There's not a lot to do in space, so bored people in close quarters resort to what they always do, and have large families as a result. The small families that are the norm in today's developed world are a historical anomaly.

In rural China, it was nearly impossible to police a "one child" rule until they became more economically developed, because rural people had nothing to take away. If evicted from one bush, they just sleep under another one. Jail meant more and better food, and better shelter.

Your miners are in a similar position, jail time trades hard dangerous work with some liberty for easy living and guaranteed meals and marginally less liberty. With the loss of income, the government takes on responsibility to feed and care for the family. The government can't take anything away without it costing them (the government) more than they gain.

Attrition

Also one of the reasons rural medieval families were usually large ones. Because space stations represent the frontier, the most hazardous jobs and the workforce handling those jobs would be found there. A high attrition rate could come from pioneering exploration into unknown and likely dangerous environments, technology being inadequate to keep the colonists safe, or even a willful disregard for the safety of the colonists from the Hegemony. The Hegemony could be too corrupt to consistently enforce proper quality control and safety standards, or they could simply not care about their lives enough to bother.

Throw in the expectation that children would be expected to join the workforce from an early age to round out the explanation.

Eugenics.

Most people are not cut out for long term space life. A lot of people degenerate within a few years in low gravity - malignant osteoporosis, accelerated atherosclerosis and other diseases. After wasting a lot of effort moving people to space only to have them sicken and die, the Hegemony systematically studied the genetic differences between the rare people who can tolerate long term space life and the majority who cannot.

People with the optimal genotype for space were really hard to find. They are less than 0.1% of the population, and many were too old or had other issues precluding a move to space. But now that there is a working population of the correct genotype in space, the Hegemony is very interested in breeding more. Space folk with their superior genes are encouraged to have as many space-ready kids as they can.

Why did people in the past had large number of kids?

Simply said: higher infant mortality, so one family needed a lot of born kids to be sure that a few could make it to adult age.

I see no reason why space should be different. Whoever has had a baby around can testify how incredibly "dumb" they can be when it comes to dealing with dangers:

• sticking a pointy object in an electric plug


• swallow a mouthful of acid or bleach


• walk over a step just because


• pushing a bean up their nostril


• etc.

The above are things for which any parent on Earth has to be vigilant.

Now you are putting those things called kids into a mining environment in space!

Space is lethal already for well trained people. The difference between life and death is often the blink of an eye.

Mines are lethal too, no need to explain why.

To me it simply makes sense that kids in space will have higher mortality just because they are kids. Therefore, instead of devoting additional energies to chase those fluffy fruits of their loins, the Spacers find more effective to put more effort in making more kids. At the end there will be no surplus.

Additionally to this, as Joe Bloggs pointed out in comments, simply growing up in microgravity has a whole host of complications. Can you clear mucus from a cold off a child’s lungs if their airways are blocked because their cardiovascular system is designed for constant acceleration, or will it be lethal?

Going agaisnt the Grain of answers here, but I don't think so. Space is different from living on a planet. You can go a month without food, 2 days without water and probably 5 minutes without air.

Resources are limited. You are either relying on the outside world bringing you the resources you want or you need machines to produce and recycle everything. Either way, the amount you can get and store is limited. Having lots of kids means more resources are invested into the Kids and aren't returned back to the colony for a long time. There is the traditional Food, Air and Water you need to keep the extra person alive, but you also need someone to look after the children. Thats someone not working (especially during the first couple of years) on producing the product which keeps the colony viable and afloat. You also need a bunch of extra resources that your working population don't need. Baby Formula, Nappies, baby clothes, cribs, stroller, toys, childrens books. So children act as a giant resource sink for a colony which more often than not, needed to produce goods to keep receiving essential supplies and stay alive.

You will also eventually run into a resource wall. When your 3 kids each have 3 kids of their own, and these 3 kids have 3 more. Every generation your population increases by 50%. Thats 50% more resources consumed (probably more if you factor in construction to make space of the extra people). Eventually your population (without proper control) is going to outgrow what you can produce and start to cost you money to keep alive.

