A growing number of researchers and observers are beginning to take note of a huge demographic reality — those who take belief in God most seriously tend to have more babies. For many years, the conventional wisdom has held that demography determines destiny. Well, now it appears that theology determines demography.

Writing in Intelligent Life, reporter Anthony Gottlieb notes that some belief systems “are dramatically outbreeding others.” As he acknowledges, this “outbreeding” will impact the course of nations and societies.

The background to all this, of course, is the contraceptive revolution. Once human beings developed the ability to separate sex from procreation, children became optional. For the first time in human history, couples could engage in sex without even the thought of babies. Once these technologies spread, a decision to reproduce became just that — a decision.

The task of social scientists is to observe, describe, and possibly explain social behavior. One behavior that has become more and more apparent is the fact that couples in most nations are having fewer children. Indeed, falling birthrates have become a major concern in many societies. Once social scientists look closer, it also becomes apparent that this pattern is not uniform.

As Anthony Gottlieb explains, those who are more “religious” (meaning those who hold to a worldview that is based in belief in God) have more children. This appears to be a constant across cultures and religious lines.

This fact will change the face of society at home and around the world. In the United States, high Mormon birthrates and low Jewish birthrates mean that there will soon be more Mormons than Jews in America — a startling development. In Europe, falling birthrates among secularized couples must be placed alongside the much higher average birthrates of Muslim couples. Europe, Gottlieb observes, will inevitably become more religious in years ahead. But the religion that is on the ascent is Islam.

Interestingly, these patterns play out within denominations and religious families as well. More liberal Jews tend to under-reproduce, but the Orthodox reproduce at much higher rates. Evangelical birthrates outstrip those of more liberal Protestants. Traditionalist Roman Catholics are far more likely to have large families than is the case with more liberal Catholics.

The differential in birthrates can lead to fundamental changes within societies. In Israel, the “Ultra-Orthodox” reproduce at much higher rates than the general population. Thus, Gottlieb reports that by 2025 the Ultra-Orthodox will account for “at least a quarter of Israel’s population of under-17s.” That spells big changes for the nation.

Gottlieb asks the big question: Does having a big family make you more “religious,” or does being more “religious” make you more likely to have more children?

In the end, Gottlieb seems to favor the first option. He cites the work of Mary Eberstadt of the Hoover Institution and argues that the experience of having children makes parents more concerned with spiritual matters and, in essence, more conservative in theology.

I would argue that this argument could just as easily be reversed. From a Christian perspective, it is easy to see how the belief that children are gifts from God would lead believers to have more rather than less children on average.

In any event, Gottlieb’s article serves as a powerful reminder that theology matters. Just look at the fertility factor.