Science fiction worldbuilding often features space colonization, but only seldomly is it considered through a demographic lens. This is likely because most science fiction writers and artists find other aspects of their worlds much more interesting to build, and basing the demographics of various places in outer space on their terrestrial analogues works well enough for most purposes. However, when you think about it, pushing the boundaries of demographics in various space colony settings can lead to quite interesting results.
Space Shakers: Low Fertility Cultures
To take perhaps the simplest example, a colony with a population that has a fertility rate of zero (i.e. the people there have no children) would undergo a natural shrinkage equal to the mortality rate. This depends on the age structure of the population; a population of young adults will start with much lower mortality than an exclusively elderly population, for instance. However, assuming an age structure in the normal range (28% under the age of 20, 16% over the age of 60, and a median age of roughly 35) and no immigration or emigration, the population will shrink by 50% in 40 years, 90% in 90 years, and almost certainly become completely extinct in no more than 120 years. Of course communities that refrain from reproduction and take in no new members over the course of a century will likely be rare in any realistic space colonization context. The only exceptions to this would tend to be very small groups that remain isolated for a century or longer for whatever reason. A scenario like the premise of Lost in Space might have demographics like this.
A much more realistic scenario would be a celibate community that does accept new members, such as a monastery or a community like the Shakers. In this case, the population growth will be dictated by the mortality rate plus the net migration rate. Assuming a mortality rate within the range seen in developed countries, 1% per year, net immigration equal to 1% of the population will keep the population stable. In this situation a colony consisting of, say, 1000 people would on average have 10 of its members die each year, replenished by 10 new immigrants (on net). The same net immigration of course could be maintained by, say, 30 people leaving and 40 people coming in per year on average. It goes without saying there are no guarantees. If the net immigration rate falls below 1% the population will shrink over time, and if there is net emigration over a long enough period the colony will eventually be completely depopulated.
This demography might seem like an exotic province of religious sects, but when you think about it ships at sea, and in the future space, work in the same way: low to zero birth rates on board the ship but with crewmen coming in to replenish fatalities. Obviously the first exploratory ships into the broader solar system will be run with naval-style discipline with few if any children and families, much like the Mayflower. Eventually that will change as the colonization phase begins, much like Plymouth. How much could this change? Instead of the Shakers, in the case of the opposite extreme we should look at the Amish.
Space Amish: High Fertility Cultures
The Amish and science fiction might seem like a strange combination, but the Space Amish trope is common enough in the literature, perhaps because the voluntary eschewing of technology becomes all the more interesting at higher technology levels. In addition, as the TVTropes page points out, a surprising amount of science fiction uses high technology as a justification for recreating a pre-modern world of frontiers, tribes, and hierarchies. There are ample reasons to believe that this will be the case in the real future; opening up a new frontier is particularly obvious, as the space to retreat to in the face of encroachments by governments and hostile “civilized” forces is effectively infinite in space, an option that has been closed on Earth for centuries, if not millennia.
Where the Amish example comes into play in terms of demography is that they are rather famous for being the largest group in the West with genuinely high fertility rates, currently at six to seven children per woman. It is somewhat less well known that there are other Anabaptist sects that are similar in this respect, albeit smaller and with lower fertility, such as the Mennonites and the Hutterites. The Hutterite example is particularly interesting, since they mostly settled in the Canadian Prairies, an environment harsher and thus more space-like than where the Amish settled, and in the 1950s they attained a fertility rate of ten per woman, which is considered to be a world record and is often used as the benchmark for the maximum human fertility rate.
All these Anabaptist groups live in countryside colonies with a higher degree of self-sufficiency than the average rural community, and make their living off agriculture and (more recently in the case of the Amish) more-traditional trade work such as carpentry. These characteristics would make for good space colonists, though more technological sophistication would be strongly advised, especially in the initial stages before you can buy a more or less self-maintaining O’Neill cylinder and asteroid mining rig off-the-shelf.
