Rory]

And, finally, here is the table of data included in Cutler's 1987 paper, which brings together studies from four separate years:

As we can see, the numbers involved are not only tiny, but often zero - so it is little wonder that statistical anomalies were found.

 

[qed]

Is there really no link?

The Regulation of Menstrual Cycle and its Relationship to the Moon

A synchronous relationship between the menstrual cycle and lunar rhythm was confirmed by

1. Investigative data: Among the 826 female volunteers with a normal menstrual cycle, aged between 16 and 25 years, a large proportion of menstruations occurred around the new moon (28.3%), while at other times during the lunar month the proportion of menstruations occurring ranged between 8.5–12.6%; the difference was significant (p<0.01).

2. Laboratory findings: The 6-hydroxymelatonin levels in the urina sanguinis of 3 female volunteers reached their zenith prior to and during menstruation, gradually declining to their nadir during ovulation. The difference in 6-hydroxymelatonin between menstruation and ovulation was significant (p<0.01). Two of these three volunteers had their zenith in the period of the new moon and nadir 3—4 days prior to the full moon respectively.

3. Clinical experience: The lunar-menses-regulatory therapy in treatment of Nephropenic secondary amenorrhea revealed 4 clinical cure, 5 marked effect, 8 menogogue and 3 inef-fect out of 20 cases.

Acta Obstetricia et Gynecologica Scandinavica

 

[Rory]

Have you looked at the raw data for that study? What does "around the new moon" mean? Is there any evidence of bias?

Even just going by the abstract, what it shows is: 1. the exact opposite of what Cutler believed she had found; and 2. a disagreement with a sample size of 1.5 million.

 

[deirdre]

Have you looked at the raw data for that study?

i just did. link here

self reported Chinese study published 1986 in a scandinavian journal with an impact factor of only 2.4 (New England Journal of Medicine is impact factor 79). It doesnt seem they recorded data for consecutive months. Again sounds self reported girls and yound women aged 16-25. No breakdown of ages. Methodology information lacking.

The "cure" is silly. they dose the women with drugs at certain moon times, but there is no control group or testing of a group (group is n=20) giving drugs at different times. So that data is worse than useless as far as lunar connection.

 

[deirdre]

And while googling that i ran across a related 'myth' that more babies are born during the full moon period. I think this is closely related enough to add to this thread.

There is, however, a limited body of more empirical evidence for the existence of the phenomenon. A 1959 study broke the month into consecutive three-day periods and found that the three days of a "full moon window" -- the day before, day of and day after a full moon -- had more births than any other single three-day period [source: Shulman]. Another study, this one published in 1966, studied birth rates by moon phase -- full, half, one-quarter and three-quarter. The authors found that within the study period, more births centered around the full-moon phase than any other [source: Shulman].

But as you get deeper into the scientific study of the connection between birth and the full moon, it quickly becomes clear that these studies supporting the lunar effect are an anomaly. The vast majority of evidence reveals the lunar effect to be myth, not scientific reality.

https://health.howstuffworks.com/pr...regnancy/labor-delivery/births-full-moon1.htm

page 3 of this article linked then gives more studies (looking at large populations)that do not show a link.

 

[Rory]

Also, as noted in the OP, the Clue analysis states that:

"Statistically speaking, with a random distribution of period start dates, about 1 in 2 women will have their period start ±3 days from either the full or new moon."

https://helloclue.com/articles/cycle-a-z/myth-moon-phases-menstruation

If statistics predict 50% of women will menstruate ±3 days around either the full or new moon, I suppose we can logically deduce that 25% would be predicted to menstruate around either - which is not so different from the 28.3% the SP Law paper declares.

That said, this apparent correlation is most likely a coincidence - Law has only 11.5% occurring around the full moon - and the paper really falls foul to the same statistical shortcomings that Cutler's does.

I think sample size and methodology are the key issues here.

 

[Rory]

Both Cutler's four studies (when combined) and Law's study use over 800 volunteers to gather data. That might seem like a lot but, as an example of why a sample size of even several hundred is an insufficient number on which to base a conclusion, I just looked at which month my facebook friends' birthdays fall in.

