Countries where life is difficult have high birthrates, because birth control is harder, etc.
Countries where life is easy have low birthrates, because children are just stressful and a waste of money you could spend on yourself!
The exception: religious people in well-off countries have very high birthrates. [1] Being religious provides all sorts of motives to have more kids: less hedonism, more sense of a role for children in one's own life, and a duty to God to produce religious children.
These are generalizations, but it's not too difficult to extrapolate: religions that dissolve in the face of modernism will become less powerful with each generation, whereas religions that become stronger in the face of modernism will become more powerful.
So, look forward to seeing more of the following groups in the years to come:
- Orthodox Jews
- Evangelical Christians
- Muslims
- Catholics