Here is something a bit more rigorous related to my Kepler Bb Civilization

I thought 100 humanoids transported with these criteria:

  • 1:1 male:female ratio
  • No children below 15
  • No ill or otherwise diseased people
  • and No pregnant women

would be enough. But now I don't think it is.

Here is a more detailed view of why I think it isn't. Primarily population growth.


First of all, their pregnancy is long. More than an earth year in all cases but lets assume Kepler years are being used. If 1 year is 20 months then their singleton pregnancy is 9 months(just to simplify things).

Here is a list of pregnancy lengths related to number of babies:

  • Singleton: 9 months
  • Twins: 12 months
  • Triplets: 15 months
  • Quads: 18 months
  • Quintuplets: 21 months or 1 year + 1 month
  • Sextuplets: 24 months or 1 year + 4 months
  • Septuplets: 27 months or 1 year + 7 months
  • Octuplets: 30 months or 1 year + 10 months
  • Nonuplets: 33 months or 1 year + 13 months
  • Decuplets: 36 months or 1 year + 16 months
  • Undecuplets: 39 months or 1 year + 19 months
  • Dodecuplets: 42 months or 2 years + 2 months

    This is all the possible numbers of babies in a viable pregnancy from 1 to 12 and their respective lengths.

Now how are all these viable? Well as you can see, for each baby added, the pregnancy extends by 3 months. At the 6 month mark all except 1 baby stops growing. But these are preserved in the womb. Once 1 baby is born another begins growing again until it is born and this continues for every baby in the pregnancy. The singleton is the only special case here. With singleton pregnancies there is no diapause whatsoever.

Inbreeding vs Outbreeding

Outbreeding is what I am aiming for to be the majority. This is why I put the 1:1 male:female ratio as 1 of the criteria. I knew that deviations in either direction from a 1:1 sex ratio lead to more inbreeding. And inbreeding while beneficial in some cases to increase diversity in a low diversity population or to enhance a trait is usually not what you are aiming for.

I have been told to have a 1:10 ratio where every man has 10 women. But this would lead to massive inbreeding in the second generation and beyond and eventually nobody.

Let me give you an example of 22 people in a 1:10 ratio.

Everything goes fine in the first generation but there is lower diversity than there should be because of the low number of males(just 2). In the second generation, not everybody can outbreed and so inbreeding occurs and lowers diversity. Inbreeding leads to more inbreeding until the whole population dies off. Not good.

Graphically it would look something like this after several generations:

Exponential decay

An exponential decay graph

With a 1:1 sex ratio however, there is much more outbreeding than inbreeding in all generations and this would lead to more diversity and higher population.

Graphically, it would look something like this:

Exponential growth

An exponential growth graph

Clearly, the closer the ratio is to 1:1, the better off for the civilization as a whole.

Other Factors

Lots of other things factor in but Illness, despite there being a severe viral illness called Viral TB that can and does affect all systems in more or less a cycle, isn't that much of a contributor to death rate. The reason? These people have a super tough immune system and a fast acting immune response to venom and poison that is beneficial, not harmful, at least in most cases.

Babies and immunocompromised people(in this case immunocompromised would pretty much only be genetic or as a side effect of treatment) are really the only people here with a significant chance of death from illness.

All the other major factors are positive ones(increasing birth rate):

Young and old both. Old moms have a higher chance of multiples and young moms have a higher chance of pregnancy.

Diet. Their diet varies but is abundant in vitamins, minerals, and protein, good and essential for supporting pregnancy with as few negatives as possible. In fact these people can digest bone slowly but surely into calcium, phosphorus, iron, and other minerals as well as whatever proteins, carbs, fats, and nucleotides are in bone(as far as percentage is concerned for those macronutrients). This helps them give the needed calcium and everything else to the baby while using as little of their own bone as possible.

Year length. This one is obvious. The longer the year and the more months in a year, the more that can happen in that year. For example, 1 incredible mom can have 3 pregnancies start in 1 year, all singletons or 2 twin pregnancies start in a year, etc. while a different mom, one that needs wet nurses to be with her at all times, might have 1 pregnancy in 2 years but have 12 babies in that pregnancy. I am not sure what a reasonable percentage is for each type of pregnancy, nonetheless subtypes(MoMo twins, Identical twins + singleton(type of triplet pregnancy), etc.).

Given all of this and that I don't think 100 people is enough(excluding Robin, Lisa, Alma, and the twins) what is a reasonable starting population? I don't want it too big(like a million would be too big for starting a civilization in early times) but I also don't want it too small(like a hundred would, I think, be too small for inbreeding to not be a significant minority(at or more than 20% of total pregnancy rate)).


