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The scenario

In the scenario that I'm imagining, a small group of people (ideally 100 or less) travel to a parallel universe and begin a new society. The new planet that they land on has one land mass surrounded by water, with sufficient freshwater resources and enough fertile land to build a prosperous agriculturally-based society. There are no existing cultures on this planet, and they have no further way to interact with any other societies. In other words, no one can immigrate to them, and they can't emigrate to other places.

The question

The population needs to reach the continent's capacity (we'll say that's about 5-10 million), then remain stable. What kind of laws or societal norms would have to be enforced to ensure this quick rise and then leveling out of the population? How would this affect their society? For example, is it plausible that a society could be so focused on increasing the population, then do a quick 180 turn and change their rules/norms/familial institutions to focus on population stability?

Assumptions about the society

My initial thought was that, once they reach their population capacity, the familial structures would put less focus on having children biologically, and more focus on contributing to a child's upbringing as a teacher, or by mentoring them, for example. I also imagined that the society wouldn't be heteronormative because having children isn't a priority, and that the people who do have kids wouldn't be encouraged to have more than one.

However, after doing some reading on here about other isolated/stable populations (such as here: Maintaining a constant population in a country), I've learned that these assumptions might be wrong. For example, in order to continue replacing the population, if not everyone is having kids, you might need an average of 4 kids per woman who does choose to reproduce.

More info about the world I'm trying to build

To get to this world, the pioneers travel using magic that is nearly impossible to wield. So, they can't really use it to build their new society. Also, since they don't want their descendants to find a way back to the world they escaped from, they don't pass on the knowledge of the magic's existence.

The world that they come from has a medieval or renaissance level of technology.

The new world is 'egalitarian' in nature, or at least, its inhabitants buy into the illusion that it is. The society doesn't experience wars and has very little crime (that the general population knows about, anyway), but there is some corruption bubbling under the surface, that will come to fruition in the story's plot. In terms of how the world is set up, though, it needs to appear as though everything is peaceful and carefully planned.

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    $\begingroup$ Medicine is technology: medieval or renaissance level of technology implicitly means very low population growth anyway; infant mortality and perinatal mortality and diseases and lack of antibiotics take care of that. From 1500 to 1600 the population of Europe latissimo sensu grew from 78 to 112 million, about 3.5% per year; your world will rise above Renaissance-level technology long before population growth will become noticeable. Moreover, the carrying capacity of a piece of land will grow as the tech level progresses. $\endgroup$ – AlexP Oct 13 '18 at 19:41
  • $\begingroup$ That's a good point - I hadn't thought of that. I'm definitely open to adjusting the technology level (and therefore, level of medicine) if that makes more sense for the story I'm trying to tell. $\endgroup$ – JubaSuperman Oct 13 '18 at 20:45
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    $\begingroup$ "I also imagined that the society wouldn't be heteronormative" before or after they reach "the continent's capacity"? $\endgroup$ – RonJohn Oct 13 '18 at 23:52
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    $\begingroup$ A "continent's capacity (of) about 5-10 million" is grossly and unreasonably tiny when it has "sufficient freshwater resources and enough fertile land to build a prosperous agriculturally-based society" and the abundance of animals that are part of a region with fertile land. $\endgroup$ – RonJohn Oct 13 '18 at 23:57
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    $\begingroup$ @SRM no, the continent Zealandia is separate from Australia. Most of it is sunken. stuff.co.nz/national/94922642/… Only the highest peaks remain above water. $\endgroup$ – RonJohn Oct 14 '18 at 10:00
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Growing your population is easy

Humans (I'm assuming humans or folks behaviorally similar to humans) naturally enjoy doing this, and like any creature on any planet, they will expand to fill the limits of their resources. Give them food, water, a bit of privacy, and a tax benefit, and humans'll breed like rabbits (and frequently will even without all that stuff).

It's those limitations that are needed for the second step

Stabilizing a population is the trick. There's nothing inherently natural about stopping growth. Growth is what life does. In other words, you need something antiethical to life to get it to stop.

Life-Giving Resource Limitation: Run out of water or food and your population begins to starve.

External Motivation: China's one-child policy, war, disease, forced sterilization, eugenics, or any other external force can be applied to whittle-away at the population. Thanks to a comment from SRM: depending on tech level you can add animal predators to this list.

What there isn't is a nice, polite, humane solution. Life wants to grow. Stopping it from growing is intrinsically mean-spirited, and it's no wonder that all the solutions for "stabilizing" a population are awful. Simply put, there is no way to naturally stop growth that doesn't result in pain.

Edit

My thanks to a comment from AlexP, another solution that might be more humane (from a certain point of view) is economics. When combined with a lack of social welfare programs, a rising standard of living would have the consequence of reducing the number of children born to the poor and middle classes.

