Conclusion:

1. Human populations are under a change of cohorts. I think people who follow the rules of communities can increase their own number. Such people can be well off in their communities. And this world is getting closer to that before the 20th century.

2. Related contents:

2.1. Elizabeth Brown and Alfred Dittgen "Fertility of married and unmarried couples in Europe"

2.2. Why birth rates are dropping everywhere (This may be more helpful: Why the world’s population is heading for collapse )