Sub-replacement fertility

Sub-replacement fertility is a total fertility rate (TFR) that (if sustained) leads to each new generation being less populous than the older, previous one in a given area. The United Nations Population Division defines sub-replacement fertility as any rate below approximately 2.2 children born per woman of childbearing age, but the threshold can be as high as 3.4 in some developing countries because of higher mortality rates. Taken globally, the total fertility rate at replacement was 2.33 children per woman in 2003. This can be "translated" as 2 children per woman to replace the parents, plus a "third of a child" to make up for the higher probability of males born and mortality prior to the end of a person's fertile life. In 2024, the global average fertility rate was around 2.2 children born per woman. Replacement-level fertility in terms of the net reproduction rate (NRR) is exactly one, because the NRR takes both mortality rates and sex ratios at birth into account. As of 2010, about 48% (3.3 billion people) of the world population lives in nations with sub-replacement fertility. Nonetheless most of these countries still have growing populations due to immigration, population momentum and increased life expectancy. This includes most nations of Europe, Canada, Australia, Brazil, Russia, Iran, China, India, the United States and many others. In 2024, all European Union countries had a sub-replacement fertility rate, ranging from a low of 1.0 in Poland and Lithuania and 1.1 in Latvia, Estonia and Spain, to a high of 1.5 in Slovenia, Croatia, Denmark and Ireland, 1.6 in France and 1.7 in Bulgaria. The countries or areas that have the lowest fertility are in developed parts of East and Southeast Asia: Singapore, China and South Korea. Only a few countries have had, for the time being, sufficiently sustained sub-replacement fertility (sometimes combined with other population factors like higher emigration than immigration) to have population decline, which are South Korea, China, Japan, Germany, Lithuania, Latvia, Estonia, Poland, Finland, Romania, Bulgaria and Ukraine. As of 2024, the total fertility rate varied from 0.72 in South Korea and 0.84 (2025) in China to 6.0 in Chad, Somalia, Central African Republic and the Democratic Republic of the Congo.

History

The Greek historian Polybius largely blamed the decline of the Hellenistic world on low fertility rates, writing in his work The Histories that:

In our time all Greece was visited by a dearth of children and generally a decay of population, owing to which the cities were denuded of inhabitants, and a failure of productiveness resulted, though there were no long-continued wars or serious pestilences among us.... For this evil grew upon us rapidly, and without attracting attention, by our men becoming perverted to a passion for show and money and the pleasures of an idle life, and accordingly either not marrying at all, or, if they did marry, refusing to rear the children that were born, or at most one or two out of a great number, for the sake of leaving them well off or bringing them up in extravagant luxury. In a speech to Roman nobles, the Emperor Augustus commented on the low birthrates of the Roman elite:

We liberate slaves chiefly for the purpose of making out of them as many citizens as possible. We give our allies a share in the government that our numbers may increase; yet you, Romans of the original stock, including Quintii, Valerii, Iulii, are eager that your families and names at once shall perish with you. Upon the establishment of the Roman Empire, Emperor Augustus would introduce legislation to increase the birthrates of the Roman nobility. Some believe that not only the Great Recession, but the Great Depression, may have been the result of a decline in birthrates overall. Clarence L. Barber, an economist at the University of Manitoba, pointed out how demand for housing in the US, for example, began to decline in 1926, due to a decline in 'household formation' (marriage), due, he believed, to the effects of World War I upon society. In early 1929, US housing demand declined precipitously and the stock market crash followed in October of that same year.

Causes

Higher education

The fact that more people are going to colleges and universities, and are working to obtain more post-graduate degrees there, along with the soaring costs of education, have contributed greatly to postponing marriage in many cases, and bearing children at all, or fewer numbers of children, and the fact that the number of women getting higher education has increased has contributed to fewer of them getting married younger, if at all. In the US, for example, females make up more than half of all college students, which is a reversal from a few decades back. The relationship between higher education and childbearing varies by country: for example, in Switzerland by age 40, childlessness among women who had completed tertiary education is 40%, while in France it is only 15%. In some countries, childlessness has a longer tradition, and was common even before educational levels increased, but in others, such as Southern European ones, it is a recent phenomenon; for instance in Spain the childlessness rate for women aged 40–44 in 2011 was 21.60%, but historically throughout the 20th century it was around 10%. Not all countries show a relationship between low fertility and education: in Czech Republic, of women born in 1961–1965, low educated women were more likely to be childless than high educated women. In the United States in 2022, women between the ages of 15 and 50 with a graduate or professional degree had the highest birthrate (62 births per 1,000 women), whereas women between the ages of 15 and 50 with less than a high school graduate had the lowest birthrate (32 births per 1,000 women) among all levels of educational attainment of the mother.

A general trade-off

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