I've been wondering what a world similar to ours but with the leading species (otherwise similar to humans) having a reproductive system that naturally avoided overpopulation.

My idea is that (1) almost every conception would produce fraternal twins, (2) almost every birth render the mother infertile (maybe the fallopian tubes would pass the cervix and snap during birth), and (3) the sex ratio of viable sperm depend on the local population levels (by pheromonal effects? or stress levels, perhaps), such that when circumstances are good, more girls are born, carrying more wombs and allowing the population to grow in the following generation.

So the first two differences would mean that reproduction would be around replacement level, with the possibility of gradual growth; the third one would mean that overcrowding would reduce the reproductive rate.

I can see that society would have to have some matching changes; for example, all women of this species would have to be expected to "do their duty" (like Roman matrons); it wouldn't work well with monogamy, but perhaps the species would regard the twin-bond as the primary bond, and reproductive bonding as less important (or even as perverse).

Could this work? Would it produce a pleasant world? Or would they die out quickly? Would the maths of the reproductive rates work, or would they oscillate wildly, with undesirable social effects?

  • 2
    $\begingroup$ Define "over-population". And while you're at it, why are you coming from the point of view that humans are suffering from over-population? In point of fact, high birth rates are generally a response that "balances" high mortality rates, and resource scarcity is a pretty effectively balance against over-population... which brings us full circle to the question of over-population compared to what? $\endgroup$ Jul 11, 2016 at 19:54
  • $\begingroup$ Fecundity is one of the basic features of evolution on earth. You are proposing a species that violates that feature. Does that mean all species on this world are self limiting in this way? This matters because the details of the species will need to change based on the assumption. If you don't care, that's fine. But if its critical to you one way or the other, please let us know. $\endgroup$
    – Jim
    Jul 11, 2016 at 22:57
  • $\begingroup$ The propagation of a species is only restrained by the resources it can collect. You have to make a connection to resources to explain this convoluted reproduction inhibitors as "beneficial". $\endgroup$
    – user458
    Oct 26, 2023 at 19:07

2 Answers 2


One thing to consider is the child mortality rate within this species. http://data.worldbank.org/indicator/SH.DYN.MORT?end=2015&start=1960&view=map If like humans, early on the mortality rate would be extremely high, and the species would die out. And this is just "before 5." They still have to reach mating age.

Basically it would be the ratio of females to males, F/M, multiplied by the percentage of females that survived long enough to mate (and then not see pregnancy issues that killed the children).


(F/M)*(Percent of female survived to reproduce) * 2 >=1

...would be a perfectly mathematically stable species. Less than 1 would imply a decline, assuming all females did, indeed, mate once of age.

Without knowing the pre-reproduction mortality rate and the exact female to male ratio I can't say.


As a side note, in rural families in the not-too-distant past, it was common to have many, many children to help out on the farm, expecting a significant portion of them not to survive early childhood. I know I heard stories about brothers and sisters dying from both my late grandmother and grandfather (both of whom grew up in rural America).


First off, the world you are in would have to be very pleasant. A replacement rate near 2 is quite extraordinary. First world humans don't even have a replacement rate that low. I've heard its around 2.6.

The trick you mentioned about balancing the sexes would be the key. Since the interesting physiology you are exploring is specific to the women, the species would need to maintain a replacement rate of women higher than 1-1. Our females on our world need to have a replacement ration of 1.3 daughters per mother to keep up. This can be accomplished if your sex determination system was balanced towards 65% females, 35% male.

Of course, the society would be very dependent on the exact rate. If you increase the rate of females, it becomes less important for any one female to "do their duty," because the system can catch up as long as the average number of daughters per mother stay high enough. If a war or famine ravaged the female part of the community, "duty" may become a bigger issue.

One side effect of this is your alien species would, without a doubt, not share the nuclear family we are oft used to here. There simply aren't enough males around. If they were to have the equivalent of our nuclear family, it would be one husband with two wives.

I'll let you decide whether that would be "pleasant" or not.

  • $\begingroup$ Your ratio is probably too conservative. I suspect each male would have to mate with 3 or more women. And since the pregnancy would only occur once, would the male even stick around? This breeding population may more resemble a lion pride than a human community. Males would undergo a rigorous culling process and the fittest would get to knock up a lot of ladies in a "once in a generation" orgy that produced mostly daughters and a few males that would get to participate in the next orgy when they matured. $\endgroup$
    – Jason K
    Jul 12, 2016 at 13:59

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