Total fertility rate in the context of "Christians"

In world demographics, the world population is the total number of humans currently alive. It was estimated by the United Nations to have exceeded eight billion in mid-November 2022. It took around 300,000 years of human prehistory and history for the human population to reach a billion and only 218 more years from there to reach 8 billion.

The human population has experienced continuous growth following the Great Famine of 1315–1317 and the end of the Black Death in 1350, when it was nearly 370,000,000. The highest global population growth rates, with increases of over 1.8% per year, occurred between 1955 and 1975, peaking at 2.1% between 1965 and 1970. The growth rate declined to 1.1% between 2015 and 2020 and is projected to decline further in the 21st century. The global population is still increasing, but there is significant uncertainty about its long-term trajectory due to changing fertility and mortality rates. The UN Department of Economics and Social Affairs projects between 9 and 10 billion people by 2050 and gives an 80% confidence interval of 10–12 billion by the end of the 21st century, with a growth rate by then of zero. Other demographers predict that the human population will begin to decline in the second half of the 21st century.

Population decline, also known as depopulation, is a reduction in a human population size. Throughout history, Earth's total human population has continued to grow, but projections suggest this long-term trend may be coming to an end. From antiquity (10th century BCE–500 CE) until the beginning of the Industrial Revolution in early modern Europe (late 18th–early 19th centuries), the global population grew very slowly, at about 0.04% per year. After about 1800 the growth rate accelerated to a peak of 2.1% annually during the mid-20th-century baby boom (1945–1968 period), but since then, due to the worldwide collapse of the total fertility rate, it has slowed to 0.9% as of 2023. The global growth rate in absolute numbers accelerated to a peak of 92.8 million in 1990, but has since slowed to 70.4 million in 2023.

Long-term projections indicate that the growth rate of the human population on the planet will continue to slow down, and that before the end of the 21st century it will reach growth zero. Examples of this emerging trend are Japan, whose population is currently (2023) declining at the rate of 0.5% per year, and China, whose population has peaked and is currently (2023) declining at the rate of about 0.2% per year. By 2050, Europe's population is projected to be declining at the rate of 0.3% per year. Population growth has declined mainly due to the abrupt decline in the global total fertility rate, from 5.3 in 1963 to 2.2 in 2023. The decline in the total fertility rate has occurred in every region of the world and is a result of a process known as demographic transition. To maintain its population, ignoring migration, a country on average requires a minimum fertility rate of 2.2 children per woman of childbearing age (the number is slightly greater than two because not all children live to adulthood). However, most societies experience a drop in fertility to well below two as they grow wealthier.

As of 2025, Afghanistan has an estimated population of around 49.5 million people. The country is characterized by a highly diverse, multi-ethnic, and multilingual society, reflecting its historical position at the crossroads of Central, South, and Western Asia, along ancient trade and invasion routes. The four major ethnic categories in the country are Pashtun, Tajik/Farsiwan, Hazara, and Uzbek. In addition, Afghanistan has a plethora of minor ethnic categories, including Turkmen, Baloch, Aimaq, Nuristani, Arab and many others. Together they make up the contemporary Afghan people.

Approximately 43% of the population was under the age of 15, and about 74% of Afghans lived in rural areas as of 2016. The country has one of the highest fertility rates in the world outside of Africa, with the average woman giving birth to five children over her lifetime. Around 6.8% of infants die during childbirth or early infancy. According to the World Health Organization (WHO), the nation's average life expectancy was estimated at about 63 years as of 2019. The prevalence of HIV remains very low, affecting only about 0.04% of the population.

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 2.1 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 2023, 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.

Population momentum or demographic inertia is the tendency of the raw birth rate to rise as a result of past high fertility rates, even after fertility rates have fallen, or vice-versa. This occurs because a current increase in fertility rates causes an increase in the number of women of childbearing age roughly twenty-to-forty years later, meaning population growth figures tend to lag substantially behind fertility rates. Well-known examples include the Echo Boom (the increase in the total number of births as baby boomers reached child-rearing age) and Chinese population growth throughout the era of the one-child policy (from 1979 until 2021).

Population momentum explains why a population will continue to grow even if the fertility rate declines or continues to decline even if the fertility rate grows. Population momentum occurs because it is not only the number of children per woman that determine population growth, but also the number of women of reproductive age. Eventually, when the fertility rate reaches the replacement rate and the population size of women in the reproductive age bracket stabilizes, the population achieves equilibrium and population momentum comes to an end. Population momentum is defined as the ratio of the size of the population at that new equilibrium level to the size of the initial population.

The ageing of Europe, also known as the greying of Europe, is a demographic phenomenon in Europe characterised by a decrease in fertility, a decrease in mortality rate, and a higher life expectancy among European populations. Low birth rates and higher life expectancy contribute to the transformation of Europe's population pyramid shape. The most significant change is the transition towards a much older population structure, resulting in a decrease in the proportion of the working age while the number of the retired population increases. The total number of the older population is projected to increase greatly within the coming decades, with rising proportions of the post-war baby-boom generations reaching retirement. This will cause a high burden on the working age population as they provide for the increasing number of the older population.

Throughout history many states have worked to keep high birth rates in order to have moderate taxes, more economic activity and more troops for their military.