351 share their birthdate information, resulting in an average of 29 birthdays per month. In July, however, there were only 19, while in October there were 38 - quite large statistical variations.

That doesn't mean that there's something special about those months, though - they don't in fact correlate with the actual least and most popular months to be born (February and September) - just that, if I want a more accurate picture, I'm going to need a much larger sample size. Something like this, detailing over 13,000,000 birthdays:

Source: https://www.ons.gov.uk/peoplepopula.../articles/howpopularisyourbirthday/2015-12-18

 

[Rory]

It's interesting that a PhD such as Cutler, who does apply statistical analysis to her data, could get it so wrong.

In her 1987 paper she states:

A random distribution [of cycle dates] would yield a flat density since it would be equally likely for women to begin to menstruate on any day of the lunar cycle. A departure from a straight line thus indicates non-randomness.

And in the 1980 paper:

Under the assumption of the null hypothesis, in this binomial situation, one would expect one half of the entries (one half of 68) to fall in each half of the lunar cycle.

In a nutshell, she imagines that, were menstruation random, every day of the month should return the same total, and that even with a sample size of only 68, a balanced spread should be observed.

Actually, the opposite is true: for example, the chances of randomly selecting 31 people and finding they were all born on different days of the month is fantastically small. Likewise, given a similar sized binomial situation I wouldn't ever expect a fifty-fifty outcome - the odds of tossing a coin 68 times and it not landing 34 times on each side are a whopping 93.2%.

As the Clue data shows, "a departure from a straight line" is exactly what we ought to expect from the randomness of the natural world.

 

[Whitebeard]

That doesn't mean that there's something special about those months, though

Apart from the fact folks get horny at Christmas.

 

[Rory]

Apart from the fact folks get horny at Christmas.

Naw, October babies are conceived in January/February. Christmas holidays are the reason most babies (in Christmas-celebrating countries) are born in September - even in Australia.

 

[Leifer]

However, menstruation cycles in other mammals are longer or shorter.
Is there something special about us humans ?

https://en.wikipedia.org/wiki/Menstruation_(mammal)#Animal_estrous_cycles

Is there a tendency or bias among people that wants to link us, to the Moon or the heavens ? .....and helps us "search for similarities" in the data ?
Astrology and religious or spiritual thinking, demonstrates there has always been a yearning for a connection to the heavens (sky, moon, sun, stars).

 

[Whitebeard]

Naw, October babies are conceived in January/February. Christmas holidays are the reason most babies (in Christmas-celebrating countries) are born in September - even in Australia.

so cold winter nights in the north and sultry summer passion down under.

 

[Rory]

So cold winter nights in the north and sultry summer passion down under.

It's more likely the same reasons in the Southern Hemisphere as it is in the Northern - Australia's three most popular birth months are September, March, and February. So Christmas holidays followed by winter snuggles. In the north we get the double whammy of Christmas and winter combined.

Source: http://stat.data.abs.gov.au/Index.aspx?DataSetCode=BIRTHS_MONTH_OCCURRENCE

(If anyone checks the data, you'll have to weight it for number of days per month)

 

[Rory]

However, menstruation cycles in other mammals are longer or shorter.

https://en.wikipedia.org/wiki/Menstruation_(mammal)#Animal_estrous_cycles

Yep. As mentioned above, with reference to mammals that menstruate in similar ways to humans. This list, however, is of mammals' estrous cycles, which is something different.

Is there a tendency or bias among people that [...] helps us "search for similarities" in the data?

Which "similarities in the data" do you mean? Apart from when sample sizes are too small, there don't seem to be any similarities, patterns, or significant anomalies, as far as this proposed connection is concerned.

There does definitely seem to be a tendency among some people to want to believe there's a connection between the moon and certain aspects of human life, which I suppose is fueled by a mixture of confirmation bias, pattern seeking, anecdotal evidence, and desire for meaning.

