Total Fertility Rate

Total fertility rate, or TFR, is the average number of children born per woman over her reproductive lifetime in a given population. Two parents need to produce two children just to replace themselves, but TFR has to sit slightly above 2 to hold a population steady, because some children die before growing up to have kids of their own. In wealthy countries with low infant mortality, that number is about 2.1. In countries where more children die young, replacement fertility is higher, sometimes 2.5 or more.

TFR falls as countries develop. More years of schooling, especially for women, delays childbearing and shrinks desired family size. Access to contraception lets people act on that smaller desired family size. Falling infant mortality removes the old incentive to have extra children as insurance against loss, so parents have fewer kids and invest more in each one. These forces usually move together, which is why TFR tracks so closely with development.

A country can hit exactly 2.1 and keep growing for decades anyway. This is population momentum: if past fertility was high, there's already a large bulge of children and teenagers alive who haven't reached their reproductive years yet. They still have to grow up, have their own kids, and age through the population before growth levels off. This connects directly to age-structure pyramids: a wide base today locks in future growth no matter what fertility does next, and a TFR that drops far below replacement, like 1.2, eventually produces the opposite problem, a shrinking workforce supporting a large retired population.