Well, there is a mechanism that largely prevents this, but it isn't genetic. Say you're a rabbit, and you have a bunch of rabbit babies, and they eat all the g

Well, there is a mechanism that largely prevents this, but it isn't genetic. Say you're a rabbit, and you have a bunch of rabbit babies, and they eat all the grass and leave less grass growing. Their babies will die young and the population will stabilize around necessary resource availability. What happens is increasing infant mortality and starvation. The interesting thing is, you never actually get to a global state of overpopulation, you get osculating population numbers between generations that are very stable. The stabilization of human population now is the same phenomenon, but also has to do with technological advancement. As standard of living rises for a population, they become accustomed to abundance. This abundance is different from times past, in the past the more children you had the more labor you had so the more abundance you had. This created more pressure to reproduce than to reduce the number of mouths you need to feed. Since resource availability is becoming decoupled from availability of human labor the opposite incentive now dominates, and people naturally reduce reproduction to maintain personal comfort. This has the emergent, not deliberate effect of doing the same for the larger population.