This is a good article about lunar misconceptions, and how past studies may have provided misleading evidence: https://www.csicop.org/si/show/the_moon_was_full_and_nothing_happened

Skepdic also has a decent more recent article (though it does contain a few errors): http://skepdic.com/fullmoon.html

 

[Whitebeard]

So Christmas holidays followed by winter snuggles.

Well my folks always said I was the product of the winter of 1963.

 

[Rory]

I ran across a related 'myth' that more babies are born during the full moon period. I think this is closely related enough to add to this thread. Page 3 of this article gives more studies (looking at large populations) that do not show a link.

Checking in on that, I found that by far the largest study done did claim to show a link - a 1998 paper produced by an astrophysicist team at Appalachian State University, looking at data for over forty million live births, and published in I.A.P.P.P. Communications (NASA/Harvard affiliated?).

Rather than a spike around the full moon, though, the paper states that peak birthrate was found to be around 21 days (third quarter) and minimum near day 7 (first quarter).

It seemed that the writer(s) were somewhat surprised by these findings; as was I. But given the establishment, the lack of obvious bias, and the sample size, it seemed worth looking into.

Anyway, I found daily birth statistics for the US from 1994 to 2014 and just finished cross-checking them with the lunar cycles for those years - 89.5 million births over a total of more than 250 lunar cycles: most likely the largest analysis on the subject to date.

Result: it's basically a flat-line graph.

 

[deirdre]

I found that by far the largest study done did claim to show a link

the paper states that peak birthrate was found to be around 21 days (third quarter)

without looking at the paper now.. so the link wasnt for births at the full moon. the link was for the third quarter?

 

[Rory]

That's what they put forward. But there's no correlation from the data I've checked. Not sure where they went wrong.

 

[deirdre]

Not sure where they went wrong.

what makes you think 'they' went wrong?

anyway, they normalized for Tuesdays. and they didnt incorporate yet data before 0.5 and afer 29.5

 

[deirdre]

I was born just at the the third quarter. But knowing my mom i was supposed to born on the New Moon. (maybe the Full Moon, even. Depends on what the doctor would allow. That might explain while i'm so small. hhmmm. )

 

[Steve S]

Both Cutler's four studies (when combined) and Law's study use over 800 volunteers to gather data. That might seem like a lot but, as an example of why a sample size of even several hundred is an insufficient number on which to base a conclusion, I just looked at which month my facebook friends' birthdays fall in.

351 share their birthdate information, resulting in an average of 29 birthdays per month. In July, however, there were only 19, while in October there were 38 - quite large statistical variations.

That doesn't mean that there's something special about those months, though - they don't in fact correlate with the actual least and most popular months to be born (February and September) - just that, if I want a more accurate picture, I'm going to need a much larger sample size. Something like this, detailing over 13,000,000 birthdays:

View attachment 35170
Source: https://www.ons.gov.uk/peoplepopula.../articles/howpopularisyourbirthday/2015-12-18

Looks like pregnant women are crossing their legs and holding on during xmas and new year. Those look to be significant drops in birth rates!

 

[Rory]

Looks like pregnant women are crossing their legs and holding on during xmas and new year. Those look to be significant drops in birth rates!

I've got a few jokes I could make here, but I'll just be sensible instead.

Less hospital staff working, and less c-sections scheduled (in UK and US that accounts for ~25 and ~35% of births respectively) - same reason why weekends are always lower.

I made a chart of US births similar to the one above and some of the troughs were quite striking:

Source for data: https://github.com/fivethirtyeight/data/tree/master/births

In order from left to right: New Year's Day; April 1st; Memorial Day; July 4th; Labor Day; Halloween; Thanksgiving; Christmas; New Year's Eve.

Surprisingly - as in the UK - the 13th of each month is markedly lower too.

Not sure whether this is superstition on the part of the parents or the staff.

Valentine's Day was another surprise - big peak compared to the surrounding days and weeks. Though part of this may be those skipping the 13th.

 

[Rory]

What makes you think 'they' went wrong?

