An interesting generalization from fertility data (chart below) at Our World in Data:
We also see from the chart that the speed with which countries can achieve low fertility has increased over time. A century ago it took the United Kingdom 95 years and the US 82 years to reduce fertility from more than 6 to less than 3. This is a pattern that we see often in development: those countries that first experience social change take much longer for transitions than those who catch up later. Countries that were catching up increased life expectancy much faster, they reduced child mortality more quickly and were able to grow their incomes much more rapidly.
I’d say this is rather intuitive. At the least, it jives with our life experiences with regard to any new frontier–a new skill, a new method, a new process or way of thinking. The pioneers of anything have the hardest time, carving paths where none existed before. The rest of us follow on trodden, smoother ground.
Social change, especially, isn’t easy. Lots of fear involved. Lots of barrier-breaking and throwing off long-held traditions. But once the guinea pigs make it through unharmed, others gain confidence.