The initial 100 people transported are not really transported in the sense that a space ship takes them from their home planet to a similar planet relatively close by. They are teleported from their home planet to Kepler Bb which happens to have a similar environment and gravity. Pregnancy could cause complications in any form of interplanetary or intergalactic transport, even if there were no complications from outer space itself. And 15 is the lower boundary for adolescents of this species and obviously if you have ill or otherwise diseased people, this can cause complications whether it be an illness spreading or passing on the disease via genetics or worse, death from that illness or disease.

  • $\begingroup$ Could you elaborate on the initial "100 people transported" for clarification within the question? $\endgroup$
    – Zxyrra
    Nov 30, 2016 at 4:27
  • 4
    $\begingroup$ I think your pregnancy model fails the plausibility check. If life on Earth is any indication, number of children per pregnancy within a given species is not materially connected to the duration of pregnancy. Wolves, dogs, etc., have litters of variable size, yet pregnancy remains the same length regardless of the number of puppies born. In humans, twins or triplets do not result in longer pregnancies than the normal single births. You're free to make up whatever suits your world, but this question suggests you're interested in some level of realism. $\endgroup$
    – Palarran
    Nov 30, 2016 at 4:31
  • 3
    $\begingroup$ Also, genetic diversity is determined by the starting individuals; 100 people is 100 people, whether they have one child a year or five. I'd suggest looking at questions on minimum viable population. That figure varies based on the level of guided breeding (arranged marriages, genetic testing), but I believe the figure is around 5000 genetically distinct individuals for completely undirected breeding. Still, Earth's history suggests even 100 can be survivable, albeit with hazards (see: diseases in the New World spread by Europeans and the ruinous effects on indigenous populations). $\endgroup$
    – Palarran
    Nov 30, 2016 at 4:39
  • 2
    $\begingroup$ Humans have been recorded as having numerous children from a single pregnancy (out of cases where all involved survived, the record is eight children). That had no effect on pregnancy duration, although I will concede that it was very hard on the mother. It's an extreme case, but the point is that variable pregnancy length doesn't make sense unless you have separate pregnancies that are being mistaken for a single extended one: look up superfetation for cases of mammals that can conceive while pregnant (the link talks about a species of rabbit). $\endgroup$
    – Palarran
    Nov 30, 2016 at 4:48
  • 1
    $\begingroup$ Can I ask why you decided to have (for say triplets) sex once, triplets conceived and then diapause + 3 spaced out births, instead of sex then conception & birth of child 1, more sex, conception & birth of child 2, more sex, conception & birth of child 3? Also I hope your Kepler creatures aren't mammals - lactation (breast feeding) is far more energy demanding than pregnancy. Mum is probably going to expire trying to supply milk for 4 babies of octuplets and sustaining the pregnancy for the other 4 at the same time. $\endgroup$
    – DrBob
    Nov 30, 2016 at 21:03

4 Answers 4


Before we address the question, there are some errors that may need to be addressed within your plans. Creative, yes, but not yet perfect.

Pregnancy lengths related to number of babies are costly. You describe keeping infants in the womb but preserving them there after they're fully developed; to keep them as true "twins" "triplets" etc. I am assuming all infants are released at the same time. There are many problems, however, with keeping infants in the womb too long. The chance of still birth will skyrocket if infants are kept in-utero for years (which you suggest in some cases); you will be denying access to stimuli for the most important developmental years in their lives; the moms will become extremely heavy, if not immobile and vulnerable, which evolution will weed out; numerous other systems must be calibrated with gestation to get the species working. Phew. It's easiest to develop them in one litter, with one set time for any number of births, like dogs.

A 1:1 gender ratio may not be ideal. While I didn't completely follow your thinking, I can assure you that inbreeding is inevitable no matter what proportion you use - and the degrees "better" it can be with a 1:1 ratio are negligible. Consider allowing for a slightly increased amount of inbreeding (more women, less men) at the very least to get more workers going in the colony - then diversify if/when new colonists arrive, or use genetic engineering, or recombination, or something to fix the problems inherent with inbreeding.

On to the question

There are many sources that will claim many different "magic numbers" - I will provide the sources, and you can choose what to use.

This question on Worldbuilding placed the number at a minimum of 160 people, but it assumed they would eventually return home to re-mix. Consider doubling it for safety's sake.

This fleshed-out version says you can theoretically make it work with 50 if you regulate the population harshly, 500 if you regulate it somewhat, and 5000 if you seek no regulation / want natural breeding to occur.

See also this Wikipedia page for required vocabulary / information on the topic of viable population.

