Population Issues on the Moon

I made a pretty big claim in class today -- that a society consisting of 90% women would quickly reach the equilibrium point of approximately 50% males and 50% females. I was able to find some research backing that up in the work of the primary sex-ratio scholars Fisher, Bodmer and Edwards (who argue that species will always lean towards 1:1 ratios of males to females as it offers the highest return on their energy investment in terms of reproduction). Feel free to look up their work on JSTOR. In the meantime, I decided to use arithmetic to help explain my principles.

To do so, we will first demonstrate how Luna likely functions in terms of population growth and sex ratios. Let's start with some basics. We know that, at the time of the narrative, Luna (or at least Luna City) is 90% male. To simplify things, we'll use population figures of 10 men for every 1 women, with a starting population of 100 men and 10 women. We'll assume that in Loony society, each woman takes at least two husbands, and each woman has at least two children. We'll also assume a 1:1 ratio of male to female births, and we won't take into account old age (meaning a member of any generation can marry a member of any other generation), but we will allow marriages to only occur once. Finally, we'll add 10 men and one woman to each generation to simulate the introduction of new convicts into the environment. It won't seem like much after a few generations, but I think a 10% increase in population at first is reasonable, and there's no reason to think it will increase any more over time. This will, theoretically, give us quite a long time before the population is brought to 50/50 male/female. Let's calculate a few generations....

Generation 1
Available Men: 100
Available Women: 10

Unmarried Men: 80
Male children: 10
Female children: 10
Male convicts added: 10
Female convicts added: 1

Results:
Total Men: 120
Total Women: 21
Percentage Women: 15%

Generation 2
Available Men: 100
Available Women: 11

Unmarried Men: 78
Male children: 11
Female children: 11
Male convicts: 10
Female convicts: 1

Results:
Total Men: 141
Total Women: 33
Percentage Women: 19%

Generation 3
Available Men: 99
Available Women: 12

Unmarried Men: 75
Male children: 12
Female children: 12
Male convicts: 10
Female convicts: 1

Results:
Total Men: 163
Total Women: 46
Percentage Women: 22%

Generation 4
Available Men: 97
Available Women: 13

Unmarried Men: 71
Male children: 13
Female children: 13
Male convicts: 10
Female convicts: 1

Results:
Total Men: 186
Total Women: 60
Percentage Women: 24%

And so on. I'll save you the tedium of the rest of the operations, but suffice to say that it takes 25 generations before hitting 40% women, and a full 200 generations before hitting 48% (by which point our total population is around 56,000, and equilibrium has already been well enough achieved -- trust me). Clearly, Heinlein's depiction of Luna City has it bad, but not so bad as he makes it seem...using my model, they go from 10 to 1 all the way down to 4 to 1 in just a few generations, and that's using very conservative estimates. My model also didn't include any individual woman having more than two children (which made the count take longer) or any men or women dying of old age (which shouldn't have effected the statistics anyway, since there was always a surplus of men in the gene pool). In other words, in a society without these limitations (like Heinlein's), there would be a lot more children, and therefore things would get corrected much faster. So while 10 to 1 males to females is pretty bad, it self-corrects relatively quickly.

My argument, however, was that a society of 10 to 1 females to males would reproduce and self-correct so quickly as to hardly matter. So let's flip my system on its head; 100 women and only 10 men. Each man takes 2 wives, each woman has 2 children. Every generation we'll add an extra 10 females and 1 male. Let's see how this goes down.


Generation 1
Available Men: 10
Available Women: 100

Unmarried Women: 80
Male children: 20
Female children: 20
Male convicts: 1
Female convicts: 10

Results:
Total Men: 31
Total Women: 130
Percentage Men: 19%

Generation 2
Available Men: 21
Available Women: 110

Unmarried Women: 68
Male children: 42
Female children: 42
Male convicts: 1
Female convicts: 10

Results:
Total Men: 74
Total Women: 182
Percentage Men: 28%

Unbelievable! I'll spare you the calculation...suffice to say that this model reaches equilibrium in just 7 generations, interestingly with a total population of about 7,000. In fact, this model reaches perfect equilibrium (exactly 50/50) in only 10 generations...which would have taken the first model over 50,000, more than all of human history has taken already! And that's assuming that each man takes no more than two wives, and each woman has no more than two children. If men take as many as 4 wives, you can reach equilibrium in 3 generations, or even 40% in only 2. And you'll STILL have a huge surplus of women. I think it's safe to say that equilibrium comes plenty fast in a society like that; men wouldn't be treasured articles of value for very long.

* These operations were performed using a program I wrote in QBASIC. If you'd like to see the code, please let me know and I could post it here. It's a very short program.