The replacement fertility rate is the average number of live births per woman required for a population to replace itself from one generation of parents to the next, assuming prevailing mortality conditions and no migration. In demographic analysis, the concept is usually expressed through the total fertility rate, abbreviated TFR, and is commonly described as approximately 2.1 births per woman in populations with low mortality. The figure of 2.1 is not a universal biological constant.
It reflects the fact that two children are needed, in broad numerical terms, to replace two parents, while a small additional fraction allows for children who die before reaching reproductive age and for the fact that slightly more boys than girls are normally born. Replacement therefore concerns the number of daughters ultimately surviving to reproductive ages rather than simply the number of children born. The distinction is important because fertility at exactly 2.0 births per woman does not necessarily produce exact generational replacement, even in a population with low mortality.
The replacement fertility rate varies according to mortality and demographic conditions. In populations where virtually all girls survive to adulthood and reproductive ages, replacement fertility is close to 2.1 births per woman. Where childhood and young adult mortality are substantially higher, more births are required for each generation to replace itself, so the replacement level can be considerably above 2.1. Historical populations with high mortality therefore needed much higher fertility to achieve replacement.
Improvements in nutrition, sanitation, vaccination, medical treatment, maternal health and child survival generally reduce the fertility level needed for replacement. Replacement fertility is consequently a demographic concept that combines fertility with survival rather than a simple statement that every woman must have exactly two children. Researchers can calculate more precise replacement levels for particular populations by considering sex ratios at birth, age specific mortality and the probability that children survive to the ages at which they themselves reproduce.
Replacement fertility is closely related to the total fertility rate but the two terms are not interchangeable. The total fertility rate estimates the average number of children a woman would have if she experienced a specified set of age specific fertility rates throughout her reproductive lifetime. Replacement fertility is the fertility level associated with replacing one generation under specified mortality and sex ratio conditions.
A TFR below replacement therefore indicates that fertility is insufficient for exact generational replacement in the absence of other demographic forces, but it does not mean that the population immediately begins to shrink. Population size depends on age structure, mortality and migration as well as fertility. A population with a below replacement TFR can continue growing for decades if it contains a large number of young people who are entering reproductive ages. Conversely, a population with fertility near replacement can experience population decline if its age structure contains relatively few women of reproductive age or if substantial net migration out of the population occurs.
The phrase replacement level fertility is often used in discussions of population stabilisation, but replacement does not mean immediate population stability. Demographers distinguish between replacement fertility and zero population growth. Zero population growth occurs when births and other additions to the population are balanced by deaths and departures, whereas replacement fertility concerns the reproduction of successive generations. A population can have replacement level fertility while continuing to grow because of population momentum.
Population momentum occurs when a relatively large cohort of young people reaches reproductive age and produces children, creating further growth even after fertility has fallen to replacement. The opposite situation can also occur. A population that previously experienced low fertility may develop an age structure with comparatively few people in younger reproductive cohorts, producing continued decline even if fertility later rises towards replacement. This distinction explains why changes in fertility do not translate directly into immediate changes in total population size.
Replacement fertility has changed over historical periods because survival patterns have changed. In preindustrial societies, mortality was often high, including substantial mortality during infancy, childhood and early adulthood, so populations needed considerably more than two births per woman to maintain their numbers over generations. During the demographic transition, mortality declined before fertility declined in many societies. As survival improved, the replacement level moved closer to the familiar value of approximately 2.1 in low mortality populations.
The demographic transition subsequently involved declining fertility, urbanisation, improvements in education, changes in family structure, increased availability of contraception, declining desired family size and changing economic incentives. Many countries now have fertility rates substantially below replacement. Other countries, particularly some lower income populations, continue to have fertility levels well above replacement. Replacement fertility therefore has to be interpreted within the demographic conditions of the population being studied rather than treated as an identical threshold everywhere.
The calculation of replacement fertility also depends on whether the analysis concerns a closed population or incorporates migration. Replacement fertility itself is fundamentally a fertility and survival concept, while migration is an external component of population change. In a closed population with no migration, sustained fertility below replacement tends eventually to produce population decline, although the timing depends heavily on age structure and mortality.
In an open population, immigration can offset fertility below replacement and maintain or increase population size, while emigration can reinforce the effects of low fertility. This is why statements such as a country being below replacement should not automatically be interpreted as statements that its population is currently shrinking. Demographic projections normally combine fertility, mortality and migration assumptions and model their effects by age and sex. Replacement fertility is therefore one component of population analysis rather than a complete population forecast.
A common misconception is that replacement fertility means every family needs to have exactly two children. It does not. The measure is an average across women and generations, and individual families can have zero, one, two, three or more children. Some women may never have children, while others have several children, and the population can still have an average fertility rate near replacement. Another misconception is that replacement fertility guarantees that a population will remain permanently constant.
Population age structure, mortality, migration and population momentum can produce growth or decline even when fertility is around replacement. It is also incorrect to assume that a TFR below replacement automatically means demographic collapse. The consequences depend on the depth and duration of low fertility, age structure, longevity and migration. Replacement fertility is therefore best understood as a demographic benchmark rather than a prediction, target or universal rule about family behaviour.
Replacement fertility is particularly important in analysing long term population trends because persistent fertility below replacement changes the age distribution of future generations. If low fertility continues for many decades and is not offset by migration, successive birth cohorts become smaller, the population eventually becomes older on average, and the number of people entering reproductive ages can decrease. This can reinforce population decline because fewer potential parents produce children even if fertility rates per woman remain unchanged.
The process can create what demographers sometimes call a demographic feedback effect, although the strength and direction of such effects vary between societies. Governments therefore sometimes monitor replacement fertility when considering policies concerning pensions, healthcare, education, labour supply and population structure. However, replacement fertility is a descriptive demographic concept, not an inherent social optimum, and the social consequences of different fertility levels depend on economic institutions, migration, productivity, longevity, family policy and other circumstances.
