In the early years of the twenty-first century, religion seemed to be on the rise. The collapse of both communism and the Soviet Union had left an ideological vacuum that was being filled by Orthodox Christianity in Russia and other post-Soviet states. The election in the United States of President George W. Bush, an evangelical Christian who made no secret of his piety, suggested that evangelical Christianity was rising as a political force in the country. And the 9/11 attacks directed international attention to the power of political Islam in the Muslim world.

A dozen years ago, my colleague Pippa Norris and I analyzed data on religious trends in 49 countries, including a few subnational territories such as Northern Ireland, from which survey evidence was available from 1981 to 2007 (these countries contained 60 percent of the world’s population). We did not find a universal resurgence of religion, despite claims to that effect—most high-income countries became less religious—but we did find that in 33 of the 49 countries we studied, people became more religious during those years. This was true in most former communist countries, in most developing countries, and even in a number of high-income countries. Our findings made it clear that industrialization and the spread of scientific knowledge were not causing religion to disappear, as some scholars had once assumed.

But since 2007, things have changed with surprising speed. From about 2007 to 2019, the overwhelming majority of the countries we studied—43 out of 49—became less religious. The decline in belief was not confined to high-income countries and appeared across most of the world.

Growing numbers of people no longer find religion a necessary source of support and meaning in their lives. Even the United States—long cited as proof that an economically advanced society can be strongly religious—has now joined other wealthy countries in moving away from religion. Several forces are driving this trend, but the most powerful one is the waning hold of a set of beliefs closely linked to the imperative of maintaining high birthrates. Modern societies have become less religious in part because they no longer need to uphold the kinds of gender and sexual norms that the major world religions have instilled for centuries.