The Demographic Transition Model describes the typical shift of a population, as a country develops, from high birth and death rates to low birth and death rates, passing through recognisable stages of population growth.
- Stage 1 (high stationary): high birth rate and high death rate roughly cancel out, so population is large but grows slowly, as in pre-industrial societies.
- Stage 2 (early expanding): the death rate falls sharply (better food, medicine, sanitation) while the birth rate stays high, so population grows fast.
- Stage 3 (late expanding): the birth rate now also falls (urbanisation, education, family planning) while the death rate is already low, so growth slows.
- Stage 4 (low stationary): both birth and death rates are low, so population is large but stable; some models add a Stage 5 of decline where the birth rate falls below the death rate (as in Japan and parts of Europe).
- India is broadly in Stage 3, with falling fertility; several southern states have reached replacement-level fertility (a total fertility rate of about 2.1), while the population is ageing slowly, linked to the demographic dividend.
The four (or five) stages and the order in which birth and death rates fall, the high-growth Stage 2 cause (death rate falls first), and India's broad Stage 3 position connect to population geography and the demographic dividend.
In Stage 2 it is the death rate that falls first (not the birth rate), which is why population explodes; replacement-level fertility (about 2.1) is not zero growth instantly because of population momentum; Stage 4 is stable, Stage 5 is declining.
Population shift through stages as a country develops: high-high, then death rate falls (rapid growth), then birth rate falls, then low-low (stable).