Back in the 1800s, women all over the world had, on average, 6 babies.
European women were the first to start having fewer babies.
Why?
The Industrial Revolution started in Europe.
This graph shows that as each region developed, women in that region have fewer babies. Today, highly developed nations have low total fertility rates: 1.4 in Europe, 1.8 in the US, even 0.8 in South Korea. Regions that have a higher birth rate simply started developing later.