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I am currently writing a short story about a world that is about to go into nuclear annihilation and ultimately destroy themselves through global thermonuclear war. Therefore, the government of one country finds a very deserted island that would be an appropriate place for some of their citizens to inhabit. This island has new laws that is different from their homeland in order to preserve their own species from going haywire and extinct.

If a country created a law that wanted to keep a completely constant population, how many children would they be allowed to have?

Let's assume that this country provides healthcare that is extremely advanced so there are no diseases. In other words, this new civilization would be extremely ideal without conflict.

My guess would be that you would simply need to have only one girl and one boy in your family but no more. However, I was thinking that there could be a problem with incest or some genetic redundancy that could cause birth defects.

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    $\begingroup$ If there is no disease, then there is no need to worry about incest. $\endgroup$ – A. C. A. C. Dec 27 '17 at 23:56
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    $\begingroup$ I meant external diseases like flies, birds, fish, etc. that carry disease. These humans are not immune to diseases but the diseases are curable. $\endgroup$ – KingDuken Dec 27 '17 at 23:57
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    $\begingroup$ Sex ratio at birth is about 1.07:1.00 boys to girls; so from the beginning you need a little more than 2 children per woman just to keep the number of females constant. Then you have some mortality from diseases (not all diseases can be cured, even with the most advanced medicine imaginable), accidents, murders, etc. so in the end the replacement fertility rate is usually considered to be about 2.1 children per woman (for advanced countries). This is all very easy to find. $\endgroup$ – AlexP Dec 28 '17 at 1:21
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    $\begingroup$ You should wait at least 24 hours before accepting an answer to encourage people from every time zone to weigh in on your question. $\endgroup$ – sphennings Dec 28 '17 at 1:37
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    $\begingroup$ A strict "completely constant population" is impossible. That would require people to have babies at the same moment someone else dies, which is unrealistic, to say the least. Some variation will be necessary, unless you plan on killing people to keep things 100% stable. $\endgroup$ – computercarguy Dec 28 '17 at 14:40
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Replacement rate is about 2.05

Even without diseases, you can still die of things, assuming your people are mortal. For every million Americans, there are 432 accidental deaths, 133 suicides, and 57 homicides each year. By my count from that link above, due to just accidents and homicide, about 594 of every 100000 people will not make it to adulthood. Therefore, every 100th family or so will need to have 3 kids to keep the population up.

Also keep in mind, that if your people are not immortal, then there will be some unexpected deaths for various sources. Being too tough or stubborn to go to the doctor can kill you, if you have a ruptured appendix or really bad food poisoning. Also, some people can get sick while alone and have no one available to get help for them.

All in all, some people will not make it to baby-making age. You will need some families to have more.

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  • $\begingroup$ I've never heard of that terminology, "Replacement rate". That's very interesting. Thank you for the feedback. $\endgroup$ – KingDuken Dec 28 '17 at 1:22
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    $\begingroup$ Could make for some interesting Ender's Game kind of development, where only families with very good first two children can have a third $\endgroup$ – bendl Dec 28 '17 at 18:09
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    $\begingroup$ That figure might have to be adjusted upward a little. Besides premature death, consider also that not every single heterosexual pair can bear the requisite number of children - some couples may need to pick up the slack for other couples if they are sterile, or indeed if any adults simply do not wish to procreate. $\endgroup$ – Xenocacia Dec 29 '17 at 8:53
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You ask how many children to allow by law, but you also say you want a "completely constant" population. These conflict; a rate set by law will get you an approximately steady population, but people die, some people don't have kids, some people live a really long time... so your population will vary.

For a strict steady rate, then, you need something other than family-size laws. Hugh Howey's Silo books show a way to mandate a constant population -- though be warned that it comes at the cost of some personal liberties. In his world, what remains of humanity is living in an underground structure (the "silo"), with carefully-balanced food production, living space, waste management, air, etc. And the rule is: when somebody dies, a lottery is held to choose the next couple to produce a child. They get some amount of time in which to try, and if they don't succeed, somebody else gets a try. This is enforced by a mandatory contraceptive device that is implanted at puberty and suppressed somehow for couples that are allowed to be fertile.