  • $\begingroup$ The long pregnancies have births separated because of the diapause of all but 1 baby at the 6 month mark and then the birth of 1 baby triggering the next to go out of diapause and develop and when that baby is born, another baby being triggered to go out of diapause and develop etc. until the last baby grows, develops, and is born. This I view as better for large pregnancies(more than 6 babies) than 1 single litter with 1 single birth. I mean if you have 12 babies in 1 pregnancy, either diapause is needed, extremely fast development in-utero is needed, or luck is needed for 100% full term. $\endgroup$
    – Caters
    Nov 30, 2016 at 5:07
  • $\begingroup$ @Caters For clarification how developed do you intend those in diapause to be? $\endgroup$
    – Zxyrra
    Nov 30, 2016 at 5:10
  • $\begingroup$ I intend those in diapause to be able to move their muscles, hear, feel with their skin, and basically do anything a 6 month old human fetus can do(with more development in the muscle coordination so that 2 months after birth they can climb and 3 months after birth they can speak(both of which require control of a lot of muscles whether it be the whole body or just the head and neck muscles and vocal cords). So highly developed in muscle coordination and about the same as a 6 month old human fetus in things like breathing and digestion taking into consideration anatomical differences. $\endgroup$
    – Caters
    Nov 30, 2016 at 5:18
  • $\begingroup$ @Caters That helps but note that the stimuli problem is completely present still $\endgroup$
    – Zxyrra
    Nov 30, 2016 at 10:00
  • 1
    $\begingroup$ Let us continue this discussion in chat. $\endgroup$
    – Zxyrra
    Nov 30, 2016 at 22:55

One factor which is different in your system, so may distort the sex ratio and absolute population size figures, is the need for wet-nurses to support large litters. That would suggest that you need more than 1:1 women to men. If around half the pregnancies were large enough to require two lactating women, then you would need 60% women to 40% men to make that work.

Having given birth to twins, I will also say that it is completely unbelievable that a human woman could function with three or four full-sized infants inside, and another 1-8 growing. My two were relatively small for full-term babies (6 pounds 4 and 5 pounds 11), and I was pretty much useless for the final eight weeks, apart from minor tasks like showering, making food, etc. On doctor's orders I had to lie down at least two hours every afternoon. Doing colonist tasks like chopping wood, farming, etc would definitely have put me into early labour, probably too early for the babies to survive. A third baby in there, and I would have needed a carer to look after me from about month seven of the pregnancy.

I know a woman who had sextuplets, and she was immobilised in bed from about the fifth month, and ran out of space before reaching seven months gestation, so they did a C-section.

If you are planning on having biological humans, I don't think your pregnancy plan will fly.


100 hundred people is the minimum number to avoid any long term problems with inbreeding. Even if you only lose one person, then your population will feel some of the effects of inbreeding. Which is why I recommend you bring more people. To ensure the maximum Groth over the shortest amount time I would recommend about 1500.

Also I would lose the idea of having woman bear more than three children at once. Multiple births but not only the children but also the mother at risk. The more children birthed at once the larger the risk. Now this risk can be somewhat mitigated buy modern technology but still it would be more efficient just to bring more people.


One problem I can see with your story is that you've given your population three or four superpowers, and those powers don't even solve your story's problem. It makes your ability to justify, or handwave, other story necessities that much harder.

For example, your people can choose when they get pregnant, and how many kids they're going to have at once? That's a big deal, it can let them plan their population, avoid giving birth in dangerous times (local scarcity or seasonal extremes, like winter), quickly grow their population when there's plenty and they will need manpower, or keep the population the same when things are tight and there's not extra resources. Of course, it can be a problem when someone misjudges, and there would likely be a harsher stance on infertility (culturally speaking) but that can happen in any society.

Your people can choose to pause a pregnancy (presumably historically used when the tribe is threatened), to avoid giving birth during crises or seasonal extremes? That's a big deal, they can plan birthing times for when there's seasonal abundance, or delay births so as to have more able bodies (not disabled by being heavy with child) when seasonal labor or the threat of battle requires all the bodies that society can field. The equivalent of six months is a really odd time to be pausing a pregnancy, though, it would make more sense to have pregnancies paused as early as possible, preferably before the equivalent of three months (that is, when the fetus is almost totally undeveloped, and the carrying body is neither showing nor unusually vulnerable), or else that it can be paused at any time...though again you'd want a limit on how long, since there are problems with staying in the womb too long, and lack of stimulation at later stages.

Your people are capable of getting pregnant in advance, and bearing kids long after their father has passed away? That's a big deal - especially since people will be lost in a survival situation, and this might mean they can still contribute their genes to the next generation (especially if they can fertilize a dozen or more kids at once). Historically, this kind of adaption would be put into use before wars, or dangerous hunts, or even at the start of married life - assuming that the timing of when the kids actually develop can be somehow regulated or decided (getting or not getting nutrient X or producing or suppressing hormone Y to let the body know when to pause or restart pregnancies).