I guess mostly because their results don't agree with reality, and then partly because their results didn't agree with other large scale studies, including bigger ones. Also it wasn't really presented in a very convincing way, and they expressed doubts themselves about whether they'd calculated the data right.

Now I've found that one of the authors had redone the study a few years later, with improved methodology, and concluded that there was no correlation after all.

Not before I'd completed my own ginormous analysis and write up though!

Posted here: https://www.metabunk.org/posts/226953/

Surprisingly - as in the UK - the 13th of each month is markedly lower too.

Bonus material - births by day of the month in the US and UK (scaled for visual aid):

Is it common knowledge that less people are born on the 13th of each month? I'd never known that till I made these graphs. It's not just Fridays either.

(Other dates are down because of holidays.)

Well my folks always said I was the product of the winter of 1963.

And a product they were proud of, I'm sure.

 

[Jez]

Hi, fascinating discussion. I signed up here just to chime in.

This is timely, as I was recently told that the slaughter of pigs to coincide with the phases of the moon are important for optimal flavour. This was by a female chemist, who assured me that she could feel the influence of the moon on her body (I assumed to do with water retention, but didn't want to pry for fear of being creepy). I couldn't argue with her personal experience, as I had neither my own personal experience nor scientific data. Now I have the latter.

I have a few points to contribute, that don't seem to come up in all the usual places where this is discussed. If anyone can point to authoritative references to support (or refute) these, I'd be grateful.

My first point is that "full moon" doesn't mean "maximal gravitational influence of the moon". The amount of light falling on the moon doesn't change how much pull the moon exerts. So during a lunar eclipse, the tides don't suddenly drop.

The second point is the misunderstanding that gravity somehow acts on water only, or somewhat more than other (dry) matter. Gravity acts equally on all matter of equal mass, regardless of the molecules that make up the matter.

The third point is a corollary. At the location of high tide, where the moon is directly above the peak of the tide (yes this is not quite true, but I'm simplifying for the storyline) the moon would exert equal pull on the water as it would on the ground directly beneath that water. Unless the body of water is relatively deep in relation to the distance to the moon, the differential will be practically zero. Therefore, any pull on the water would produce an equal pull on the ground below, and any caused motion of the water would be matched by the ground below.

Another way to put this is that the earth is falling down the gravity well (towards the earth) just as much and just as quickly as the water above it. So a direct gravity influence on the world nearest the moon is not what causes the lunar tides.

This effect, where equal pull on the water and the ground beneath, is negligible because the depth of the water as measured towards the moon's gravity well (or more correctly, the gravity well that is mostly influenced by the mass of the moon) is very small compared with the size of the Earth and the distance to the moon. Comparing the gravity differential between a molecule of water at the surface of the water and one at the bottom of the water would yield a non-zero result due to one being further down the gravity well, but the depth of even the deepest ocean is extremely small compared with the size of the earth and the distance to the moon.

However, this is not the case at points 90% from high tide. At such points, a water depth measurement would be approximately at right angles to a line directed towards the gravity well. If we measure the water at one such points being the distance heading down the gravity well (or approximately, which passes through the moon), we aren't measuring the depth of the ocean, but some distance ACROSS the ocean. This would be a much larger distance, by possibly three or four orders of magnitude.

And therefore, my fourth and final point is to assert that the effect of gravity on the tides is due to a sideways force on the water, greatest at points half way between the two tides, that are minuscule, but add up to displacement that exceeds that of the earth (remembering that the earth is pulled towards the moon also). All of those water molecules are giving tiny sideways "pushes" or "lean" towards the moon, and the accumulation of these trillions of trillions of leans in the same direction all add up to a measurable motion that is greater than that of the earth. On the non-moon side of the earth, the leanings towards the moon are less than the motion of the earth, causing the opposite bulge.

I searched for citations supporting this model, and came up with this article: https://www.sciencealert.com/the-moon-s-gravity-does-not-fully-explain-how-ocean-tides-work

 

[Rory]

Nice post, Jez, and welcome to the forum.

My first point is that "full moon" doesn't mean "maximal gravitational influence of the moon". The amount of light falling on the moon doesn't change how much pull the moon exerts. So during a lunar eclipse, the tides don't suddenly drop.

