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?