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Note: This is the first in an occasional series on “cultural indicators”—measures and metrics that Christians should know about that signal potential shifts or changes in society.

The Indicator: The fertility rate

What It Means: Demography is the science of populations, which seeks to understand changes in population by considering three main demographic processes: birth, migration, and aging (including death). In demography, an important metric related to birth and population is the fertility rate. (Demographers use the term “fertility” to refer to the product or output of reproduction, rather than the ability to have children, which they refer to as “fecundity.”)

What It Measures: The term “fertility rate” is used to refer to several different ways of measuring fertility. Here are four of the most common uses:

General fertility rate (GFR): This metric is the number of births per 1,000 women of childbearing age (i.e., aged 15–44).

Age-Specific Fertility Rate (ASFR): This metric is the number of live births per 1,000 women in a specific age group for a specified geographic area and for a specific point in time, usually a calendar year. For example, all the births to women age 20 to 24 in the United States in 2015.

Completed fertility rate (CFR): This metric is the number of children actually born per woman in a cohort of women (typically those born in a country around the same time) up to the end of their childbearing years.

Total fertility rate (TFR): This metric refers to the total number of children who have been born or are likely to be born to a woman in her lifetime if she were subject to the prevailing rate of age-specific fertility in the population.

An awareness of the different ways the term is used is important for understanding the claims and predictions made about fertility. For example, last year numerous media outlets reported the United States had reached a record-low fertility rate. A New York Times headline read “U.S. Fertility Rate Reaches a Record Low” and the first line claimed, “In 2016, the fertility rate in the United States was the lowest it has ever been.”

This was a true claim since the general fertility rate (GFR) hit a record low in 2016. But the completed fertility rate reached a record low in 2006, and the total fertility rate in the United States reached its record low in 1976. This is also why The New York Times can also report, as it did earlier this month, that, “The U.S. Fertility Rate Is Down, Yet More Women Are Mothers.” (They were referring to the GFR, which is down, and the CFR, which is up slightly from previous years.)

To avoid confusion, throughout the rest of this article we’ll focus on the total fertility rate (TFR) within the United States.

Current measure/indication: As of 2015 (the last year data is available), the total fertility rate in the United States is 1.80 children per woman.

Why It Matters: The total fertility rate is a key factor in the growth of a country’s population. If we assume there is no net migration (i.e., the number of people who immigrate to a country is equal to the number who emigrate to another country) and mortality is unchanged (i.e., there are no major wars or diseases wiping out the population), a total fertility rate of 2.1 children per woman ensures a broadly stable population (in the United States the necessary rate is closer to 2.08). This is known as the replacement rate, since the total number of children replaces both the mother and the father and accounts for the number of children who won’t live to childbearing age.

What this means is that if we want the U.S. population to remain stable we need, on average, women of childbearing age to have 2.08 children during their lifetimes. Currently, though, we are falling short with a rate of 1.80. This means we are on track to replace only 87 percent of our population.

Why does it matter if the population declines? Because the decline only affects one end of the population scale. Rather than having about equal numbers of people continuously going through the aging process (e.g, childhood, adolescence, early adulthood, midline, late adulthood), the population skews toward the older cohorts.

One reason why this matters is that young people are a form of “old-age insurance.” In every nation and every era, older people have relied on the young for support. But in the United States the social safety net in America is predicated on having a higher number of younger people of working age relative to those of retirement age. Take, for example, the Social Security system. As the Peter G. Peterson Foundation explains, the Social Security program is financed largely on a pay-as-you-go basis, which means that today’s workers pay Social Security taxes into the program and money immediately flows back out as monthly income to beneficiaries. A pay-as-you-go system works well as long as there are enough workers contributing to the system to cover its costs.

In 1945, there were 41.9 workers for every retiree receiving Social Security. But in 2010 the ratio was 2.9 workers for every retiree, and it’s expected to drop to 2.3 by 2030. Because of a falling fertility rate, that number could drop close to 1:1 by the time young millennials get ready to retire.

For young, upwardly mobile workers with a solid 401K plan, this may not seem like much of a concern. But for society as a whole, this change could have massive and troubling effects. Because the elderly poor will be an outsized drain on tax-funded resources, they could be viewed as being a burden on society. At best they’ll be devalued and, at worse, could be pressured into submitting to “voluntary euthanasia.” (This is already becoming a problem in some European countries where fertility rates have fallen and the acceptance of euthanasia has risen.)

The change in fertility also affects society in various other ways. Areas of the country that are already losing population, such as the Midwest and Appalachia, would decline even further. This would lead to economic stagnation and hardship for those who remain in those areas. These factors could lead to a spiral of instability that can have a spiritual effect on communities that last for generations.

If the fertility rate continues to decline, Christians will need to prepare for the long-term effects it will have on a variety of issues, ranging from human dignity to immigration to rural church planting.

What to Watch For: People tend to postpone having children during economic downturns like that of the Great Recession (2007–2012), which causes the fertility rate to decline. The rate usually picks back up during more prosperous times, though we haven’t seen it increase in the last decade. It may still recover as the economy improves, or it could flatten out, as it did in the 1980s. Watch also to see if it drops below 1.70, which could signal a new long-term negative trend.