I'll try to keep this concise and focused on the question at hand.

A generation spaceship leaves Earth around the year 2060 on a journey to Alpha Centauri A. Thanks to fusion power and other new tech irrelevant to this question, the trip should take 110 years. There is only a max crew of 54 to keep ship size down. Genetic diversity is available in frozen embryos, to be grown with new tech once they are dirtside.

One of the many rules to ensure survival with such finite resources is a Birthing Queue. Birth control is mandatory until a death occurs, and the next hopeful mother that voluntarily registered on the Queue is allowed to become pregnant.

Reproduction rights are a very controversial issue on Earth. Enforcing it is no easy task. The first crew (born on Earth) may be highly trained or mission motivated... but subsequent generations did not choose to be stuck on a spaceship. Intermediate crews that will never see a planet may not respect the abstract idea of a government from some mythical home planet. I can see a lot of things going wrong on ship, and this issue will definitely be a part of the branching storyline of the RPG I'm working on. I'm curious what others think. I've also read some interesting books with generation ships, such as Ark and An Unkindness of Ghosts most recently.

Would the crew of a generation spaceship obey population control for a century, or would they feel justified (for various reasons) to break it?

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    $\begingroup$ I suspect this will be closed on the grounds that it asks to speculate about possible character action instead of making the situation work. $\endgroup$
    – rek
    Mar 17, 2021 at 4:08
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    $\begingroup$ This question really needs to be changed a lot. Asking "what would happen" and "how would people react" is not really answerable. It's story-based - if you think that generation 3 will spend their time in different tribes, then you can write that story. However, you can ask how to keep the generations focused on the objective. $\endgroup$
    – VLAZ
    Mar 17, 2021 at 8:08
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    $\begingroup$ It seems that you base your reasoning on an assumption that your crew members will definitely want to have children and oppose birth control. This is not going to be the case at all. Perhaps, your male crewmembers are those macho men who are unaware of contraception and do not want to use it. But this will not apply to women on your ship. Birth control use and low reproduction rates strongly correlate with levels of education of women: The higher level of educational attainment is the fewer children women have. Moreover, highly educated women tend to postpone pregnancies and plan them [cont.] $\endgroup$
    – Otkin
    Mar 17, 2021 at 19:48
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    $\begingroup$ [...] in advance. Since your ship only has a skeleton crew (54 people is a very low number for a generation ship), everyone will have to have extremely high levels of education. We are talking 2-3 degrees for each crew member. The crew will also be very busy even if your ship has a much higher degree of automation than what we have now. In such a situation it is hard to imagine that fertility on your ship will be above the replacement level (about 2.1 children per woman if the sex ratio is 1:1). So, it is likely that not only there would be no problems with the Birthing Queue, but [cont.] $\endgroup$
    – Otkin
    Mar 17, 2021 at 19:54
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    $\begingroup$ [...] your female crewmembers would need to be incentivised into getting pregnant and raising children. This will be the case for all generations if educational levels continue to be high for women. Your ship and crew are too small to develop a culture where women are not allowed to get an education, so this should not be a problem. Your real problem is lack of children and even more so after the arrival at the final destination. Unless you have artificial wombs it would be very hard to have the high birthrates necessary to expand the population quickly. $\endgroup$
    – Otkin
    Mar 17, 2021 at 20:02

4 Answers 4


Mostly yes

There are 3 crew dynamics that will work for a long term multiple generation mission like this:

