The traditional demographic transition model is commonly presented as four stages, although some versions add a fifth stage. Stage 1 is characterised by high birth rates and high death rates, producing relatively slow population growth because births and deaths are both numerous. Stage 2 begins when death rates fall substantially while birth rates remain high, producing rapid natural population growth. Stage 3 occurs when birth rates begin to decline and population growth slows.
Stage 4 is characterised by low birth rates and low death rates, with population growth approaching zero or remaining relatively small. Some modern versions describe a Stage 5 in which fertility remains below the replacement level and deaths can exceed births, producing natural population decline and increasing population ageing. These stages are analytical categories rather than mandatory steps that every country must experience in an identical way. The model is particularly useful for explaining changes in age structure and population growth over long periods.
Stage 1 is often associated with preindustrial populations in which both fertility and mortality are high. Families may have many children because child mortality is substantial, children can contribute to household production and there are limited means of controlling fertility. Death rates can be high because of infectious disease, poor sanitation, food insecurity, limited medical care and periodic famine or epidemic disease. Life expectancy is comparatively low, although individuals who survive childhood can sometimes live to relatively old ages.
Population growth remains limited because high fertility is offset by high mortality. Stage 1 is mainly of historical importance for contemporary demographic analysis because very few countries now maintain the complete combination of persistently high fertility and high mortality characteristic of the classical preindustrial regime. The stage nevertheless provides the starting point for understanding the transition.
Stage 2 begins when mortality falls while fertility remains high. Improvements in sanitation, clean water, nutrition, food distribution, vaccination, maternal healthcare and disease prevention can cause death rates to decline rapidly. Birth rates may remain high because fertility preferences, social norms and family structures change more slowly than mortality conditions. The resulting gap between births and deaths produces rapid natural increase. Populations in Stage 2 are generally youthful because large numbers of children survive and fertility remains high.
This stage can produce rapid expansion of schools, housing, healthcare systems, transport networks and labour supply. It also creates population momentum because large child cohorts eventually reach reproductive ages. Stage 2 therefore represents the period of greatest potential natural population growth in the classical model.
Stage 3 is characterised by declining birth rates and continuing relatively low death rates. Fertility falls for a combination of reasons including urbanisation, female education, increased employment opportunities for women, improved access to contraception, delayed marriage, declining child mortality, changing cultural preferences and the increasing financial cost of raising children. Population growth continues but generally slows because the gap between births and deaths becomes smaller.
The age structure begins to change because fewer children enter each new generation while large earlier generations move into working ages. The proportion of working age people can therefore increase substantially, creating conditions in which a demographic dividend may become possible. The dividend itself is not automatic because employment, education, health and economic policies determine whether the changing age structure produces economic benefits.
Stage 4 is characterised by low fertility and low mortality. Population growth becomes slow, stable or close to zero, although migration can still produce substantial changes in total population. Life expectancy is generally high, and a large proportion of people survive into older ages. The age structure becomes much less pyramid shaped and can become relatively rectangular because children constitute a smaller share while adults and older people constitute larger shares.
Countries in or near Stage 4 often face issues such as population ageing, pension financing, healthcare demand and slower growth of the working age population. However, low population growth does not necessarily mean economic stagnation. Productivity growth, technology, higher labour force participation, migration and changes in retirement patterns can influence economic performance independently of population size.
Stage 5 is used in some versions of the demographic transition model to describe populations where fertility remains below replacement level for a prolonged period and the number of deaths eventually exceeds the number of births. Such populations can experience natural decrease and substantial ageing. Japan, Italy, Germany and South Korea are examples often discussed in connection with very low fertility and population ageing, although their demographic circumstances differ and stage classifications depend on the particular model being used.
A Stage 5 population may shrink without immigration and may experience a rising old age dependency ratio. The distinction between Stage 4 and Stage 5 is therefore primarily about whether natural population change remains approximately stable or becomes persistently negative. The fifth stage is not part of every textbook version of the model and should be treated as an extension rather than an undisputed component of the original framework.
The stages are connected through changes in fertility, mortality and age structure, but actual countries do not always follow a neat progression. War, famine, epidemics, migration, political intervention, economic shocks and cultural changes can alter demographic patterns. Some countries have experienced very rapid fertility decline, while others have remained at high fertility for much longer. Migration can also make a country's total population grow even when natural increase is low or negative.
The model is therefore best used to identify broad demographic conditions rather than to assign countries permanently to a single stage. The same country can also display regional differences, with urban populations having lower fertility and older age structures than rural populations. A national stage classification can conceal substantial internal variation.
The stages of demographic transition are useful because they provide a framework for understanding why population growth changes over time. Stage 1 involves high fertility and high mortality with limited growth. Stage 2 involves falling mortality and rapid growth. Stage 3 involves falling fertility and slowing growth. Stage 4 involves low fertility and low mortality with stable or slow growth. Stage 5, where used, involves sustained below replacement fertility, natural decrease and increasing ageing.
These stages help connect demographic processes with economic development, dependency ratios, labour supply and population structure. They should not be interpreted as a deterministic ladder in which every society must behave identically. Modern demography combines the model with detailed data on births, deaths, migration, fertility preferences, life expectancy, age structure and socioeconomic conditions to understand how individual populations actually change.
