[danieltillett]

The number of children you have is selection (in an evolutionary sense) and because it is not a trait being selected for directly by anyone (we are not trying to breed people that want or don't children), the selection is being done by nature.

The environment of humans has radically changed and we are undergoing very strong selection and adaption to this new environment. In the past the number of children you had survive to adulthood was tied to the amount of food you could bring in (as with most animals). In the new human environment the number of children you have is a consequence of how resistant you are to the cultural meme that children are undesirable.