Lesser-known rural groups of this type include the Old Believers of Russia, who have high birth rates (though not quite as high as the Amish from what I’ve heard) and mostly live in remote parts of Siberia to avoid persecution, a strategy that worked all the way through the Soviet era. The most extreme instance of this was perhaps the six-member Lykov family, who lived alone in the Altai Mountains 160 miles from the closest settlement and encountered no one for 42 years. Siberia is particularly interesting because of all the regions of Earth it is the closest environment to outer space that has agricultural settlements. Verkhoyansk’s extreme records range from -90 to +99 Fahrenheit, and its averages range from -55 to +74, a range that isn’t that different from large parts of Mars. Gale Crater’s extremes range from -197 to +68, and winter afternoons are actually warmer in Gale Crater (-9) than in Verkhoyansk (-44). Due to the thin air Gale Crater is much colder at night, but even with that the daily mean of -67 there in winter is not much colder than Verkhoyansk’s -49. On Mars the airlessness and distance from Earth are the problem, not the climate. A people used to the rigors of Siberia may well have an advantage in colonizing other planets.
High fertility groups aren’t confined to the countryside; the Laestadian Lutherans of Scandinavia live in all environments including deep within urban areas, do not eschew certain modern technologies in the way the Amish do, and in the 1980s had a fertility rate of over five per woman in Finland (compared to 1.5 or so for the general population). Haredi Jews in Israel have a fertility rate of around seven per woman, and predominately live in urban settings.
The Power of Population Growth
Still, there is a correlation between population density and fertility; it is reasonable to suppose that, all other things being equal, people have more children in wide-open rural nature than in the cramped artifice of the city. Indeed, it is a tendency in nature across the plant and animal kingdoms for pioneer populations to expand more rapidly than more confined ones, and according to one study focusing on colonial Quebec this applies to people as well. While the first spaceships may be cramped and thus more city-like, as space manufacturing matures in the future the construction of spacious habitats like O’Neill cylinders may become relatively cheap. Even smaller-scale one-family ships may have access to technology that can process raw asteroidal materials into living space very cheaply. In 2018 technology to manufacture a 650 square foot home in a day for under $10000 using 3-D printing was demonstrated; using something like lunar concrete as the raw material, a space-rated version could construct large homes on asteroids, planets, or in free space very cheaply. When we go into space our living quarters will not be cramped.
A community in space that has need of new housing for a booming population won’t have much trouble; aside from the cost of the machinery, the energy needed to extract raw material will be the primary cost. So how much will a community with truly high fertility rates grow over time? At a rate of eight children per woman, the population grows by 3.7% per year, doubling about once every twenty years. This is about the rate the Amish have grown over the past century. After a hundred years, the population grows by about 38 times; a population of just 1000 expands to 37830. After another hundred years, it grows to 1,431,109.
Of course, we know that eight per woman is not the maximum birth rate possible. The Hutterites, as mentioned above, exceeded ten per woman at one point. There is speculation that parts of New England and (especially) Quebec may have reached eleven per woman at some points during the colonial era. At a fertility rate of ten per woman, populations grow at 4.5% per year, doubling about once every 16 years. After a hundred years the population grows by 81 times; a population of 1000 expands to 81580. Another hundred years brings it up to 8,389,359. Only two extra children per woman relative to the eight per woman scenario almost octuples the population after two centuries have passed; after only a century the number is almost tripled. As far as I am aware no group has maintained ten children per woman for even one century, let alone two, but it does show the power of exponential growth.
Natural Fertility: The Ultimate Limit
It also raises the question of what the ultimate limit is. Of course in a science-fiction world genetic engineering and/or artificially-enhanced methods of reproduction can be used to achieve almost any fertility rate if the society has the will, but what the figure would be without any constraints (positive or negative) is a much more interesting question. This is what demographers call “natural fertility“, the fertility rate under maximally favorable environmental conditions and with no deliberate controls. The Hutterite’s peak of 10.9 is often used as the benchmark for this, but one study finds good reason to believe this benchmark is too low, as some controls appeared to be practiced even by the 1950s Hutterites.
The average woman experiences menarche at 13.5 and menopause at 49.2, leaving 35.7 childbearing years. Given a nine-month gestation period, you might think the number of children is 47.6, but this doesn’t take into account the average time needed for conception to occur, or (much more crucially) the natural contraception provided by breastfeeding, the natural way of feeding a baby, which adds about an additional ten months between births. Taking that into account, the ultimate limit is around twenty per woman. Even with a period of two years of natural birth control, the fertility rate would still come to almost thirteen children per woman.