  • Military. High discipline and chain of command. Children's education works towards military recruitment in late teens. People will be following orders their entire life with clear disciplinary consequences for breaking them. Refusal to be on birth control, or abort an unexpected pregnancy, is disobeying orders. An example of this in sci fi "Ascension".
  • Religion. Pilgrims go on a mission to spread the word of God (or their own view on it anyway) through missionary preaching of scripture (if life is found), otherwise through their action, or just being fruitful and multiplying. Religion is the centrepiece of ship community and everyone is religious and a spiritual motivation for the journey is taught to children. Population control usually conflicts with religion but a "for the will of god" argument can be made to allow it. An example of this in sci fi is mormons in "the expanse".
  • Tight small social group, < 100 people. Everyone has a very close connection with let's say 10 people (parents, partner, children, close friends, closest work mate), and through 2 levels you get almost the entire ship (the graph will be imperfect but close). Motivation here isnt by some external factor like Earth or God, you are doing your part so that your close connections survive, and so that they keep their connections alive. Nobody wants to see their partner suffer the loss of their mother, or their child suffer the loss of a best friend. "Good of the collective" arguments work very well when you have 2 degrees of intimate connection to the entire collective.

So yes people will mostly stick to birth control, to either: avoid being court martialed, avoid letting their God down, or avoid jeopardising the safety of their entire social network.

The odd accident will happen, condoms will break, implants will run out of hormones earlier than expected, people will be allergic to one optimal form of control and use a less effective one, and children will have sex earlier than expected. These will be quickly fixed by the doctor, typically using chemical abortion.

The odd intentional rule breaker typically can't hide their pregnancy in such a small community - all 53 others would essentially need to be complicit in it, and act against their own self interest by allowing it to continue.

  • $\begingroup$ A 100 is less than Dunbar's number, so even not worth 2 handshakes, they will know each other inside out $\endgroup$
    – MolbOrg
    Mar 17, 2021 at 7:58
  • $\begingroup$ There is no need for all of this at all. High levels of education for women naturally lead to low birthrates when effective contraception methods are known and available on demand. It holds true for every culture on Earth. $\endgroup$
    – Otkin
    Mar 17, 2021 at 20:09
  • $\begingroup$ great points ash, and 3 very different aspects that can contribute to the society, ty! $\endgroup$
    – Koon W
    Mar 18, 2021 at 1:33

but subsequent generations did not choose to be stuck on a spaceship.

For all practical purpose, subsequent generations ties with Earth's are weaker than that with the spaceship. They derive their memories, culture and norms growing up with other highly trained staff of the spaceships, which helps perpetuate the required values within the newer set.

Also, its not like a few individuals can turn the ship around by just birthing a few more babies out of embryos and no one notices while that happens.

Would the crew of a generation spaceship obey population control for a century?

You can look into social marital systems for inspiration. In feudal ages, it was accepted practice for the young lords and nobles to marry within nobility. While exceptions would always be there, the general mindset would be to follow these norms. These kind of marital arrangements remained in place for 100s of years, and there is good chance that that can be the case with accepted cultural practices on your spaceship (list for motherhood).

or would they feel justified (for various reasons) to break it?

Individuals may break it, but the larger group will not, because ultimately such selfish acts affect the stability of the ecosystem itself. At 54 crew members, everyone knows everyone, so IMO, the rewards for such an act knowingly are grave (you end up loosing someone you know).

To counter this,

  1. Pregnancy shows up on various scans, and thus, if there is a fear of such individual acts, the ship can have a policy of monthly gynec checks for the females

  2. The ship can be all female (since frozen embryos are used, no need for males per se, and embryos can have sex selection), which can also keep hormonal interactions in check.


You are assuming every pregnancy goes through successfully, and that all women have equal probability of conception (even through embryonic transfer). That is rarely the case in real life, and the probability could be further reduced by adverse events on pregnancy due to zero gravity. So rather, you would have a probability of success, (say 0.25), and accordingly get 4 women pregnant, and plan for resources accordingly (in case all 4 succeed).



Allow me to introduce you to a phrase I coined: Technology Dichotomy. Your question represents such a dichotomy.

Your civilization has the technological capability to successfully send a generational ship to another star system. Since no technology evolves in a vacuum (independent of all other technologies) and given the reality that such a journey would involve pretty much every technology ever invented by humanity, then you obviously have the technology to introduce whatever chemical/nano-tech/subliminal-messaging/etc is necessary to guarantee controlled conception.

