Aliens have abducted 10,000 people and placed them on a generation ship, a really big one, for the purpose of seeding the stars with our species, which is nice of them. What ratio of men to women should the aliens choose in order to end with an even sex ratio at then end of a journey lasting 500 years?

Assume any form of society you please. Awesome alien tech takes care of all sorts of otherwise intractable problems.

Edit -> More detail:

Our alien friends want to deposit a balanced population on a habitable world before sending the craft back to earth to pick up another load. But to their way of thinking, having excess males when one can impregnate many females is a wasteful extravagance. They are looking for efficient population growth.

Think of chicken husbandry as an analogy if you wish: many hens, one rooster (or in this case, many roosters and many, many hens).

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    $\begingroup$ As ZioByte says, the ratio will quickly even out. Each generation will be roughly 50% and the only skewing will be in the starting generation who will all be dead within 100 years. Why are the numbers even skewed in the first place? Are they trying to maintain a population of 10000 for the entire journey, are they looking for maximum diversity, or starting with 10k but hoping to end with millions? I suggest adding more detail of what you are looking for. $\endgroup$ – user2259716 Jul 27 '17 at 22:05
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    $\begingroup$ You're assuming that the natural sex ratio (50:50) is somehow not the optimal ratio to ensure our continued survival? Even though it was selected over eons by a system geared specificially to doing so? $\endgroup$ – AngelPray Jul 27 '17 at 22:05
  • $\begingroup$ What is your the capacity of the ship? What are the humans expected to do for the 500 years. If they hope to have a billion humans on arrival they may want to pick a lot of ladies to start, but if we are cargo hoped to be constant it really doesn't matter. $\endgroup$ – user25818 Jul 27 '17 at 22:06
  • $\begingroup$ close voters: i think this question is perfectly answerable. $\endgroup$ – Burki Jul 28 '17 at 9:06
  • $\begingroup$ @Michael Morrison: around here it is often encouraged to wait a little longer before accepting an answer. Often interesting approaches to a topic are being posted within a day of asking. $\endgroup$ – Burki Jul 28 '17 at 9:08

Well, with 500 years or so, you are talking about 25 generations. Over that many generations, You might take 9,999 Women and one man and you could theoretically make it to your 50/50 gender distribution. You would have all kinds of genetic problems arising from a single male ancestor, but that doesn't affect the math.

Gender, biologically speaking, should fall out roughly 50/50. The Female contributes x chromosomes. The Male can contribute one x or one y to determine the sex of the offspring. None of this is shocking, it's middle school biology.

So if each new generation has a roughly 50/50 split, it's only going to take a handful of generations to render the beginning distribution irrelevant.

The beginning distribution does matter when it comes to a whole host of potential problems that could come from in-breeding though. A single common male ancestor, 25 generations later, can cause a whole host of genetic problems.

In addition, 10,000 may not be a viable starting population. I can't remember where I read it, but an ideal starting population was somewhere around 42,000 to ensure a minimal ideal amount of genetic diversity. Something to think about.

  • $\begingroup$ Thank you for your excellent answer. Genetic disorders are a concern so your reply is helpful. Perhaps a starting population of 50,000 is more reasonable. How many men to insure sufficient generic diversity? $\endgroup$ – Michael Morrison Jul 27 '17 at 22:31
  • $\begingroup$ Fallout Bible mentions Vaults 68 and 69 with populations 999-1 and 1-999 male-female respectively. Let's just say that leaving 999 women with 1 man didn't end well. $\endgroup$ – Mr Scapegrace Jul 28 '17 at 9:03
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    $\begingroup$ Genetic diversity would be basically met with 50 or so men. That's not actually a serious factor. The bigger one is how fast your ship of limited size can fill up. 50 men to 9,950 women (assuming our eager men are (ahem) up to the task) can maximally produce 9,950 babies every 9 months for a period up to, say, 10 years (eventually the woman's body gives out). That means in 5 years you have a maximum population of about 60,000 people... almost all of whom are under the age of 5. That means child care is a huge issue. Almost everything is more important to the scenario than genetics. $\endgroup$ – JBH Jul 28 '17 at 9:14
  • $\begingroup$ @JBH but what about the...eeeewwwwww factor :) You may be right about the genetic diversity thing, but I would think that the closer the parity initially, the better off you would be if you are talking about a normal spread of potentially bad genetic markers. Now if your benevolent aliens screened for all those potentially bad genetic issues you could get away with a massive imbalance. $\endgroup$ – Paul TIKI Jul 28 '17 at 13:07
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    $\begingroup$ It's quite a bit less than that. 1500 is sufficient without controlling pairings. 50 if you do. $\endgroup$ – Draco18s Jul 28 '17 at 14:07

It doesn't matter what you chose, ratio will be 50% after one generation regardless (unless aliens do something really nasty like gender-related abortions).

If this is not the answer You're looking for please specify better.


For all vertebrates reproductive balance is 50/50, with no exceptions.

Chicks are born evenly matched (so many would-be-rooters and the same number, on average, would-be-hens). Reason why you have "many hens, one rooster" is because we kill (and sell as meat) all males. In the wild males use several techniques to scare away or simply kill off competition. Exception is when it's needed a combined effort to give next generation better chance to survive; there you have stable couples (family, if you want).

