“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 lament how much lower than that it is. A 2.1 TFR would equate to replacement level only in the rare population that is neither increasing nor decreasing. It is typically conflated with zero population growth—a natural increase of zero—either out of ignorance or equivocation.
A Total Fertility Rate (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 15-44 year-olds lowers it. Despite a wide disparity, 2.1 TFR is generally assumed for all populations.
Nations’ fertility rates can remain well below a TFR of 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 never been zero.
Figure 3. shows the lag globally between nations below “replacement level fertility” and an actual decreasing of population in nations.
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.
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.
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 name for a person who is 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 many factors influence timing, limiting its usefulness.
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.
The best use of the term “Replacement Level Fertility” is as a red flag signaling potential pronatalist propaganda, although even population-awareness organizations mistakenly use it. When they don't include actual population size and growth numbers, and make dire warnings of too few workers to support massive elderly population size we know it’s pronatalist nonsense.