So letting people have many (3+) kids isn't a good idea. Its a resource sink which you will likely see no or little benefit to and it costs you a ton of resources and ties up part of your potential workforce in daycare. It would be better for that to occur on a planet where you don't need to worry about running out of Oxygen, Water, Food or Space and then shipping proper working adults into the actual colonies to do proper work.

Organs

Medicine has advanced to such a degree that most people can live for many, many years. But their internal organs keep failing and need to be replaced from time to time. Not a problem on Earth, where there are clonal factories, but these factories are too difficult to set up in space (and there are crippling licensing fees as well). So, you harvest organs from your children.

Initially, there were competition rites during coming of age, but people soon realized that physical competition is counterproductive - the healthiest, fittest survive, and you get the organs of the weak ones. So by now, the competition is purely mental, the children know it's either them or their siblings, and the most intelligent wins. After generations, natural evolution starts to assert itself - most of the miners are weak, with failing health (requiring constant supply of new organs), highly intelligent and completely sociopathic.

And, since you never know when you need a replacement, people always live in fear that their parents might "harvest" them, even in their adulthood. So they prefer to sire children as soon as possible, to offer grandsons instead, in time of need (genetically, they are a bit less desirable, but still - and they do not fight back).

My father comes from a family of 12. His parents were farmers high in the mountains of Puerto Rico. The reason why would appear fairly simple to begin with. The need for laborers in a large coffee farm was great. But the more I think about it the more I suspect there were other reasons. Initially each additional child would add a sizable drain on resources without contributing anything to the economy of the farmstead. Farmers are used to waiting for results but still...

Isolation contributed to the situation. Most children where born at home since a hospital would have been a whole day's drive at the time. This would have meant little to no access to contraceptives. The lack of other entertainments may have also been a factor, but as someone who has father two children I know how difficult it is to find the time or energy to create the second one. Isolation would have also provided plenty of space to spread.

In my opinion there is only one real reason why a space society would disregard limited resources and have oodles of children in spite of the risks. Religion. In New Zealand, where I now live, there is a large community that call themself "Gloriavale Christian Community." This 500 member Christian cult consist of 55 families with about 8 children each. Nothing in human history has led more people to make decisions contrary to their best interest as a strong religious belief.

So to review: labor, isolation, and/or religious belief.

Space has less distractions. No internet and no shopping means we go back to a primitive lifestyle.

Source: https://www.smbc-comics.com/comic/cattle

While I doubt the following is true, it could be a logical reason in your universe.

Zero-G conception causes much higher occurrences of twins, triplets, etc. in humans.

They produce too much food.

Maybe your colonies, for having planned too much greenhouses in their prime design (or after mining an edible asteroid, whatever fits your world) grew an excedent of perishable commodities with no way to preserve it appropriately (some food do spoil in a freezer). So instead of throwing it in space, your settlers eat it as it comes, and the overall abundance ended in more children than previously planned.

Which in turn, produced more people working in your "space farms" and results as an increase of perishable food, and so on...

Note that having "too much of something somewhere" would also explain the trading exchanges with the others colonies that lacks this "something".

They need the work force

Perhaps your miners are very tribal in nature. The colony is very territorial. You and your tribe own a claim. You and your tribe work the claim. Need more workers? There's only one way to make that happen. Why does the colony support this? Because they draft some of the children into military service. So they need the work force too.

There are a few aspects to consider, the short version being: the purely utilitarian "more babies mean more survive", the moral aspect of "who appointed you to decide how many babies I can make", and the simple fact you need a roughly equal amount of men and women to make the equation $man+woman=baby$ work.

Would you like to know more?

One child policies have problems, and the most obvious one is demography in the form of sex ratio imbalance.

Historically, working the farm is a boy thing. The generally patriarchal society might also make it more desirable to have a son, who will lead the next generation of the family, rather than a daughter whose role is to make babies and little else. While a more modern society might not have these considerations immediately in mind, traditions and prejudices die hard.

China, which recently abandoned its one-child policy, had a ratio of up to 117 males for 100 females (thanks Wikipedia). The consequences down the line are potentially dozens of millions of males unable to find a mate, unless they go find one abroad (which means living the country which isn't necessarily good for the economy, or importing one with all the human traficking problems it might imply), or unless the family changes towards a more polyandric model which then brings us back to traditions and prejudices that could make such a society unthinkable.

