“Replacement Level Fertility” is a virtually meaningless remnant of theoretical demographics. And yet, articles about human population dynamics rarely fail to include it, with a designation of 2.1 births per woman. They usually raise the alarm that it’s so much lower than that, signaling a looming population crisis.
A 2.1 Total Fertility Rate (TFR) would equate to replacement level only in the rare population that is neither increasing nor decreasing. Vatican City for example. It is typically conflated with zero population growth—a natural increase of zero—either out of ignorance or equivocation.
A TFR that would achieve exact replacement depends on a nation’s mortality and previous fecundity. A high child mortality rate and short life span requires a higher birth rate for stability. A high proportion of 15 to 44 year-old females and a longer lifespan lowers it. Despite a wide disparity, 2.1 TFR is generally assumed for all populations.
Nations’ fertility rates can remain well below 2.1 for decades before population stops increasing. South Korea took 37 years. China took 31 years (1991-2022), and Japan, 34 years (1975-2009). The US has been below 2.1 since 1974, and natural increase has yet to achieve zero.
Comparing charts of projected TFR and population growth reveal how population increases despite improved fertility. In more developed regions, populations are projected to hold steady, contrary to pronatalists’ warnings of a fertility crisis.
A fun interactive population pyramid simulation allows viewing a variety of TFRs over time. Click on ‘free simulation’.
Previously high fertility causes population momentum: population continues to increase even when TFR falls. Global TFR in 1955 was 5.0 and population was increasing by 49 million per year. In 2020, world TFR was less than half at 2.35 and population increased by 76 million that year. A lower rate is multiplied by a larger base.
Chart below shows population growth rate as a percent of total population and actual increase. It reveals this same momentum: lower rate x larger base.
A chart of global births and deaths gives a more realistic view of what’s happening, which is why it’s not used by those who want to make it seem as if human population is about to plummet. At this rate, we won’t stop increasing until the mid 2080s.
The definition of Replacement Level Fertility is confusing, even for knowledgeable people, as these examples show.
Marie Boran, Technology Reporter at Newsweek, writes, “A TFR of 2.1 is considered the ‘replacement level’ in developed countries, meaning the population would remain stable (ignoring migration and mortality factors).” Migration is ignored in determining natural increase, but mortality is a critical factor that can’t be ignored.
Anna Rotkirch, of the Population Research Institute in Finland writes: “The so-called ‘theoretical replacement rate’ of 2.1 children per woman is just what it says: a theoretical number. In real life, mortality and migration also shape population change.” Yes mortality is important, but so is the proportion of a population that is female 15-44 years old.
Niall Ferguson, Bloomberg Columnist, writes: “… the 2.1 threshold (the ‘replacement rate,’ allowing for childhood deaths and sex imbalances), below which the population is bound to decline.” Eventually. Child mortality is important as is all mortality in determining a stable population. Sex imbalance, as in China, influences fertility by lowering the number of females 15-44, however, existence of proportionately more males has little effect.
Paul Poast of MSN writes: “… below the rate needed to replace the existing population, commonly set at 2.1 children per person who is able to give birth, global population will begin to decline.” Eventually. There’s a convenient term for people who are able to give birth.
Jess Cockerill in Science Alert writes: “… within just 25 years, over two-thirds of countries’ populations will be in decline.” No, two-thirds will have a TFR below 2.1, but fewer than half will be in decline.
The Lancet reports: “The replacement level is generally accepted to be a TFR of at least 2·1, although the true replacement level depends on the specific mortality rate and sex ratio at birth in a population.” If sex ratio had a meaningful effect, it would start 15 years after birth.
“Our estimates indicate that there is approximately a 30-year gap between the time when TFR falls below 2·1 and when the natural rate of population increase turns negative.” 30 years could be an average gap, but with several factors influencing timing, its usefulness is almost zero.
The US Census Bureau adds a little clarity with: “It is widely believed that 2.1 is the replacement-level fertility rate — the number of births a woman would need to have in her lifetime to replace herself and the father. However, the precise level of fertility necessary for long-term replacement varies between countries due to different mortality rates.” It also varies with the population size of females 15-44 years old.
Population Research and Policy Review explains “The Surprising Global Variation in Replacement Fertility” in December 2003.
“It is frequently assumed by the general public and also by some population experts that the value of replacement-level fertility is everywhere an average of 2.1 lifetime births per woman. Nothing could be further from the truth. The global variation in replacement fertility is substantial, ranging by almost 1.4 live births from less than 2.1 to nearly 3.5. This range is due almost entirely to cross-country differences in mortality, concentrated in the less developed world.”
Amen, but let’s not forget momentum, something most African nations have an abundance of. In many populations, 50% are below 18 years old.
“Policy makers need to be sensitive to own-country replacement rates. Failure to do so could result infertility levels that are below replacement and lead to long-run population decline. For example, the current replacement total fertility rate for the East Africa region is 2.94. Lowering fertility to 2.10 would, under current mortality conditions, result in a regional birthrate 29 percent below replacement.”
Maybe eventually, and a few decades later, their populations might go into negative growth, or “decline,” as they like to call it. The idea that policy makers could or would improve fertility levels to 2.10 is questionable, but not impossible, with true reproductive freedom. Entrenched patriarchy is a hindrance.
The best use of the term “Replacement Level Fertility” is as a red flag signaling potential pronatalist propaganda, although even population-awareness advocates and organizations mistakenly use it. When actual population size and growth numbers aren’t included, and they make dire warnings of too few workers to support massive elderly population size, we know it’s pronatalist nonsense.
For example, Dean Spears, of the Elon Musk-funded Population Wellbeing Initiative at the University of Texas, writes in 2023, “Two-thirds of people now live in a country where fertility rates are not high enough, on average, to prevent depopulation. By depopulation, we mean that the number of people alive exponentially declines, generation after generation. The dividing line that separates population growth from depopulation is whether fertility is consistently above or below 2 children per adult woman, on average globally. Below that critical threshold, the next generation will not replace the last. Nearly all rich countries are already below.” And a few are actually declining in population.
Population activists will help raise awareness by not using it, and by disregarding it when seeing or hearing it. It reinforces the popular myth from 1968, when Zero Population Growth’s campaign slogan was “Stop at two.” When a couple co-creates two more of us, it increases their impact by at least 100%, not counting their offspring’s progeny. People today still pretend that it doesn’t increase population, just replaces mom and pop.
Les appreciates feedback, corrections, and additions.
