As countries develop, they gain access to education, healthcare, economic development, and urbanization. This leads to prioritization of careers, no need for children to help on farms, and more women in school. The fertility rate in developed countries hovers at 2 whereas the fertility rate in developing countries hovers at 5. If fertility rate is 2 this is called replacement fertility because the 2 people that brought 2 kids into this world replaced themselves after they died. The whole puzzle changes once lifespans are increased as population is predicted to inevitably grow at that point. But who cares about head count once we're a sustainable interplanetary species? We'll never run out of flying rocks orbiting hot balls of hydrogen to colonize.
