Glossary term
Fertility transition
The long-run decline in a country's total fertility rate from high pre-modern levels (typically 5–7 children per woman) to low post-transition levels (below 2.1, the replacement threshold). It follows the mortality transition and is the primary mechanism by which rapid population growth eventually slows — though in some countries fertility has fallen far below replacement, creating new demographic challenges.
What drives the fertility transition
The fertility transition is not a single-cause phenomenon. Research from the International Institute for Applied Systems Analysis (IIASA) and the World Bank identifies five main drivers that typically operate simultaneously:
Falling child mortality. When infant mortality falls, the "insurance" demand for extra births falls too. Parents no longer need many children to ensure some survive. This is often the trigger that initiates the transition.
Female education. Each additional year of female secondary schooling is associated with a TFR reduction of 0.3–0.5. The mechanism operates through delayed marriage, increased economic opportunities outside the home, and greater decision-making power over childbearing.
Urbanisation. Urban fertility is consistently lower than rural fertility everywhere in the world — driven by higher housing costs, later marriage, greater female labour force participation, and access to contraception.
Access to modern contraception. The UNFPA estimates 218 million women have unmet need for family planning. Where contraceptive access expands alongside demand, fertility falls faster.
Changing social norms. Once small-family norms become culturally established — typically once urbanisation reaches ~50% — they tend to be self-reinforcing across generations.
Speed of transition: lessons from history
Early transitions in Europe and North America took 50–100 years. France's TFR fell from about 5.5 in 1750 to 2.5 by 1900. Sweden's transition was similar. More recent transitions have been dramatically faster. Iran achieved a TFR decline from 6.5 to 1.8 in just 26 years (1980–2006) — one of the fastest in history — through a combination of expanded female education, rapid urbanisation, and a government that reversed its pro-natalist stance post-revolution and embraced family planning. Lancet 2024 global fertility projections show that rapid transitions are now possible in 20–30 years where investment in female education and healthcare is sustained.
Bangladesh went from TFR 6.9 in 1975 to 2.3 by 2010 despite remaining a low-income country. DHS Programme survey data shows this was driven primarily by expanded community health worker networks delivering family planning services and female education investment. The Bangladeshi case is often cited as evidence that income growth is not a necessary precondition for fertility decline — targeted health investment can trigger the transition even in poor countries.
When fertility goes too far
A "successful" fertility transition ends with TFR near replacement (~2.1). But in many East Asian and European countries, TFR has fallen far below this — a phenomenon demographers call "lowest-low fertility." South Korea (0.72), Japan (1.2), Italy (1.20), and Spain (1.16) are the most prominent examples. Read our detailed analysis of the South Korea fertility crisis and China's one-child policy legacy.
The demographic transition model originally assumed fertility would stabilise around replacement level — a Stage 4 equilibrium. The emergence of lowest-low fertility has prompted some demographers to propose a "second demographic transition" driven by post-materialist values, individual autonomy, and the incompatibility of demanding labour markets with parenthood in societies that have not adapted social infrastructure to support working parents.
Frequently asked questions
What is the fertility transition?
The fertility transition is the long-run decline in a country's TFR from high pre-modern levels (5–7) to low post-transition levels (below 2.1). It follows the mortality transition and is the primary mechanism by which rapid population growth eventually slows.
What drives the fertility transition?
Five main interacting drivers: (1) falling infant and child mortality, (2) rising female education, (3) urbanisation, (4) access to modern contraception, and (5) changing social norms around family size. Where all five operate simultaneously, transitions can happen in 20–30 years.
How fast can a fertility transition happen?
Early European transitions took 50–100 years. More recent ones have been faster: South Korea TFR 6.0 → 2.1 in 24 years; Iran 6.5 → 1.8 in 26 years; Bangladesh 6.9 → 2.3 in 35 years. The Lancet 2024 shows rapid transitions are possible where female education and healthcare investment is sustained.
Can a fertility transition go too far?
Yes. South Korea (TFR 0.72), Japan (1.2), and much of Southern Europe are experiencing "lowest-low fertility" — far below replacement. This leads to population ageing, workforce shrinkage, and eventual decline. See our articles on the South Korea fertility crisis and Europe's population decline.