Thirteen to twenty children per woman is considerably higher than any known culture, mostly because the math assumes every single woman will have a father for her children by the age of thirteen and continue in that state for thirty-six more years. This does not happen in real life, so even a culture that deliberately tried to maximize natural fertility would fall short of twenty. Even the friskiest historical cultures didn’t deliberately try to maximize fertility, so they fell short by even more, hence the ten to eleven maximum observed historically. But what if there were a culture that deliberately maximized natural fertility? What would be the highest fertility rate a culture could realistically attain using natural methods?
Universal marriage by age thirteen is unrealistic; even cultures that practice arranged marriages where future spouses are scoped out from childhood fall short of that goal. However, modern Westerners might be surprised to know that you could come most of the way there; the average age at which women marry in hunter-gatherer societies is fourteen. This varies quite a bit by culture, but the fourteen figure is met even by some cultures that do not practice arranged marriage. While this is close to thirteen, this is one year later and thus on average subtract one child from the total; additional subtractions will occur due to the large number of women that marry a few years later than average. Even in the most favorable environment for family formation a small number of marriages will involve deaths, divorces, infertility, or sterility, and a small number of people will never marry. This will drag down the numbers a bit more. My best guess is that fifteen per women would be the maximum fertility rate that is easily achievable by a culture that deliberately maximized natural fertility.
What implications does this have for science fiction worldbuilding? Well, first of all the population grows at a torrid pace of 6.1% given fifteen children per woman, doubling the population every twelve years. Over a hundred years they grow by a whopping 356 times; a thousand people would become 356000. Over a second century they would grow to a mind-boggling 126 million. A culture that took to space colonization that bred this fast would quickly become a force to be reckoned with across the solar system even if they had a small population to start and couldn’t maintain that torrid pace for that long.
A Young Society
Another implication for cultures of this nature, which is brought to an extreme with 6.1% per year population growth, is the age structure introducing some problems. We are used to fretting about the old-age dependency ratio, but there is also a childhood dependency ratio; in this society based on my rough calculations only 3% of the population would be over the age of sixty but 65% of the population would be under the age of twenty. 38% would be under the age of ten, and a whopping 20% would be under five. The total dependency ratio, as measured by the percentage of under-20s and over-60s, would be 68%; roughly 50% was the average in the 19th century and will be later in the 21st century, substantially less than this figure.
The median age in this society would be twelve or thirteen, which would be younger than any currently existing society, but interestingly isn’t all that much younger than Niger’s fifteen, the currently youngest country. Playing around with the numbers, truly extreme (and unrealistic) fertility rates are required to push the median age much below the teenage bracket. These sort of societies are dominated by the young.
Another interesting feature of this society is that the only way to lower the dependency ratio to historically normal levels (around 50%, which is still higher than the modern United States) is to lower the working age from twenty to at most twelve, perhaps closer to ten. 47% of the population is between twelve and sixty in this scenario, and perhaps up to 10% is between ten and twelve. If the average age of entry into the workforce were between ten and twelve, the demographics would be financially sustainable; this would also provide a solid economic base to couples in their early to mid teens, something that is nonexistent for all but a tiny minority in modern Western countries.
Back to the worldbuilding implications, a society where the average man enters the workforce at ten, the average woman marries by her mid teens and is nursing or pregnant for most of her adult life, the average person is twelve years of age, the population doubles every twelve years, and the average family has fifteen children looks very different from the norm Western (or by now even Eastern) worldbuilders are accustomed to just by virtue of the demography alone.
Rewilding through Space
Needless to say the kind of culture capable of stimulating ultra-high natural fertility would introduce even more differences; the culture would certainly prize and perhaps outright venerate fertility and childbearing. These are strongly associated with marriage and family and so they too are very likely to be revered, along with sex and beauty. It stands to reason that cultures and belief systems that venerate nature and natural living, perhaps to the point of primitivism, are the most likely to reach the realistic maximum of natural fertility; in real life this mentality usually goes with a rejection of modern foods, modern lighting, modern materials, and modern media. This natural primitivism might seem contradictory to spaceflight, but an ideology or theology of expanding the biosphere into a biospace, enhancing biodiversity, creating life, and protecting against catastrophic risks, meshes very well with space colonization.
There is also the fact that the maximum population Earth can support by hunting and gathering is 136 million or so according to Hunger Math. Thus 99.6% of the population would need to die off in order to rewild mankind’s food supply, a grim prospect to say the least. However, inverting the math we see seventy Earths worth of land are needed; while the solar system doesn’t contain that much Earth-like land, more than enough raw material exists to construct it in the form of space habitats. While it would be an extremely heavy lift on the scale of any realistic near-future space industry, it is technically feasible, especially given centuries worth of build-out. Once again, rewilding and spaceflight work together very well.