That's true. The size of the moon's gravitational pull on earth will be as a result of distance, which changes by around 17,000 miles either side of an average of 238,000 miles. Relative position and distance to the sun are also a factor.

The tidal misunderstanding is a good one, and the reason why a lot of people think the human body could be affected by the position of the moon.

Your post makes me think that, for believers of a 'lunar effect', perigee and apogee of the moon would be a more suitable place to search for a correlation than lunar cycle. Though, as the data from Clue and the birth rate studies show, there's no evidence for a cyclical pattern of any kind.

 

[Steve S]

I've got a few jokes I could make here, but I'll just be sensible instead.

Less hospital staff working, and less c-sections scheduled (in UK and US that accounts for ~25 and ~35% of births respectively) - same reason why weekends are always lower.

I made a chart of US births similar to the one above and some of the troughs were quite striking:View attachment 35182
Source for data: https://github.com/fivethirtyeight/data/tree/master/births

In order from left to right: New Year's Day; April 1st; Memorial Day; July 4th; Labor Day; Halloween; Thanksgiving; Christmas; New Year's Eve.

Surprisingly - as in the UK - the 13th of each month is markedly lower too.

Not sure whether this is superstition on the part of the parents or the staff.

Valentine's Day was another surprise - big peak compared to the surrounding days and weeks. Though part of this may be those skipping the 13th.

Yes C-sections plus scheduled inductions must make up to quite a proportion of births. Our second daughter was induced, do the date was planned. Our first daughter though came in the early hours of the morning (on my birthday!) and there was no choice in the matter!

 

[Rory]

C-sections plus scheduled inductions must make up to quite a proportion of births.

Disturbingly high - over 45% in some countries (emergency and recommended rate is around 10-15%).

It's interesting that a PhD such as Cutler, who does apply statistical analysis to her data, could get it so wrong.

One of my biggest takeaways from researching this is to really question the statistics. A PhD puts out a paper, says "that's statistically significant", and throws in the old "p<0.001" and I might think, hm, no arguing with that. But the two papers above show very clearly how misleading this can be - even with a sample size of over 800.

Here's a good example - a random coin toss site:

http://www.shodor.org/interactivate/activities/Coin/

On my fourth click I got a result not far off Cutler's original study - 47 to 21 - which was given a P-value equating to "high statistical significance" - less than a one in a thousand chance of being wrong.

Wary. Very wary indeed.

 

[Leifer]

Well my folks always said I was the product of the winter of 1963.

Me too....I was born in the 9th month of 1963, so....conception was in late January, same year.

 

[Leifer]

For what it's worth, my close family friend is a retired primate researcher at Emory University.

(Ronald D. Nadler, PhD
Professor Emeritus
Yerkes National Primate Research Center)

I emailed him for his thoughts on the study, and he consulted a colleague.
This was Ronald's reply...

Hi Randy,
I didn't want to spend the time trying to evaluate critically Cutler's article. I knew of her, but didn't think much of her. I did send your message to a friend of mine who worked in the area and knows a lot about statistics. This is his opinion:

Ron,
Cutler is, IMHO, a non-scientist. Even today, she advertises *pheromones* in the back pages of the New York Review of Books. The menstrual synchrony - lunar menstrual cycle story is pretty unimpressive, in my opinion. When someone heavily filters the data, as Cutler does, and then comes up with a story that has not been corroborated even 4 decades later, then I think you can basically write the idea off.

I have to admit that I don't follow all of the twists and turns of the attached article's criticism (I read it twice), but I have always thought the Cutler research (and a lot of other research in the area) was a fad.

 

[Rory]

It seems that the idea of a human menstrual-lunar cycle connection isn't quite dead: a new study came out earlier this year looking at it from a slightly different angle. The paper is here:

https://advances.sciencemag.org/content/7/5/eabe1358

And it puts forward the idea that some women sometimes appear to synch with the full or new moon, based on patterns such as these:

Key: the vertical blue lines represent new moons, the yellow lines full moons, and the black dots when periods began (other symbols are explained at the link, but those seem to be the most important ones).