Your people can overlap pregnancies, and so kids can be born quicker, with less need for extended vulnerability, when the parents (or society) are in a state to support that growth? That's a big deal, especially when you need a population boom, or when you need to regulate the timing of said births. There are times and situations where a single extended vulnerable period might well be a better choice than two separate, longer but less vulnerable pregnancies. Or to have one person at a time down for longer but with many kids, instead of many down for a shorter time with one kid each. Of course, it might make more sense to achieve this by allowing secondary conception while pregnant, which will also give you the choice to vary genetic contributions for the kids, but that's a separate choice.

As a side note, if the body is capable of holding twelve six-month fetuses, but is also incapable of developing even two kids to full term at the same time, I'm just going to shrug at that. Physical capacity matters. Pausing at six months specifically is weird, and the babes are developed enough at that stage to likely run into problems with lack of stimulation when being held like that for extended periods of time (years, in some cases). Six months is also fairly obviously pregnant, with accompanying disabilities and vulnerabilities - and you'd have to be able to afford these women being unable to do heavy lifting, unable to stand environmental extremes, and them and their unborn being extremely vulnerable to some kinds of damage (environmental toxins, blows to the abdomen, etc) for years on end.

On the other hand, if you're people can't make use of any of the upsides of these abilities, that is, when they get pregnant and with how many is by chance, they can't choose when or if to pause pregnancies - then your people are going to die. Because when teleported, they first and foremost need to survive, which is going to be a fulltime job for generations. They can't afford the extended disability and vulnerability in women, for years on end, they can't afford to have people unable to work, or needing extra resources, without being able to say when they'll be back (there's a difference between nine months and two years, and it is less about which it is than the ability to know, and to plan around that) - they literally can't afford even a single pregnancy if it might give a dozen babies, when they only have resources for a few. And dying in childbirth is so very common without good medical facilities, and this kind of extended pregnancy setup will make it infinitely worse, since the body will not have time to rest or recover while it is developing and birthing more kids... and the overlapping development means difficulty with any of the kids threatens all the rest who aren't developed enough to survive. At least with litters, at the most dangerous point (birth) the whole litter is developed enough to survive if they can get born, and with sequential kids only one is at risk, or needs resources, at a time.

Anyway, back to the point I was trying to make - the rules are geared towards a historical scenario, where it was extremely likely for men to die young (so their contributions had to be collected early and birthed over time), but it was also so exceedingly rare for women to be hurt, or die, or even need to do physical labor, that they could afford to be disabled and vulnerable for years on end. Also, your world where the story is being set would need to follow the same sort of setup to make any use of these abilities - maybe a world where all resources were really abundant, but the population went to war all the time? Any one of these superpowers could solve that problem, I have a hard time understanding why you needed all of them - especially since you'd need to explore the downsides if you wanted any realism.

And none of it solves your problem, of starting with a tiny population and figuring out how to make population growth stable. If you were to, say, just let women get fertilized in advance, and only let these fertilized eggs develop when they are in a place to do so - you can increase genetic diversity a lot by having a few of your women have already fertilized eggs (where the original restriction on pregnancy would be "not having a child developing", not about stored fertilized eggs which are not being developed at the time). If you were to allow conception while still pregnant, and overlapping pregnancies that way, you could have varied genetic contributions while minimizing the number of bodies unable to work. If you could pause pregnancies during crises or delay them to take advantage of environmental factors, you could increase your survival by having more able bodies when you need them, and delay vulnerability for when your society can afford it (within limits, of course).

You might also want to rethink your 1:1 gender ratio if you've got a scenario where men are several orders of magnitude more disposable than women - that is, your setup for pregnancies is really only any kind of an advantage if men are dying early and often (so they can get a full set of kids anyway), which will skew your gender ratios anyway. And you'll be having women down and vulnerable for really extended periods of time for childbearing, not to mention the extra energy it will take for nursing, and also rearing the kids (up to a couple years old, which is really active) while still pregnant and nursing and bearing their siblings? yeah, these moms will not be doing anything, anything, but producing babies. Someone else will have to be running around after the kids, playing with them, feeding everyone, cooking and cleaning, and also taking over whatever job the mom was supposed to do (and for generations, "jobs" will be about survival necessities, not things that can be easily set aside for a couple years). So you need extra people to be nursemaids and child-carers and to do the jobs left undone by women who are pregnant - if you're also gendering jobs by risk and ability (which the society would have to, why else could they afford extended vulnerability in women and demand redundancy in men) - then you might need more men since you'll be losing lots of them and having their kids postmortem, and/or more women so fewer can be pregnant at any given time, depending on how you want your story to run.


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