There's a lot of hypotheses, but most seem to fail across one domain or another. The only I've seen consistently explains collapsing fertility across all contex

There's a lot of hypotheses, but most seem to fail across one domain or another. The only I've seen consistently explains collapsing fertility across all contexts is consumerism. The modern world has introduced a completely ridiculous level of advertising everywhere. And this has really driven a boom in consumerism everywhere. What room is there for family in a world where making and consuming Product is gradually becoming the entire point in life? Children greatly constrain your time meaning less time for making Product, and also are a substantial cost themselves (above and beyond the reduction in time) meaning less ability to consume Product. So for instance this cleanly explains the Western income to fertility curve. In particular people who are poor have far more children than those who are middle to upper income, but then once you reach the ultra wealthy then we're back to having lots of children again. Both the top in bottom are unable to participate in the Product chasing game for different reasons. The bottom are unable to afford to do so, while the top have so much money than no carrot is ever out of their reach, so they must look elsewhere for meaning and purpose. It also explains the religion to fertility correlation. Religion not only provides a meaning and purpose in lieu of the almighty Product, but of course itself also tends to focus heavily on fertility and family. It also explains why nearly all modern correlations with regards to fertility simply completely fail when you look at the past where people of all income levels, education, and religious values were having lots of children. It's because consumerism, and its driving force of never-ending advertisement, had yet to really take much of any grip on the world.