Why Women’s Menstrual Cycles are Linked to the Moon

Are women’s menstrual cycles affected by the moon?
A menstrual cycle does after all span the same amount of time as a lunar cycle. The word menstrual actually means monthly, and a month is based on a lunar cycle (so it should maybe be called a moonth).
Discussion about the subject often revolves around the fact that the moon creates the tides in the oceans, so logic suggests that it probably creates tides in people too, affecting their bodies in interesting ways – and affecting their brains too, such as turning some of them into ‘lunatics’ or ‘lunar-tics’ at the full moon.
This theory falls down due to the fact that the moon’s gravitational pull on objects on the earth is extremely small. It only affects the oceans to the extent that it does because there’s a lot of ocean to affect – so you notice the effect at the edge as the water goes up and down the beech. You don’t notice the tide in a puddle on the pavement after a shower of rain (and the puddle definitely doesn’t travel several metres as a single entity across the ground following the pull of the moon, covering the same distance as is covered by the tide on a beech). In fact the gravitational pull of a tree outside your house is much greater than the pull of the moon, so the pull of the moon is effectively swamped by everyday objects here on earth.
If the moon does have an influence on menstrual cycles I think that the reason is probably quite prosaic – it’s to do with the brightness of moonlight.
Why would the brightness of the moon affect menstrual cycles?
Because it affects what people in the days before the invention of artificial light could do at night.
The full moon lights up the night sky quite significantly (and conversely, the new moon results in ink black nights on which you can’t see your hand in front of your face). When there was a full moon people could move around at night; when there was no moon they couldn’t (or at least could only do so with much more difficulty). In our modern world we can easily forget this fact, but in the world before artificial light it had a huge impact on after-dark activity.
There are probably numerous scenarios that can by posited in which moonlight and the resulting nght-time activity of humans could play a part in influencing menstrual cycles – here’s one that I’ve devised (as I’m sure have others).

Primitive humans used to live in small family groups, often in competition over territory with neighbouring groups.
Within any particular group it would be common for males and females to mate. Unfortunately, because the member of the group were almost definitely related to each other one of the consequences of mating within the group would be in-breeding, with the resulting degeneration of the genetic stock. Ideally it’s best to mate with someone who isn’t a particularly close relative in order to ensure robustness in offspring due to genetic variation.
It would be preferable for a female in a group to mate with a male from a neighbouring group (even though the members of the two groups would probably all be related to each other to some degree, but not as closely as within each individual group). Unfortunately mating with a neighbour would probably be difficult due to the fact that the groups were in competition for land and resources to the extent that when the groups met the result would be conflict.
The only way in which a female from one group could mate with a male from a different group would be if they were to meet away from the gaze of other group members, and that would probably never happen while the members of the groups were wandering around during daylight hours when it was easy for them to keep an eye on each other.
Meetings between males and females from neighbouring groups were much more likely at night, when it was harder for group members to see what other group members were doing. Not only that, but meetings were much more likely on moonlit nights, when nocturnal activity was at its height.
As a result, females would be much more likely to bear offspring from members of neighbouring groups
following meetings on those moonlit nights.
How would this make women’s menstrual cycles synchronise with the phases of the moon?

In order for a liaison to bare offspring the female has to be fertile at the time of mating. As a liaison between a female and a male from different groups is more likely at the time of the full moon it follows that for such a liaison to bare offspring the female would have to be fertile at the time of the full moon.
The offspring of liaisons between members of neighbouring groups would, on average, tend to be more robust that the offspring of liaisons between members of the same group, and would thus be more likely to grow to adulthood and breed themselves. Thus the characteristics of females who are fertile at the time of the full moon would be more likely to survive into future generations – with one of these characteristics would be the tendency to be fertile at the time of the full moon.

Not only is it advantageous for a female to be fertile at the time of the full moon, it’s equally important for the female to not be fertile at other times of the lunar cycle, when she’s much more likely to mate with members of her own, related group. So, for instance, it would be a disadvantage to be fertile twice during the lunar cycle (especially as, for one of the fertile periods to coincide with the full moon the second fertile period would probably coincide with the new moon, when the sky is at its darkest and mating between members of the same group would be most likely).

I have to impress that I’m not suggesting that the appearance of the full moon in the sky would make the members of different groups actively go out and seek members of other groups to mate with (but equally I’m not saying that that wouldn’t be the case). All I’m saying is that the time of the full moon would be the time when members of different groups mated and thus had offspring who were more robust than the more inbred offspring of mating within the same group. The process involved is simply evolution at work. No special lunar powers are evoked or involved. It’s all to do with the slight advantages of genetic robustness brought about by the production of offspring from the larger gene pool that is available at the time of the full moon.

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