This might be workable for large population like China's, or Earth's. For small populations however, if everybody makes two sons that'll work the space mines to bring in all the bucks, you'll soon run out of babies to sustain the population, and that is a real problem.

Another problem is ethics, namely what gives you the right. The one-child policy infringes on one's freedom to have a family of the shape and size they want. More than that, with the considerations of the previous point, it poses the question of the value of the life of a baby boy vs that of a baby girl.

Any advanced society should be wary of that problem, not to mention that one of the main point of settling the frontier might be the freedom it procures, away from the all-powerful government. What gives you the right indeed, cue dumping spacetea into spaceharbours.

Finally, the frontier is a harsh mistress. Space hates your gut, every new planet is just a collection of unknown dangers ready to kill you. Having spare babies just in case might be useful. That one really doesn't warrant much explaining.

Manifest Destiny

As a result of most jobs being automated almost all people have been living on Universal Basic Income for generations. As a result of this they have forgotten how to work towards hard goals like "building a family" or "having a career" instead choosing to spend their lives behind their smartphone screens.

Only a small subset of humanity still choose to work one of the few remaining jobs on earth or move to space. The people who move to space have a strong believe that it is humanity's destiny to colonize the universe. An important part of this is to raise a next generation that can continue this quest.

Filling in a niche here:

Demographics

The people going to space are primarily young people looking for a better life, with a minority of older (middle aged people). The people staying behind are primarily older people, who have settled down for good, with a minority of younger people.

So, which group is likelier to:

a) have more sex;

b) actually try for children;

c) successfully carry to term?

Launch Costs

Putting mass in orbit is expensive; currently, about $2k/lb (post-SpaceX.) If there aren't drastic improvements in booster efficiency, that means something along the lines of $200k per grunt added your spacer workforce. That's a pretty heavy hiring fee.

But... with some frugality, a kid can be raised to work-ready age for less than $200k. Not to mention, the Evil Space Corporations get the opportunity to indoctrinate- ahem, train them all in company schools. It's simple economic efficiency!

They're called maternity centers for a reason

The maternity centers are required because zero-G sex is both confusing and inefficient. Also, fetuses developing in zero-G usually lead to miscarriages or present significant deformities.

However, given how these centers are built for the purpose of facilitating pregnancy and childbirth, they make sex far more... efficient than here on Earth. Be it due to the sanitary conditions, the feromones mixed into the air supply, the technology in the post-copulatory pod, or an idiosyncrasy of the gravity-generators, the fact of the matter is that if two fertile individuals have sex in a maternity center, a baby will probably be on the way.

Combining this with the fact that spacers are still human, and that humans ain't nothing but mammals with an innate desire to have sex, you'll therefore have just as many people having sex, but with a greater rate of pregnancies.

As to why the maternity centers aren't modified to get closer to Earth-like pregnancy rates?

One option is that that's not an option: the technology used in the centers is binary. It either works very well, or it doesn't at all.

A second option is that fertility rates in space are high because the mortality rates are also very high. No technology can mitigate the fact that space is a far more dangerous environment than future Earth. Be it errant gamma-ray bursts and asteroids or just good ol' cosmic radiation, people die in space. A lot. So if you want your line to continue, you need to make multiple... investments.

In case your technology is such that the dangers of space have mostly been mitigated, you can still use this idea: the dangers might've passed, but the spacer culture grew with them and now 3 kids is just... normal. People have 3 kids because they have 3 kids.

On Earth it is limited by the government policy to mitigate overpopulation.

Housing tech

There is just not enough space on the planet with the available housing technology. Could be that they are starting to build arcologies to solve it without the limitation. This project is co-financed by the child-credits sold.

This is not a problem in space since there is a lot of ... ehm ... space ... and materials to construct artificial habitats and stations.

On the other hand, there could (and probably would) be problems with food, water, and air. However, many of the asteroids are made of ice, that could be used to get water and oxygen.

Food

There is not enough fertile land on Earth to support the population growth. Food has to be imported from space which would make the spacers basically farmers who could profit from the larger workforce. The possibility of this depends on the way you solve the energy needs of your civilization.

