A huge historical puzzle is why France, from the 1760s, underwent the demographic transition to lower fertility rates a century before the rest of Europe. It’s a puzzle because economists usually argue that fertility declines are driven by technological progress, making human capital more useful and raising the cost of kids. But pre-revolution France was backwards on most development measures, with half the literacy of England and Wales.
An interesting paper crowdsources family trees to provide the answer: secularisation. Dechristianisation saw the Roman Catholic church lose influence in 18th-century France, with religious giving falling and references to God disappearing from wills. The loosening of traditional religious moral constraints also led to the wider use of contraception (of the coitus interruptus kind).