If you want a completely constant population, you need to regulate births directly. I don't know how large your island is or how many people will be living there, but if it's large you can subdivide the group for purposes of population control.

Note that since you control who's allowed to have children at all, you can also apply whatever controls you need to prevent inbreeding, inherited diseases, etc. You get to decide who's allowed to enter the lottery, in other words.

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If a country created a law that wanted to keep a completely constant population, how many children would they be allowed to have?

As many as they like

What first sprung to mind was Kurt Vonnegut's 2BR02B about a society with a strict one in one out policy. Its a short read so I won't go into it too much.

The point is that the only way you'll ever have a "completely constant" population is with voluntary (or otherwise) deaths just as a child is about to be born.

As with most ideals, you might not like this answer but if you're going to stick to your constant population you're going to have to make some sacrifices.

Incest problems can be sorted with sperm sorting (they already do it on farms) so that you can make sure your next generation don't have to mix a closely related male and female.

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Maintaining stable population levels is actually a LOT harder than you think and there are a range of issues you have to face. First of all, not everyone wants children. Secondly, you need to cater for accidents, murders, and other unforeseen circumstances (not to mention emergent homosexuality) in your world.

Then, there's age distribution. You ideally want as large a (stable) percentage of your population in working age if at all possible. Otherwise, you end up with ageing population issues which mean that a lot of your resources are being tied up with caring for the elderly and not producing food, clothes, housing, etc.

I remember reading that in the Renaissance, the number of children per family to maintain population numbers in Europe was around 4. This was because many couples didn't have or want children, and some people were bachelors or spinsters all their lives.

If you look at China and their 1 child policy, that has led to a runaway ageing population problem that is going to be at its most severe around 2050.

But wait (I hear you say), we have around 2.1 kids per family NOW and we're facing overpopulation, aren't we?

Well, no. Since most developed countries have embraced females into the workforce, our economies have effectively doubled since the 1970s but at the price of population replacement. Most developed countries now ONLY have population growth thanks to immigration. Australia, Germany, Japan, and many others are facing ageing population issues which (while not on the scale of China) are going to cause headaches in the next 50 years. Ironically, the only countries that are contributing to global population crises are those who have a far less enlightened view of the role of women in their societies.

Ultimately, if you want your population to be constant, you probably want at least some women in your group dedicated to having and rearing children. That doesn't mean treating them as less than equal; it means making the raising of children a 'career' that is valued every bit as much as hunting, gathering, etc. in your community. If you value the raising of children, then more children will be born to your community and it's only if the numbers climb beyond the percentage allowed for pre-working age population numbers that you need to put a temporary stay on new births, until more are of working age.

Depending on the other careers available, if only around half your women actually want to have kids, then the magic number is around 4 kids per woman. If more want to have kids, then the ratio goes down. You're best working this out as a percentage of families, rather than a percentage of kids per population.

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  • $\begingroup$ The ageing population is going to cause trouble in 20 years. In 50 years the problem will have resolved itself one way or another. $\endgroup$ – Separatrix Dec 28 '17 at 14:35
  • $\begingroup$ I would like to highlight more prominently that replacement rate depends on infant mortality and general healthcare and military situation. $\endgroup$ – Oleg Lobachev Dec 28 '17 at 16:13
  • $\begingroup$ Your explanations are not entirely correct. An average number of children per family refers to the arithmetic mean, i.e. the number of all children divided by the number of families. It does not reflect how many families have children: If one family has 8 children and the other has none, the mean is 4. Please note that it is exactly the same as in the case that both families have 4 children, or that family one has 6 and family two has 2. $\endgroup$ – Olga Dec 28 '17 at 18:14
  • $\begingroup$ If babymaking is a career, then you could even have layoffs if population gets too high $\endgroup$ – bendl Dec 28 '17 at 18:15
  • $\begingroup$ Female participation in labour is not the strongest predictor of falling birth rates (i.e. women have fewer children). Female access to education is. Educated women have fewer children and they have them later in life. Contemporary research also suggests that in post-industrial societies that completed demographic transition gender equity is also an important factor contributing to birth rates: Higher gender equity, especially in the domestic sphere, translates into a greater number of children. $\endgroup$ – Olga Dec 28 '17 at 18:19

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