It's important that such a question should (and won't) have anything to do with personal motivation, ethics, or morality, because no sensible government or space agency would ever leave such a potentially catastrophic decision in such a state of unpredictable limbo. Of the many reasons that astronauts today wear body/health sensors, one is that you can't trust the human to report when there is a problem.

It's certainly true that all kinds of training (even for the intermediate generations) would occur to maintain discipline and structure — but the idea of simply training away a biological imperative has been fundamentally proven impossible by every religion practicing celibacy. You can trust that some would have the fortitude to do it. You might even trust that most would have the fortitude to do it. You can't trust that all will. Not ever.

So, on the premise that your civilization must be as medically advanced as they are in all other aspects of space-faring engineering, I must assume that the technology exists to force controlled conception. To suggest otherwise is a technology dichotomy along the lines of assuming a time travel machine could be (accidentally) activated with a single toggle switch.

I should point out that a better question to ask might be, "given what we know about human physiology and what we can anticipate as likely future technologies, what biological controls could be implemented to manage conception?" Consequently, a great sub-plot of your story would be how a love-struck couple desperately wanting a child circumvented that control, throwing the entire mission into jeopardy.

  • $\begingroup$ Downvoted because "technology to introduce whatever chemical/nano-tech/subliminal-messaging/etc is necessary" is pure handwavium. $\endgroup$
    – jamesqf
    Mar 17, 2021 at 5:26
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    $\begingroup$ @jamesqf A generational spaceship is pure handwavium. No offense. How the technology is manifest is only window dressing (unless you know something about the world's current space faring tech I don't know about). Did you miss my point that if you have tech A then you must necessarily have tech B? Or did you just focus on the fact that I didn't explain how tech B would be manifest? $\endgroup$
    – JBH
    Mar 17, 2021 at 5:28
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    $\begingroup$ Upvoted because it directly refutes the Question's assumption that today's controversy must be an eternal question. $\endgroup$
    – user535733
    Mar 17, 2021 at 13:45

TL; DR: It's an authorial fiat, but there are ways:

Small population:

With a population of 54, a prescribed queue similar to what you describe is a likely cause of problems. 54 people in a very confined space are going to know each other's lives intimately, especially since privacy is likely to be at a premium. It's is also a small enough group that their culture is subject to a significant amount of change, depending on any single popularity contest. You might have at most two degrees of separation between people. Everyone will know.


The attitude toward Mission Control will also play a part. There are likely to be some who willfully reject "the Mission", especially when the light-lag increases and replies take almost nine Earth years. However, up until the final few years, keeping additional children is going to get everyone on board killed eventually, including the parent and child.


Suppose that someone - perhaps the doctors, so they can keep it secret for a while - decides that the birth-control system is not for them. Their decision affects everyone, and there is a strong chance that the crew will have to make some kind of traumatic decision.

Mission Control:

Over 110 years, with future-medicine, you'll have perhaps two generations bred. As the destination nears, a mostly-female crew will be useful, to breed as fast as possible. Carrying embryos almost eliminates the genetic-diversity problems usually associated with this, since the children aren't genetically related. Mission Control is likely to promote a culture of "breeding is duty" or "is good" to help the colony grow, and promote an anticipation of the Arrival as when it should begin. So most females will breed about twice in their shipboard life.

"Arrival" culture, which is built on the ship:

That small number of males in the early colony means that popular-sire syndrome (few males breed disproportionately often) is a real risk. The frozen-embryo stock will eventually run out, so it'll also be important to keep a few males mixed into the population throughout in case of later cultural shocks. Why is this important? Culture doesn't change overnight, and keeping your males permanently sterile is the most effective way to ensure that your genetically-diverse frozen population is the one which is born. This would begin on the ship, and possibly on Earth.


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