In the 19th century, a watershed demographic transition took place in several countries: fertility rates dropped significantly. Before the transition, and for most of human history, populations were trapped into cycles of over-expansion and stagnation — but during the 18th and 19th centuries, several countries underwent sweeping transformations. These transitions ultimately brought unprecedented technological progress and mass education, which led to the development of urbanization and essentially paved the way for modern times.

But in France, this demographic change started a century earlier. It’s one of the most puzzling events in human history, and one that, until now, had no satisfying explanation.

According to a new study, there wasn’t any biological or economic reason why this demographic trend started. It was secularization — people caring less about religion.

When we think about the major changes experienced by human society over the centuries, we tend to focus around inventions or technological paradigm shifts. We talk about the “Iron” age, or about the Industrial Revolution. But demographic changes are just as impactful. Before birth rates dropped in the past two centuries, improvements in the standard of living were offset by demographic expansion, says Guillaume Blanc, a researcher with the University of Manchester.

It’s extremely curious that the demographic transition, an essential condition for development, took hold in France more than 100 years earlier than in any other country. The first consideration would be the Industrial Revolution, the transformative period that marked a significant shift from agrarian, handcrafted economies to ones dominated by industry and machine manufacturing. But the Industrial Revolution started in the United Kingdom, not in France.

When its demographic transition started, France was relatively poor (compared to its peers). It was also rural and largely illiterate. In fact, the UK saw a temporary increase in fertility right after the industrial revolution (around 1760), so this couldn’t have caused the decline in France.

So then what was it?

Blanc believes it has a lot to do with religion. In his research, he used crowdsourced genealogies to comprehensively document the decline in fertility & identify its origins for the first time.

“In aggregate-level evidence, relying on the work of historians I document an important process of secularization that took hold at the same time in France,” the researcher explained on Twitter.

“With dechristianization (in the mid-18th century) the Catholic Church lost influence and could not oppose fertility control—coitus interruptus—anymore,” the researcher adds.

The loss of influence of the Catholic Church is first attributed to a backlash against elites. In particular, people started becoming more antagonistic towards religious authorities associated with an absolutist, “divine right” monarchy. Even before the French Revolution happened, this antagonism grew.

Secularization had a larger impact in more densely populated places, and wherever it had a larger impact, fertility rates dropped faster.

This carries some remarkable implications. For starters, if France had followed the same demographic trends as the UK (the two countries are similar in many economic ways and are often compared), it would have 250 million inhabitants now. Currently, it has under 70 million. Secondly, although the UK was the cradle of the Industrial Revolution, and although the country made impressive inventions that elevated the country’s economy, the economies of France and the UK grew at a comparable rate during the Industrial Revolution.

This demographic change in France was a pivotal moment not just for the country, but for the entire continent, given how influential France has been in Europe. In fact, given that what followed was a colonial age where both the UK and France played active roles, this event probably affected the entire world.

It’s not just technology that enabled an increase in living standards. Lower birth rates were a major reason for increasing standards of living for most of mankind’s history.