Once you also consider that space is the last frontier left to homestead, and that any space settlement not located on a planet (like an O’Neill cylinder) is inherently nomadic, just as hunter-gatherer bands are, it becomes easy to envision an ideology or theology of off-world rewilding becoming popular enough to support whole space colonies.
Building a whole O’Neill cylinder to use it for hunting and gathering might seem extreme, and indeed at first will likely be uneconomical, but as space infrastructure develops and construction becomes cheaper it may come within the range of affordability. Island Three, a big O’Neil cylinder, has 125 square miles worth of habitable area, and Paleolithic population densities averaged 0.1-1 people per square mile, which implies 10-100 people or so per cylinder, an awfully small number of people. An ideal environment, which may be what they are working with, can support up to six people per square mile by hunting and gathering, which implies 750 people per cylinder.
In any case a doubling of the population every twelve years would require constructing a new habitat long-term every twelve years or so; in such a culture the preparation for and construction of new habitats would be a major feature of life and the economy, much like the construction of new Hutterite colonies.
Sustaining an Economy
This means that new resources would need to be actively gathered, ideally on a consistent basis from asteroids or comets, one of which would likely be attached to the space habitat hosting the colony at any given time. The raw materials could be processed into finished components suitable for space habitat construction on-site; this has shades of the Amish or Hutterites except with mining forming the economic base rather than farming. Alternatively, our people here might trade mined materials for space habitat parts, log forests to sate extraterrestrial demand for lumber, or engage in some sort of specialized manufacturing (growing better insulin crystals comes to mind as the most obvious example, but there are many more), service industries (most likely remote), or some sort of scientific or academic research. This has shades of the Shakers or the Jews rather than the Amish, and would certainly provide many interesting possibilities.
Active income earning of some sort will likely be required, as with fifteen children per couple even if each spouse brought in an equal trust fund each child will only receive about one-seventh as much as each parent. Put another way, investments would have to grow by six percent a year just to keep up with population growth; while this is certainly possible, especially in an era of higher economic growth, it means that the relative return to working versus being a trust fund baby is much greater for these people than it would be for an Earthling of the same era. A combination of homesteaded resource extraction and in-house processing, active income earning through work performed for outsiders, and returns from investments will likely be utilized to support the colony’s lifestyle.
The workforce that will be doing these jobs will almost certainly be predominately men, as the women will be pregnant or nursing babies well into middle age, and thus less able to work outside the home. Additionally, the culture in order for this sort of setting to work will likely elevate women’s work and mothering as the greatest aspiration for women, negating the social status gained from working that is a feature of modern societies. This may be offset somewhat by more work being done at home and with workers choosing their own hours in futuristic settings, opening up the chance for even full-time homemakers to earn a fraction of the breadwinner’s salary. Still, this sort of society will strongly expect men to be the providers and women to be the homemakers; this stark sexual division of labor may make a strong contrast to and point of tension with more “normal” space colonies and cultures.
So we see here that just by dialing the fertility up to its natural maximum and setting such a community outside Earth in the solar system of the near future we have created a very interesting place for a science fiction story or worldbuilding project. Admittedly the “maximum natural fertility” scenario is an extreme and unlikely to actually occur in real life, but realistically shades of this could occur in a space-borne version of the high-fertility cultures.
Low- or zero-fertility cultures too, as outlined in the beginning of this post, can be quite interesting, but their demographic and economic models are much more commonly explored in worldbuilding or even in science fiction, although the monastic variant shows up more often in fantasy than it does in science fiction and in science fiction is likely still severely under-explored. Indeed, despite elaborating more on high-fertility cultures than low-fertility cultures in this post, I personally feel like “Space Shakers” holds more interesting unexplored possibilities as a concept than “Space Amish” does.
As for a culture reaching maximum natural fertility, even if it is unlikely to actually occur, sometimes the best worldbuilding occurs when one posits an extreme outcome and reasons out how such a thing could actually happen, blazing new trails for setting and story. These demographic extremes help a science fiction worldbuilder or anyone seeking to tell interesting stories about space colonies consider what possibilities might be out there, and would be a very fruitful avenue for any worldbuilder, writer, or artist to explore.