Hope this makes enough sense and helps :)

It could be a psychological design to guarantee large populations of young miners in exchange for yet another psychological mining related benefit -

Suppose there's an incentive program, not unlike a 401k.

With every hour of service in the resource pool, you gain credits toward your ascension from the common worker ranks. By the age of 45 you can take a payout of a position on a small freighter you then own and operate, which is just a disguise to keep them in the mining industry, just no longer in the physical labor sector. You can crew your ship with qualified members, but immediate family members are automatically granted a position on your family vessel if they choose to go with you or are too young to be left alone. The vessels require a minimum of 4 person crew over the age or qualification level of X, which is whatever you want it to be as the writer. Think like lost in space where the whole family works on the ship. Basic level vessels have a capacity for up to 10, so whole families can effectively retire from the labor sector.

Ships wouldn't be the only cash out too. You can have plantations managed in the same fashion where you cash out into a head position overseeing the laborers in another mining facility.

For each level of incentives, the population requirements decrease to both make it seem possible to get there without having a huge family, but having a huge family is the easiest way to no longer have to drill resources out of the ground.

At working age you will have to justify your living requirements, and the easiest way to do so is with credited time on a family vessel, or leaving the vessel and joining a mining crew to begin your own family, as would almost positively happen with generations. And as such, the cycle continues because everyone has to do their time.

So in short, having a large family will get you out of hard labor faster, thanks to the unified space ascension act of 2245.

Because They Can

Humans are biologically hardwired to want children. Having and raising them is satisfying to most individuals. Earth is at or past the sustainable carrying capacity. An educated population understands that and looks for a relatively painless way to get the population down on the long term is to breed below the sustainment level.

If the spacers can build stations out of one asteroid, they can build stations out of many asteroids. The expansion is limited by the stationbuilding capacity. The limiting factor on that? Workers. That will work until they run out of suitable asteroids, which is a long way off.

It seems to me to make a good background point you need a reason for the government of the colony to incentivise large families rather than just a reason for their being large families.

To do I suggest set up your space colonies as follows.

• Each one is attached to a large pool of raw resouces. Ethier an asteroid, inhospitable moon, giant space fungus etc.




• They have free energy. Either cold fusion or room too make as many solar panels as required




• They have free food. Yeast farms or whatever, fed by the endless supply of sunlight and CO2 from crushed rocks.




• Space travel is expensive. Shipping in machine tools and replacement parts isnt economical.




• The initial setup is expensive. Every colony has a large debt to pay back.




• Automated factories and the like initially used to produce the exports of the colony tends to break down.

Now you have a senario where unless the colony starts to produce its own replacement parts and other day to day resources it will go bankrupt.

Given even a simple pencil takes millions of people from all over the world to produce, each colony is in desperate need of crafts people to manually fabricate a million and one things.

Only colonies that become self sufficient before the initial equipment fails break even and make a profit.

So you ship with as many people as possible and encourage them to breed as fast as possible. Building shelter and feeding them all isnt a problem as long as they all learn to smelt ore, smelt steel, machine screws, weave glass fiber and generally fill in for mass production.

A side benefit of this is all your colonists are tough jack of all trade macgyvers living on the ragged edge of society

Transport Costs and timeframe

Depending on the tech you use for moving from one place to another it is very costly to transport living beings. If using current tech they are required to be fed and watered, given clean air to breath and require exercise and entertainment if they are to get to their destination in good shape. Transporting breeding stock is a lot simpler task than transporting the whole herd...

Enviromental Factors

Maybe the local environment affects the development of the unborn child. Think of local microbes entering the body of the mother passing to the foetus, changing the immune system etc that allows the child to be adapted to the local conditions from birth. As an alternative maybe some other enhancement, ie greater strength or endurance for the mining operation, arises from development and growth on the new planet.

