This may be in part by evolutionary pressure. I don't have the link anymore, but I was reading that many species have an evolutionary limit to population size -

This may be in part by evolutionary pressure. I don't have the link anymore, but I was reading that many species have an evolutionary limit to population size - and you would think it wouldn't apply to humans, but it may. The thought goes that as humans' biological needs for survival are better met, the reproductive needs trend downward.