unknown reasons People who don't want to have a bunch of kids, don't. Does it have to be more complicated than that? Unless everyone makes this decision (which

unknown reasons People who don't want to have a bunch of kids, don't. Does it have to be more complicated than that? Unless everyone makes this decision (which seems unlikely to me although that could just be because I have ten nieces and nephews) it's basically a self-correcting "problem". People who don't reproduce are replaced in the next generation by the offspring of people who do reproduce.