Why do fertility rates fall? All over the world, in rich countries, in middle-income countries, and in poor countries, fertility rates are falling below the replacement level of 2.1 lifetime births per woman.

To give an idea of how widespread the decline is, here are the figures for a few countries. In the United States, the rate is 1.8; in Australia, 1.8; in China, 1.7, in Thailand, 1.5; in Iran, 2.1, and in Russia, 1.8. Only in Africa are there countries where birth rates are far above replacement level. The rate in Kenya is 3.3; in Nigeria, 5.1; and in Niger, an astonishing 6.6.

Demographers associate the decline in fertility in advanced economies with their “demographic transition”. As a country becomes more industrialised and richer and healthier, birth rates begin to fall. There are complex reasons for this, but basically people invest in quality rather than quantity. Because more children survive, parents can afford to have fewer of them and invest more in their education.

Social scientists began to notice this in the 19th century during the Industrial Revolution. The conventional wisdom was that the standard of living was the main driver of fertility decline.

However, recent research by a French economist at the University of Manchester challenges this. Religious commitments may also be important.

In an intriguing article in the Works in Progress site, Guillaume Blanc studies fertility in 17th and 18th century France. It was in France that Europe’s demographic transition began. The great French historian Fernand Braudel once asked:

“Did France cease to be a great power not, as is usually thought, on 15 June 1815 on the field of Waterloo, but well before that, during the reign of Louis XV when the natural birth-rate was interrupted?”

Blanc points out that around 1750, before the Industrial Revolution took off, France had a population of 25 million, compared to England’s 5.5 million. The two countries were at war almost continuously between 1793 to 1815. Allied to England were Austria, Prussia and Russia. But France had so many young men of fighting age that it was able to field an army of a million men.

Nowadays, France’s population is roughly the same size as Great Britain -- 68 million compared to 56 million. But, “had France’s population increased at the same rate as England’s since 1760, there would be more than 250 million French citizens alive today,” Blanc points out. No wonder it is no longer a superpower.

What happened? Blanc points out that fertility rates began to stall in France in the 17th century and fell dramatically in the 18th century – before the Industrial Revolution. So the driver of French decline must have been culture, not a rising standard of living.

His explanation is startling – it was caused by a decline in the influence and prestige of the Catholic Church.

The Church encouraged large families. It always has. The Bible would never be selected as Planned Parenthood Book-of-the-Month. “Certainly sons are a gift from the Lord, the fruit of the womb, a reward,” says Psalm 127. “Like arrows in the hand of a warrior are the sons born in one’s youth. Blessed is the man who has filled his quiver with them.” The Catechism of the Catholic Church observes that: “Sacred Scripture and the Church's traditional practice see in large families a sign of God's blessing and the parents' generosity.”

Not everyone agrees, even in Catholic France in the 17th and 18th centuries. Primitive contraceptive methods were widely known and practised, even if they were condemned from pulpits. And if the Church lost its prestige, one would expect that more people would use contraception.

Blanc compared regional fertility rates in France with regional secularisation. He did this with the help of big data from millions of crowd-sourced family trees.

“Using this genealogical data, I estimate that the decline in fertility took hold in France in the 1760s, more than a century earlier than in any other country. The average number of children per woman declined from more than 4.5 to 3.5 in less than 40 years. In the meantime, the average English woman was bearing six children. … In England, the industrial revolution made people richer, but they spent their additional wealth having more children.”

Relying on research by other scholars, he mapped fertility rates against secularisation.

Whether it was dechristianization, secularization, or simply a loss of influence of the clergy is hard to say, but the data shows that attitudes toward life and death changed radically in the course of the eighteenth century.

At the end of the seventeenth century, most testators referred to God, Paradise, or various saints in their wills. On the eve of the French Revolution, they used more secular language and expressions, such as ‘indispensable tribute that we owe to Nature’, to discuss death. Other measures, such as requests for requiem masses (perpetual masses for the dead), bequests, offerings to the church, or even invocations of the Virgin Mary or average weight of funeral candles, all declined significantly.

And he reaches a startling conclusion:

According to the genealogical data, … the fertility transition only took place after dechristianization. I also find that the effect persisted for generations, as persons born in secular places passed their secular values on to their children, even after moving to places with different institutional and cultural norms. This means that dechristianization was not only institutional but rather, and above all, cultural.

As to what caused secularisation, Blanc does not venture a definitive answer. But he notes that the most secularised regions of France were those where the heresy of Jansenism was influential and where the Counter-Reformation was most active. Jansenism was a complex phenomenon which fostered a rigid, holier-than-thou spirituality and suspicion of Rome. Why the Counter-Reformation reforms were associated with secularisation is more puzzling, but perhaps people resented authoritarian clerics. “Secularization might have been a backlash against religious powers closely connected to absolutism,” Blanc conjectures.

The reason for the decline in fertility rates in Europe – and then the rest of the world – will probably always remain a mystery. But Blanc’s research is thought-provoking. If dechristianisation dragged fertility down, will rechistianisation raise it up? Will increasing fervour amongst faith-filled folks lead the way to a world of large families? To rescuing humanity from slow extinction?

What else will?