Basically, when black dots appear to create a vertical line pattern, that means the period is beginning on the same-ish day of each lunar cycle. So there is something intriguing there about how the black vertical lines are pretty much always aligned with the new or full moon, and often strikingly so.

I do plan to take an in-depth look at this - they also plot their data against different types of lunar cycle (gravimetric, for example) - but I haven't quite got around to it yet. A few initial notes though:

Drawbacks

1. The sample size was, again, incredibly small, with only 22 participants looked at. The reason for this is that the study examined the data of women who had recorded their period onset dates over the duration of many, many years, and women who did this were difficult to find.

2. The study cites Cutler several times, and as I think I covered pretty well in the first few posts, the Cutler papers represent some pretty awful (and biased) attempts at science.

3. There is no known mechanism to explain why such a thing would occur, and why it would occur in humans but not in other mammals.

Positives

I've written to Charlotte Helfrich-Förster, the first name on the paper, pointing her to this thread and mentioning some of my criticisms of the Cutler paper, and she was extremely receptive and gracious, and agreed with our findings on that. I also think that the paper itself is well-written and presented, very thorough, and recognises the limitations of the sample size.

Anyways, I suppose I'd better get around to giving it a proper look - but I thought I'd share the preliminaries first in case anybody else was interested.

 

[FatPhil]

And it puts forward the idea that some women sometimes appear to synch with the full or new moon

Menstrus is a very delicate thing perturbed, or driven by all kinds of different things. Controlling for all possibilities is a nightmare, so this will hopefully be a very interesting thread to follow.

I do have an example of how delicate inputs can be that can still change the outputs. Three females in shared (and fairly isolated) accomodation for a long time, in synch as expected, and one suddenly drops out of synch. The only change was that she had started to use a new (perfumed) laundry detergent. Whether they would re-synchronise with that new environment wasn't tested, she went back to her prior brand, and they eventually resyncronised again.

However, even that (involving the old "rhinocephalic cortex") is a massive change compared to the differences in the moon's gravitational attraction (tidal forces aren't even parts per million), I'd be really surprised if there was any detectable change based on gravity.

 

[deirdre]

why would 3 women in isolation living together not share laundry detergent? sounds fishy.

 

[FatPhil]

why would 3 women in isolation living together not share laundry detergent? sounds fishy.

Perhaps they did until one decided she wanted something different as she was tired of the monotony, who knows? People don't have to cooperate and collaborate on everything they co-experience, you know.

 

[deirdre]

People don't have to cooperate and collaborate on everything they co-experience, you know.

They do if they want their periods to synchronize. I'm just saying there was probably more going on then just a change in laundry detergent. Why did she change the laundry detergent? and what were the reactions to the smelly laundry detergent from the other roommates? We need more data.

 

[Rory]

Three females in shared (and fairly isolated) accomodation for a long time, in synch as expected

I'm curious about the phrase "in synch, as expected", as 'menstrual synchrony' has also been shown to lack real supporting evidence, despite what scattered anecdotes might suggest.

https://en.wikipedia.org/wiki/Menstrual_synchrony

 

[FatPhil]

They do if they want their periods to synchronize.

Where did that if clause come from? You're making up your own assumptions here.

 

[deirdre]

You're making up your own assumptions here.

when in Rome...

 

[NoParty]

I thought "They do if they want their periods to synchronize" was meant as playful.

My two cents: When I first (~1985?) heard the menstrual sync story, I thought "Oh, for crying out loud!"

But for awhile, it seemed like there was decent (study) evidence...so I was kind of won over.

Then I would watch bison in Yellowstone, virtually all give birth at the same time...which means

same conception time, which suggests some cycle sync

(or--back to the original topic--universal influence by something like the moon).

But now we also have plenty of studies that suggest little or no sync.

I don't see reason to believe the lunar theory, but the menstrual sync might have a weak effect...

nothing definitive...but just enough evidence to keep me from dismissing it entirely.