The demographic transition is the historically observed movement of populations from a regime of high fertility and high mortality to one of low fertility and low mortality, passing through an intermediate phase in which death rates have fallen but birth rates have not, producing rapid population growth. Its distinctive feature — and the source of everything interesting about it — is the lag between the two declines. Mortality falls first and fertility follows only after a delay of decades, and the size of the gap between them determines how much a population multiplies before it stabilises.
The idea was sketched by Warren Thompson in 1929 and by Adolphe Landry, who called it the révolution démographique, and given its canonical statement by Frank Notestein in 1945. It began as a description of what had happened in industrialising Europe; the trouble started when it was converted into a prediction of what must happen everywhere.
The stages
The classic model has three stages, commonly extended to four or five.
- Stage one: high stationary. Birth and death rates are both high and fluctuate with harvests and epidemics; growth is negligible over the long run. Fertility is not unregulated but is sustained by early marriage, universal marriage and the absence of contraception, and is necessary given the likelihood that children will die.
- Stage two: early expanding. Mortality falls sharply while fertility remains high. Population grows fastest here, and the age structure becomes very young.
- Stage three: late expanding. Fertility begins to fall, growth decelerates, and the age structure begins to mature. This is where the demographic dividend — a temporarily large working-age share relative to dependents — appears.
- Stage four: low stationary. Both rates are low; growth approaches zero.
- Stage five, added later, describes sub-replacement fertility, ageing and population decline, as in Japan and much of Europe.
Why mortality falls, and why fertility follows later
Mortality decline is largely exogenous to the family. It is driven by public health rather than medicine: clean water and sewerage, vector control, vaccination, famine prevention through transport and administration, and rising nutrition. It requires no change in belief or aspiration, which is why it can be imported rapidly and cheaply — the reason twentieth-century mortality decline in Asia and Africa was far faster than Europe's, and the reason stage two growth there was far more explosive.
Fertility decline requires a change in the calculus of individual families, and so takes longer. Notestein attributed it to urbanisation, industrialism and the individualism of modern life. John Caldwell (1976) offered a sharper mechanism: his theory of intergenerational wealth flows holds that fertility is high wherever the net flow of resources runs from children to parents, and falls when the flow reverses — when children become net costs through schooling, deferred earning and the emotional nucleation of the family. Fertility, on this account, is economically rational in both regimes; what changes is the direction of the flow. Mass schooling is therefore the decisive variable, and female education the most powerful single predictor, since it raises the age at marriage, the opportunity cost of childbearing and women's capacity to act on their preferences — which is where Amartya Sen's emphasis on women's agency connects to demography.
Population momentum
The transition does not end when fertility reaches replacement level. Because stage two produced a very young age structure, a large cohort of women enters childbearing age even after family size has fallen, so the absolute number of births remains high and the population continues to grow for decades. This is population momentum, and it is the single most misunderstood point in the subject. India's total fertility rate had already fallen to replacement level by the time of the National Family Health Survey of 2019–21, yet India's population will continue to grow into the middle of the century. Momentum implies that population stabilisation is a matter of age structure, not of exhortation, and that coercive fertility control cannot accelerate it much.
India's transition and its regional contrasts
India entered stage two around 1921, conventionally treated as the great divide: after that census, mortality crises ceased to cancel out growth, and the population began to rise steadily. Growth peaked in the decades after Independence as mortality fell steeply while fertility remained high, and decelerated from the 1980s onwards as fertility declined. India is now late in stage three, with a maturing age structure and a shrinking dividend window.
What makes India analytically valuable is that it contains several transitions at once. Kerala and Tamil Nadu reached replacement fertility around 1990 — Kerala without high per-capita income, through female literacy, land reform, public health outreach and late marriage, the case that made Sen's argument about social provisioning rather than growth. Andhra Pradesh followed at low income levels. Meanwhile Bihar, Uttar Pradesh, Jharkhand and Madhya Pradesh sustained higher fertility and higher infant mortality for far longer, though NFHS-5 recorded most states at or near replacement level and only Bihar clearly above it. The consequences are political as well as demographic: differential transition shifts population shares between states, which bears directly on delimitation of parliamentary seats and on devolution of central revenue. Interstate migration from the high-fertility Hindi belt to the low-fertility south is a further consequence, and a growing source of ethnic and linguistic friction.
Criticisms
Four objections are standard. Eurocentrism: the model generalises one region's history and treats deviation as delay. Determinism: it presents the sequence as inevitable, whereas the Princeton European Fertility Project directed by Coale and Watkins found that European fertility decline began at wildly varying levels of income, urbanisation and industrialisation — and clustered instead by language and culture, suggesting diffusion of ideas rather than an economic threshold. Neglect of culture and agency: the model has little to say about religion, caste, kinship structure or the position of women, yet Kerala's transition is unintelligible without them, as is the north-south contrast in Indian fertility that Dyson and Moore traced to kinship. Finally it is descriptive rather than explanatory and offers no predictive threshold — it tells us what sequence to expect but not when a transition will begin, which is precisely what a policymaker needs to know.
For the UPSC answer
Set out the stages briefly and spend your space on the lag between mortality and fertility decline, since that is what generates growth and what the model actually explains. Use Caldwell's wealth flows to give a mechanism for fertility decline rather than merely listing modernisation as a cause, and always introduce population momentum to explain why India keeps growing despite replacement-level fertility. Ground the answer in the Kerala–Bihar contrast and in the 1921 great divide, and mention the demographic dividend and its closing window. Close with the Coale and Watkins finding, which is the strongest single piece of evidence that the model describes rather than explains, and note that kinship and women's agency must be built in for it to fit India.
References & further reading
- Thompson, W. S. (1929). Population. American Journal of Sociology, 34(6).
- Notestein, F. W. (1945). Population — The Long View. In T. Schultz (ed.), Food for the World. University of Chicago Press.
- Caldwell, J. C. (1976). Toward a Restatement of Demographic Transition Theory. Population and Development Review, 2(3/4).
- Coale, A. J. and Watkins, S. C. (eds.) (1986). The Decline of Fertility in Europe. Princeton University Press.
- Dyson, T. (2010). Population and Development: The Demographic Transition. Zed Books.
- International Institute for Population Sciences (2021). National Family Health Survey (NFHS-5), 2019–21. Government of India.