America’s Growing Religious-Secular Fertility Divide

Birth rates in the United States are near record lows, but not for everyone. Indeed, under the surface of the fertility decline since 2007 is a little noticed fact: fertility has declined much more among nonreligious Americans than among the devout. Data from the National Survey of Family Growth (NSFG) from 1982 to 2019, along with data from four waves of the Demographic Intelligence Family Survey (DIFS) from 2020 to 2022, point to a widening gap in fertility rates between more religious and less religious Americans. In recent years, the fertility gap by religion has widened to unprecedented levels. But while this difference may comfort some of the faithful who hope higher fertility will ultimately yield stable membership in churches and synagogues, these hopes may be in vain. Rates of conversion into irreligion are too high, and fertility rates too low, to yield stable religious populations.¹

Past Religious Fertility

The NSFG has asked respondents about their religious attendance and their recent fertility history since 1982. In recent years, it has operated as a continuous annual survey. As a result, data from over 70,000 women surveyed from 1982 to as recently as 2019 can be used to estimate fertility rates for three broad groups of women: those without any religious affiliation, those with religious affiliation but less than weekly attendance, and those with at least weekly attendance. The estimates of fertility produced by the NSFG vary slightly from official estimates from the CDC, and so are adjusted to fit them. 

Total fertility rates are synthetic estimates of fertility: they use current birth rates by age for a given group to guess how many children a woman would end up having if she experienced those birth rates over the course of her life. In practice, birth rates by age change as women age, and of course religious identity can change over the life course, so fertility rates of this kind are unlikely to perfectly predict actual fertility outcomes.

Figure 1 shows fertility estimates from the NSFG for the three religious attendance groups.

As can be seen, fertility rates among weekly-attending Americans have never dropped much below 2 children per woman, and as recently as 2008 were around 2.4 children per woman. Fertility among religious people did decline after the 2008 recession, but by 2017 to 2019, it was once again rising. In other words, there was no long slump in births to the most devout parents.

Religious women who attend church less than weekly had lower fertility in all periods, but especially during the 2000s. And perhaps most strikingly, since 2016, fertility rates for weekly- and less-than-weekly-attending women have moved in opposite directions, with fertility falling among the more nominally religious.

Finally, fertility among nonreligious women rose considerably from 1982 to 2005, then again from 2008 to 2012, showing a very different pattern than that observed for religious women. From 2010 to 2013, nonreligious women had about the same birth rates as women who attended religious services less than weekly, before their fertility slumped through 2019. Indeed, virtually 100% of the decline in fertility in the United States from 2012 to 2019 can be explained through a combination of a growing number of religious women converting to irreligion, and declining birth rates among the nonreligious. 

Before going further, it’s worth reflecting on what a large cultural difference this represents. The most religious Americans in 2019 had similar fertility rates as women in India, Libya, Sri Lanka, Tunisia, Iran, Myanmar, Saudi Arabia, Peru, or Mexico. The least religious Americans in 2019 had similar fertility rates as women in Hong Kong, Japan, Portugal, Greece, Italy, Singapore, or Moldova. 

COVID has not reduced these differences. Across four waves of the DIFS, including about 9,000 women, from fall 2020 to spring 2022, women who never attended church reported a fertility rate of around 1.3 children per woman, vs. 2.1 for women who attended weekly: very similar to the rates found in the NSFG for 2019.

Declining Religion in America

But it’s not just fertility rates that drive birth trends: the number of women to whom those rates apply may vary as well. Figure 2 shows the share of women ages 18-44 who were in each religious grouping, according to the NSFG.

Since 2002, the share of reproductive-age women who attended church weekly or more has fallen from about 35% to 24 percent. In the DIFS data, the share was even lower: only about 18% of women. In other words, while the fertility rate among religious women has been stable, society has still become less religious overall, meaning that the overall number of births to religious mothers has trended downwards. On the other hand, though fertility rates have fallen by 26% among nonreligious women since 2005, they have grown from about 17% among reproductive age women to 30%—a 75% increase. Overall, then, births to nonreligious women have risen. Despite a widening fertility gap, the ongoing trend of younger Americans becoming more secular more than offsets the fertility advantage enjoyed by religious people. 

