BTDT wrote:
https://www.quora.com/What-does-Bill-Gates-mean-when-he-says-that-we-can-reduce-world-population-through-vaccines-healthcare-and-reproductive-health-services
It sounds counter-intuitive, but the more likely kids are to stay alive, the more likely that populations are to stabilize. In other words, if it is quite likely that your kids will grow up and have kids, the more likely that you will just have one or two kids and use birth control to avoid having any more kids. If you are poor and there are diseases that may kill your kids due to the lack of adequate vaccines or other measures, you are more likely to have a lot of kids in the hope that one or two will survive and care for you when you get old.
Economists say that populations stabilize when economies are industrialized. When nations have no industries, the resulting poor families are left to have more children in the hopes that some of them might live to adulthood and care for the families (more like-minded people foraging to survive). Industrialize the world's nations and watch the bogeyman of overpopulation fade.