3.7 Total Fertility Rate

Total fertility rate (TFR) is the average number of children each woman in a population would have during her childbearing years, based on current birth rates at each age. It's one of the best single predictors of future population growth.

Global TFR has fallen from about 5 in the 1960s to roughly 2.2–2.3 today. It varies enormously by country: some countries in sub-Saharan Africa, such as Niger, have TFRs around 6, while South Korea's has dropped below 1, among the lowest ever recorded. The U.S. TFR is about 1.6.

Replacement-level fertility is the TFR needed for each generation to exactly replace itself, so the population stays stable over the long run (ignoring migration). You might expect it to be exactly 2, one child for each parent. It's slightly higher, about 2.1 in developed countries, because some children die before reaching adulthood and slightly more boys than girls are born.

In countries where many children die young, replacement level is higher, sometimes 2.5 or more. If TFR stays below replacement for a long time, the population will eventually shrink unless immigration makes up the difference.

Infant mortality rate is the number of babies who die before their first birthday per 1,000 live births. It's a strong indicator of a country's health care, nutrition and sanitation. Babies are more likely to survive when their mothers get good health care, including care during pregnancy, and enough nutritious food.

Infant mortality and TFR are closely linked. Where many infants die, parents tend to have more children to make sure some survive. As infant mortality falls, families usually have fewer children, but there's often a lag of a generation or more, which causes a period of rapid population growth.

Cultural and religious values also affect fertility. In some countries, TFR remains high even as incomes rise, while in others it has fallen far below replacement.

• Education for girls and women: women with more schooling tend to marry later, have more job options and choose smaller families.
• Later age at first marriage and first birth: fewer childbearing years leads to fewer children.
• Access to family planning: affordable contraception lets couples choose how many children to have and when.
• Lower infant mortality: when children reliably survive, parents don't need as many.
• Economic changes: in cities and industrial economies, children are expensive and don't work on family farms, and pensions reduce the need for children to support parents in old age.
• Government policy: China's one-child policy (1980–2015) lowered births through strict limits, while other governments use incentives to encourage or discourage childbearing.