Indeed, religious fertility rates simply are not high enough to offset losses from conversion to irreligion. Data from the 2014 Pew Religious Life Survey suggest that net conversions in and out of American religions lead to about a 16% loss in religious people over the course of a generation. To offset that, religious American women would need to have, on average, 2.44 children each. Among weekly attending women, the true figure is just 2.1; adding in women who are irregular attenders to count all religious people together, religious women in 2019 had a fertility rate around 1.8 or 1.9 children each. With birth rates at just 1.8 or 1.9 children per woman vs. a conversion-adjusted “replacement rate” of 2.44, religious communities in America will tend to decline by about 25% in each generation. If these trends continue, then within three generations (that is, by the time current children in churches are elderly grandparents), religious communities in America will have shrunk by more than half, a devastating loss. On the other hand, nonreligious Americans need to only have 0.8 to 0.9 children, on average, to achieve population growth, given their conversion rates: in fact, they currently have 1.3 children, implying 50-60% population growth every generation.

If these trends continue, then within three generations (that is, by the time current children in churches are elderly grandparents), religious communities in America will have shrunk by more than half, a devastating loss.

Key Differences Across Religious Groups

However, it’s not right to lump all religious groups together when it comes to fertility. Using data on conversions and fertility from the Pew Religious Life Survey (PRLS) in 2014, it’s possible to describe some crucial differences. Thus, I separate individuals from different major world religions, and within Christianity, to break out six distinct subgroups: Roman Catholics and Eastern Orthodox (grouped together as there are few Orthodox respondents), Pentecostals, Protestant denominations in which at least 75% of respondents said they believed that “the Bible is the Word of God” vs. those under 75%, and nondenominational groupings over or under 75% on the same question. Prior research has suggested that beliefs about the Bible are a good indicator of fertility behaviors in general. 

Figure 3 below shows two indicators. First, it shows the Total Fertility Rate (TFR), or the number of children born per woman, that a religious group would need to achieve to experience growth, given its conversion rates observed in 2014 and assuming there is zero immigration into the country. The second indicator is an estimate of the Actual TFR for that group, using between-group differences observed in the PRLS in 2014, imputed to actual fertility trends in 2019. When the Needed TFR is higher than the Estimated TFR, it means that a religious group is likely to shrink over the next generation, unless it receives new members through immigration.

Liberal Protestant denominations appear to be facing a dire situation. Given high rates of conversion out of these traditions, they need to achieve fertility rates of 3 children per woman to grow over the next generation, yet currently only have about 1.5 or 1.6 children per woman. Conservative Protestants who believe the Bible is the Word of God have higher fertility rates (around 1.8 children per woman) and less-negative conversion rates, meaning they only need to have around 2.4 children per woman. As a result, liberal denominations can expect a 48% decline in a generation, vs. just a 26% decline for conservative denominations—again, assuming that no members are gained through immigration.

On the other hand, nondenominational churches have experienced robust growth. Liberal nondenominational movements have roughly the fertility rate they need for stability (1.8 or 1.9), but conservative nondenominational movements have seen extraordinary growth: they only need to have about 0.8 children per woman to grow, yet in fact have around 1.9. This means they will more than double in size over the next generation, even without immigrants.

Likewise, Pentecostal churches will grow, with actual fertility (2.1) substantially above needed fertility (1.8). On the other hand, Catholic and Orthodox churches will see appreciable decline, with an average of 1.9 children born per woman nowhere near high enough to offset high rates of conversion out of these faiths, yielding a needed fertility rate of 3.1. Roman Catholic churches can expect a 40% decline in the next generation unless immigration can offset the decline.

