Every change in a population's size can be traced to just three components — births, deaths and movement across boundaries. Fertility adds members, mortality removes them, and migration redistributes them between places. The balance of these forces determines whether a population grows, stabilises or declines, and their changing relationship over time is one of the great regularities that demographers and sociologists have sought to explain. This note examines each component, sets out the demographic transition theory that links them, and applies the framework to India's much-discussed demographic dividend.
Fertility
Fertility refers to the actual reproductive performance of a population, distinct from fecundity, which is the biological capacity to reproduce. It is commonly measured by the crude birth rate — births per thousand population — and more precisely by the total fertility rate, the average number of children a woman would bear over her lifetime at prevailing rates. India's total fertility rate has fallen dramatically, reaching and in many states dropping below the replacement level of about 2.1.
Sociologists emphasise that fertility is socially governed rather than purely biological. Age at marriage, female education, women's participation in paid work, infant survival, religious and caste norms, and the perceived economic value of children all shape reproductive behaviour. The wide gulf between the low fertility of Kerala and Tamil Nadu and the higher fertility of parts of the northern belt illustrates how social development, not geography, drives the transition.
Mortality
Mortality is measured by the crude death rate and, more revealingly, by the infant mortality rate and life expectancy at birth. India has achieved striking gains — life expectancy has more than doubled since independence, and infant mortality has fallen sharply, though it remains high by international standards. Improvements in nutrition, immunisation, sanitation, maternal care and the control of epidemic disease account for much of this progress.
Because death rates typically fall before birth rates, the gap between them widens for a period and produces rapid natural increase. Mortality decline is therefore the historical trigger of population explosion, even though it is unambiguously a social good. Persistent inequalities in mortality — between regions, classes, castes and the sexes — remain a central concern for the sociology of health.
Migration
Migration is movement that involves a change of residence across a defined boundary, whether internal or international. It has no biological ceiling and can transform the size and composition of local populations very quickly. In India, internal migration — rural to urban, and increasingly between states — dominates, driven by employment, marriage, education and distress. Migration reshapes sending and receiving areas alike, generating remittances, altering sex ratios, and creating new social tensions around belonging and citizenship.
The demographic transition theory
The demographic transition theory offers the classic model linking these components. It describes a movement through broad stages: an initial phase of high birth and high death rates yielding slow growth; a second phase in which death rates fall while birth rates stay high, producing rapid growth; a third phase in which birth rates too begin to decline; and a final phase of low birth and low death rates and renewed stability. The theory generalises the European experience and has been criticised for treating that path as universal, yet it remains a valuable heuristic for locating a society within a longer trajectory. India today sits firmly in the later stages, with mortality low and fertility approaching replacement.
India's demographic dividend
Because fertility has fallen while a large youthful cohort has already been born, India now has an unusually high share of its people in the working ages relative to dependents. This bulge is the demographic dividend — a window in which a favourable dependency ratio could accelerate savings, investment and growth. The dividend, however, is not automatic. It is realised only if the young are educated, healthy and productively employed; otherwise a youthful population becomes a source of unemployment and unrest. The window is also temporary, since the same cohorts will eventually age, shifting the burden towards elder care.
How to use this in the exam
Treat the three components as an interacting system rather than as isolated rates, and always distinguish the technical measures — crude birth rate, total fertility rate, infant mortality rate — so your answer reads as informed. Use the demographic transition theory as a scaffold but show awareness of its Eurocentric limits. On the demographic dividend, resist easy optimism: emphasise the conditionality of education, health and jobs, and note that the dividend is a closing window that ageing will follow. Regional contrasts within India are your strongest evidence of the social roots of demographic change.