The solution is capricious (people like breeding and children sometimes become the inconvenient consequence of recreation), but over time a society would begin to perceive large families are burdensome. Add to it laws that make parents legally responsible for their children's activities and suddenly poor kids stealing food landing parents in jail will lead to fewer kids.

Keep in mind that we're looking for plausible and believable, not necessarily "this is the way it happens in real life." The lowering birth rate in 1st-world countries has a great many factors, not just one, and many of them required hundreds of years to become part of the social psyche.

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  • $\begingroup$ Thanks for the information! At the risk of sounding like a sociopath, I'm open to the concept of this society using inhumane methods to control its population. I'd prefer them to be wrapped up in a way that doesn't seem inhumane to the people, though. Any further thoughts on how they might go about that using one of the methods you described, and what it would mean for the society? I'm sort of leaning toward forced sterilization - again, at the risk of sounding like a sociopath. $\endgroup$ – JubaSuperman Oct 13 '18 at 20:50
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    $\begingroup$ To be fair, our job is to help you establish the rules of your world. What you just asked for was help telling your story. But, in the spirit of one glorious sociopath helping a brother out😁: Engineer a limited-scope virus and use that virus to justify the need to sterilize elements of the population. You need to implement some randomness (e.g., you can't just infect areas that are becoming overgrown) to hide the true agenda. People will weep over their loss, but not blame the government. And there are births occuring, so all's not a loss! Patriotism abounds as only a few know the truth. $\endgroup$ – JBH Oct 13 '18 at 20:59
  • $\begingroup$ Developed countries, and countries with a medium level of development, have either a negative rate of natural population growth or, at most, have a small rate of natural population growth. No need for "one-child policy, war, disease, forced sterilization, eugenics". (There are exceptions, such as Iceland, Ireland, and Israel. Among larger developed countries, the champion is the U.S.A. with 4.3% annual natural growth rate.) The world-wide rate of population growth is decreasing, projected to fall to 5.6% p.a. in 2045–50. $\endgroup$ – AlexP Oct 13 '18 at 21:23
  • $\begingroup$ @AlexP, but when you dig into the details explaining all that you discover social animosity over world population densities and other political factors. Granted, that's one way to do it, but it's very capricious. But it does suggest another solution that I'll add to my answer: economic pressure. Make the cost of living so high that you can't support more than a few kids (and large families become a hallmark of wealth). $\endgroup$ – JBH Oct 13 '18 at 21:28
  • $\begingroup$ I have suggested an edit to fix some spelling, and also to remove extra spaces. It seems that after every period, you have two spaces. Is that intentional? $\endgroup$ – John Locke Oct 14 '18 at 0:19
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WAR

they might start off as one group but over time - especially as they grow up to ten million in number and things like food and water get scarce... not so much. the excuses will be ideological, or racial, or any convenient us vs them - doesn't take much. I'd think that actual scarcity of resources wouldn't be the reason given, at least not universally. there are plenty of secondary issues that result in a much better emotional response from the people you are trying to get fired up.

So there you have it. periodic, brutal wars keep the population in check when it stresses the ecosystem of the island. The fun caveat is the same one that applies to our isolated island with 7.5 billion people on it. Which is more people than probably should be sustainable... except that we figured out how to make more food and distribute it more effectively, and get water where it needs to go, and found more and more effective forms of energy to assist in all of the above. So how many people is too many people is kind of a moving target.

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    $\begingroup$ Good answer. MParm mentions that sustainable pop is a moving target... so is war damage. As tech increases lethality, you approach the point where war is more devastating than the population ability to recover. And that tech level can be quite low if your medical tech lags or your food/water supplies become dependent on long supply chains. When one of your characters says, “With this new X, we will end them forever,” check to see if he/she is right! (Weapons rarely stay on just one side for long!) $\endgroup$ – SRM - Reinstate Monica Oct 14 '18 at 14:21
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This answers the "grow its population" portion of the question.

"An isolated world" is no different from an isolated village.

Your little society of "100 or less" will have to be really careful in who it brings along and who mates with whom so as not to suffer in-breeding depression.

https://www.newscientist.com/article/dn1936-magic-number-for-space-pioneers-calculated/

The “magic number” of people needed to create a viable population for multi-generational space travel has been calculated by researchers. It is about the size of a small village – 160. But with some social engineering it might even be possible to halve this to 80.

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

Inbreeding depression is the reduced biological fitness in a given population as a result of inbreeding, or breeding of related individuals. Population biological fitness refers to an organism's ability to survive and perpetuate its genetic material. Inbreeding depression is often the result of a population bottleneck. In general, the higher the genetic variation or gene pool within a breeding population, the less likely it is to suffer from inbreeding depression.

Since this new world will be relatively hostile, bring more than 160 people. No children and relatively few old people. What old people you do bring will have to be more useful than young, healthy (and thus breedable) women.

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