Among Jews, fertility growth will continue, due almost entirely to high fertility among the Orthodox. But among Muslims, Buddhists, Hindus, and the “Other” category containing groups like Mormons, Unitarians, Messianic Judaism, and Jehovah’s Witnesses, Needed TFR always exceeds Actual TFR, such that all those groups can expect between a 10 and 40% decline unless offset by immigration. Notably, Hindus, Buddhists, some Christian-related sects, and Muslims do benefit from high rates of additional members through immigration, so their actual community sizes are likely to outperform this forecast.

Finally, the “Other” category for “Other world religions,” including movements as diverse as Sikhism, Native American religions, and New Age spirituality, will also see significant growth, but from a very small base. And though nonreligious fertility rates are very low, high rates of conversion into no religious affiliation will result in over 50% growth in the non-religious population over the next generation. 

In sum, religious groups in America in general face the prospect of stark decline. However, different groups will have different experiences. For institutionalized Protestantism, Catholicism, and many of the other major world religions, the decline will be swift and deep, with high rates of religious exit and modest-sized families. Some conservative denominations (most notably, a variety of conservative Presbyterian denominations like the Presbyterian Church in America) will be able to stave off serious decline for the immediate future, but most groups will still experience a decline. It is only within nondenominational Christianity, Pentecostalism, Orthodox Judaism, heterodox offshoots of Christianity, small religious movements, and secularism that significant fertility growth is observed.²

Decline Is Not Inevitable

Religiosity in America is declining for many reasons that I have described in length elsewhere. Changes in education, parenting styles, marital trends, and the geography of American neighborhoods have all conspired to suppress religiosity across the last several generations and drive conversions away from religious practice. Meanwhile, the “fertility gap” between religious and nonreligious Americans has been growing for two decades and is now the widest it has ever been. This fertility advantage, however, is nowhere near large enough to counteract losses to the rejection of faith. 

To offset these losses, religious Americans would need to have about 2.4 children each, and in some religious subgroups, even more than that. Most religious groups in America are having fewer children than they need for long-run stability, and as a result, the next few generations are likely to see a considerable decline in American religion. However, this decline is not inevitable: achieving growth again would not require American churches to do impossible things. Encouraging members to have one more child (on average), successfully integrating a modest share of immigrants into their congregations, or achieving conversion rates within the range of those experienced by some currently faster-growing religious movements could help stave off decline. 

Lyman Stone is a Research Fellow at the Institute for Family Studies, Chief Information Officer of the population research firm Demographic Intelligence, and an Adjunct Fellow at the American Enterprise Institute.

Editor's Note: Read Christianity Today's coverage of our research brief here. 

1. In this research brief, the term “conversion to irreligion” is used for several reasons. First, the religiously-unaffiliated category includes many people with a variety of religious and spiritual beliefs. Second, for the purposes of modeling, religious nonaffiliation is a same-type category as other groups, and changes across groups are defined as “conversions.” When an individual exits the unaffiliated group to join some specific group, it is a “conversion,” and so exits from some specific group into the unaffiliated group are also labeled “conversions.” 

2. Two major caveats: First, these analyses exclude immigration. Low-skilled immigrants tend to be more religious, while high-skilled immigrants tend to be more secular. Among religious high-skilled immigrants, Hinduism, Islam, and Buddhism are disproportionately common. Refugee populations often come from specific religious sects as well, which can lead to sudden changes in religious demography for those groups. Second, these analyses assume that rates of conversion observed over the 1990s, 2000s, and 2010s are stable through the next generation, and fertility rates observed in 2014-2019 are stable through the next generation. These assumptions are certainly wrong. Fertility and conversion will fluctuate, including in response to religious demography. As a group becomes larger, it will not be able to achieve as much growth through conversions and will become more dependent on fertility. Across one generation this effect is trivial, but across 5 or 10 generations, it could become considerable. Moreover, religions might respond to the dire outlook for growth by adopting new norms and behaviors related to conversion and fertility, and thus change their trajectories for growth.