> Better medical treatments and lack of wars are not child subsidies.

Lower education and religion is probably the biggest cause for these higher birth rates. In the victorian era, people would simply not get married if they didn't have the means to raise children, and they would be childless, and there was nothing strange about that. To give subsidies for everyone to have children comes from the post ww2 politics.

I think the biggest cause is actually high rates of child mortality. Humans have more children when there is a higher chance those children might not survive.

If women in developing countries had financial freedom and access to easy and convenient birth control, then they would not have high birth rates.