In our species spermatozoa (from males) are responsible for sex of newborn and our testicles produce exactly the same number of Y-bearing (male) spermatozoa as X-bearing(female) ones. Gametogenesis (Meiosis) guarantees that.

If "aliens" want to "increase throughput" have a few options:

  • Somehow kill Y-bearing spermatozoa before insemination (not easy task as there are billions of them).
  • Provoke abortion of male embryos (easy and well tested, usually on the reverse sex, as happened recently in China).
  • Sell males as slaves or raw meat (I would think such an advanced race doesn't need human slaves, but we could be a real delicacy to them, who knows?).
  • Have males to fight to death until the ratio is "right" (they would make billions of credits broadcasting live in galaxy-vision!).

All gruesome alternatives, but perfectly viable, from alien viewpoint.

Al they have to do to ensure balanced distribution on arrival is to stop interfering about 50 years before arrival.

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    $\begingroup$ Well, technically after 2 or 3 generations (1 lifetime). If everybody starts off at age 20, after 1 generation (20 years), the imbalance of the first generation will still be felt, unless you also specify that everybody dies at 40 or so. $\endgroup$ – WhatRoughBeast Jul 27 '17 at 22:49
  • $\begingroup$ If aliens are as ruthless as described I would expect them to suppress females as soon as soon as "production" slackens, exactly as happens to hens not lying eggs anymore. $\endgroup$ – ZioByte Jul 27 '17 at 23:03
  • $\begingroup$ And, sorry, but humans do not display a 50/50 ratio. The norm is about 105 to 107 males per 100 females. See Coale, "Excess Female Mortality and Balance of the Sex in the Population: an Examination of the Number of Missing Females". This is in the absence of sex-preference abortions, and occurs in populations (such as the US and Europe) which do not practice such procedures. Furthermore, post WWI, WWII, Korea and Vietnam, US births showed a temporary increase in male births wrt female births. Nobody knows why. $\endgroup$ – WhatRoughBeast Jul 27 '17 at 23:13
  • $\begingroup$ And as for "suppression", that doesn't work for high K populations such as humans. You need mothers to be around for long after birth. $\endgroup$ – WhatRoughBeast Jul 27 '17 at 23:16
  • $\begingroup$ @WhatRoughBeast: AFAIK there's a small difference between male and female spermatozoa: female ones are slightly bigger, a bit slower and more resistant. This means if copulation happens very close to ovulation or after it there's an edge for males, while if coitus happens a few days before ovulation there's an edge for females. This might account for very small deviations from equality. About "suppression": in a "high yield society" (10+ children) mother is required for no more than one year. I doubt such aliens would be really interested in correct childcare. $\endgroup$ – ZioByte Jul 27 '17 at 23:21

Assuming 1: no eugenics, or breeding for some favorable genotype 2: no considerations of cultural impact due to skewed sex ratio in the population. The humans are treated like livestock.

Consideration then is only for the maximization of genetic diversity, as maximum genetic diversity will maximize the chance of some subset of humans doing well in their new environment.

There should be 10,000 women and it should stay 10,000 women for the whole trip. Men will be represented as banked sperm, which greatly increases the effective population you can bring. Banked sperm from 250,000 men will be used to produce all pregnancies en route. Sperm sorting will be used to make sure all generations born on ship will be entirely female.

The last generation (the new world colonists) will have some males as presumably frozen sperm aboard the ship will no longer be available to the colonists once on their new planet. If only small populations are placed at any one site I think it should still be an unequal ratio (maybe 1 male / 5 females) anticipating higher mortality among females due to childbirth. If all 10,000 are deposited at one site 50/50 might be fine.

  • $\begingroup$ Good interpretation and solution of the problem. I had not considered artificial insemination. That solves the cargo problem of otherwise indolent men. $\endgroup$ – Michael Morrison Jul 27 '17 at 23:09
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    $\begingroup$ Perhaps include one robot artificial man for the trip, in case there are pickle jars to open. $\endgroup$ – Willk Jul 27 '17 at 23:32

Do they want the population to remain stable? If so, then 1:1. If they want an initial growth spurt to fill the ship then more women to men would accommodate that. Though I think that any ratio over 10:1 would get silly.

However, in the long tern, ZioByte is correct. Unless you mess with the results, the ratio will narrow to 1:1 within a generation.

In our past, difficult child birth (lack of proper medical procedures) would decrease the number of women. When the number of men got too high relative to women, we would have a war that would decrease the number of men. The ratio would yo-yo like that until we learned proper birthing procedures.

  • $\begingroup$ Thank you for your answer and pardon my insufficiently detailed question. The aliens are looking to populate a whole world so they would like lots and lots of young folks when the ship arrives. $\endgroup$ – Michael Morrison Jul 27 '17 at 22:33

500 years is a long time. Since you want a generation ship rather than cryostasis, reproduction should not be your primary concern.

What you really need is a stable population, in more than just one sense. Birth control is a thing, and as long as your initial population has access to the means of their choice, it will be applied. Where necessary, some gentle pressure may be applied, but not much will be needed.

But if you consider having a largely uneven female-to-male ratio, you are creating a very strong basis for emotional conflict that will be some order of magnitudes harder to balance than any possible birth rate problems.

Jealousy is a thing, after all.
So unless you want to encourage large-scale conflict and fighting, yo should populate your ship with a well-balanced mix of males and females.


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