Related to this, is that space is full of... space. If earth is over populated to the point where I am so close to my neighbor, that I can ask to borrow a cup of sugar through our paper thin walls, in space, the same task may involve four hour walk to your nearest neighbor... and then you need to go back home. I have lived on the East Coast of the U.S. my entire life and for the majority of that time, I have been in one of the denser parts of this area (near D.C. and Baltimore specifically). In my childhood, I lived in a house where the neighbors were "a corn field" and "a Cow pasture"... and then I moved to a more suburban neighborhood that was still so rural that the nearest "Convenient" Store was 10 minutes from my house. By car.

Coupled with the many answers that to a rural colonist of another planet, sex is one of the few available past times, kids can be farm hands and the more you have, the more workers you don't have to pay (U.S. Labor laws still do give deference to children below working age doing work in the family business) and religious reasons, it could be that your "family planning services" are to far away to justify having safe sex in the heat of the moment.

Earthlings Are Heavy

Space-based technology has just become successful at autonomous ships and mining equipment. This was based on prior efforts to automate all manufacturing capabilities. Building satellite or ground-based habitats is becoming increasingly cheap.

The government decides it's time to start populating other solar systems. Lifting mass into orbit is expensive in terms of environmental damage. That is true for people as well as ships. It just makes sense to work with mass already outside Earth's gravity well.

So, the extra population needed is bred off-Earth and sent out in ships built in space.

The actual science based trend is that spacers would have less children. As people become more technologically advanced they tend to have less children, and this is becoming increasingly exacerbated with more technology.

The only plausible solution to this problem is that space fairing people are immortal, and they have lots of children by virtue of just living so long. However their children will still all be adults.

Here is some light reading:

Mental perspective

Declining world populations

In fact one of the theories in the Fermi Paradox is that as intelligent species become more intelligent, their birthrates decline to the point of extinction. So it is not likely that a space faring human civilization would lead to many children.

1) The largest families in my home town were all Catholic. Thus, you might employ religion as a reason for more kids.

2) Also, the larger families tended to be farm families with more kids to help out on the farm. If you can find an economic benefit from putting little hands to work helping the family, and make this economic benefit exceed costs of raising the kiddos then the hidden hand of the market place will favor larger families.

I suggest you combine the two notions when designing your extra-terrestrial culture.

Connect the reason for many children to the theme of your space world.

For example, Krypton was destroyed because of irresponsible mismanagement, put hyper briefly. Cal El was born for desire to progress, expand, and survive. Kryoton's destruction serves as a monument and warning to not drestroy ourselves and Cal El is that message in a bottle delivered to us.

What is your world's message in the bottle?

What mission, "moral of the story", purpose, virtue did/does this space presence come from and serve? That is your main motive for having all these children. With that in place, the science can follow it more easily.

...Less distracting entertainment, fewer viruses, better resources, need for company and help on the approaching colony... perhaps a less-charged, less polarized social-political atmosphere where people are more pragmatic. But, all that sprouts from the initial thematic mission for this space travel.

In space, no-one can hear you breed

While in space, you live where you work. The spaceship / space station is also the family home. The crew are usually members of an extended family. There is plenty of room and the hydroponic system supplies more than enough food for everyone. Finally there is a lot of spare time.

It's limitations that reduce the size of a family. Limits on time, limits on space, limits on food, limits on money.

In a self contained, self sustaining ship, you have no real limits.

If the space jobs were higher paying then the ones on earth, space people could be higher earning and more prosperous thus could afford a big family.

If there is birthing restrictions in place on earth, it could be good for social status to have more kids.

It could be part of the culture of space people to want to increase space population.

It could be part of the culture of space people for genetic-diversity (swapping with more partners).

There could be some space foster center that removes the responsibilities of parenthood.

There could be some positive adaptation to being born in space in which space children are highly desirable.

If there were higher demand for jobs than the population.

Some combination of the above suggestions.

I will put the other way round and present some reasons why Earth based families have fewer children than space based ones:

• Due to overpopulation on Earth there is an effective birth control policy (similar to China's famous one-child policy) in action


• Some environmental factors (like chemical polution) have seriously decreased fertility on Earth


• A celibatarian cult has managed to get lots of followers on Earth

This is just a hypothesis, but giving birth is probably a lot easier in low or no gravity, for reasons similar to water birth, but with more convenience. An easier and less painful birth experience is going to make repeating the process less unattractive, resulting in more children.