Religious fundamentalists tend to have more children than other religious people. Actively religious people tend to have more children than nominally religious people, who in turn have higher fertility than seculars. Why? Different studies find different factors significant, but in essence, it is because of some combination of:

a) Pronatalist beliefs of the major religions (i.e. ‘Go forth and multiply’) and more traditional gender roles which emphasize motherhood and domesticity

b) Houses of worship tend to encourage a pro-family orientation and multi-child norm, as well as providing family support networks

c) Secular people—especially women—are more likely to delay marriage and family formation to further expressive or career goals, and often fail to achieve their desired family size as a result.

In addition, there are indirect reasons for religious growth. The fastest growing countries are poor, less educated and highly religious (i.e. Africa and Muslim world), and these also produce many immigrants. The most secular parts of the world (Europe and East Asia) have the lowest fertility and are more likely to import people. Even within countries, the poorest/less well-educated regions are often the most religious and the most fertile, such as southern Morocco, and even the U.S. South has fit this description throughout much of the 20th century.

This means that even if a certain degree of secularization of individuals occurs, the secular share of the population may decline. This is what’s happening in the world as a whole today. The most extreme case is of course Israel. The ultra-Orthodox Jews were only a few percent in 1948 and are 17 percent today, but with three times the birthrate of secular Jews they are now a third of Jewish first graders and will take over after 2050.

Eric Kaufmann is a reader at the University of London’s Birkbeck School of Politics and Sociology and author of the forthcoming Shall the Religious Inherit the Earth?