Natural population change is the change in the size of a population resulting solely from births and deaths, without including migration. It is calculated by subtracting the number of deaths from the number of births during a specified period. When births exceed deaths, the population experiences natural increase. When deaths exceed births, it experiences natural decrease. When births and deaths are equal, natural change is zero.
The concept is called natural because it refers to changes produced by the biological demographic processes of birth and death rather than movement into or out of the population. It is nevertheless important to recognise that births and deaths are strongly influenced by social, economic, medical and environmental conditions. Natural change is therefore a demographic accounting category, not a claim that births and deaths occur independently of human institutions and circumstances.
Natural change is different from total population change because total change also includes migration. The complete demographic accounting equation is population change equal to births minus deaths plus immigration minus emigration. Natural change is only the first part of this equation, births minus deaths. A country can have positive natural increase while its total population declines if emigration exceeds the combined effect of births over deaths.
Conversely, a country can have natural decrease while total population grows because immigration exceeds the natural loss. This distinction is particularly important in populations with low fertility and ageing populations, where deaths can exceed births while substantial immigration maintains population growth. Natural change therefore provides information about the internal demographic balance but does not by itself describe the overall direction of population size.
The balance between births and deaths is affected by fertility, mortality and age structure. Fertility determines how many births occur, while mortality determines how many deaths occur. Age structure affects both processes because births are concentrated among people in reproductive ages and deaths are concentrated increasingly among older ages. A young population can have many births and relatively few deaths even if individual fertility rates are not exceptionally high.
An older population can have relatively few births and many deaths even when mortality at each age is relatively low. This is one reason why natural change cannot be inferred solely from fertility or life expectancy. Demographic structure determines how population rates translate into actual numbers of births and deaths.
Natural increase was a central feature of the demographic transition in many countries. In the early stage of the transition, fertility and mortality were both high, so natural increase was limited over the long term. Mortality subsequently declined as public health and living conditions improved, while fertility initially remained high. The difference between births and deaths widened, creating rapid natural increase. Later, fertility declined and natural increase slowed. Some populations have subsequently reached a stage in which deaths exceed births, producing natural decrease. This sequence is not identical everywhere, but it provides a useful framework for understanding historical changes in the balance between births and deaths.
Natural population change can be expressed as an absolute number or as a rate. An absolute natural increase might be calculated by subtracting annual deaths from annual births. A crude rate of natural increase expresses births minus deaths relative to the population, often per 1,000 people. The crude birth rate and crude death rate can therefore be combined to estimate the rate of natural increase. However, crude rates are influenced by age structure. Two populations with identical age specific mortality can have different crude death rates if one contains many more elderly people. For detailed comparisons, demographers may therefore use age standardisation or age specific measures to separate demographic structure from underlying mortality conditions.
Natural decrease does not necessarily indicate worsening health. A population with high life expectancy and a large elderly population can have more deaths than births even though mortality rates at comparable ages are low. Deaths eventually occur because everyone is mortal, and an older population contains more people at ages when death is statistically more common. Similarly, natural increase does not necessarily mean that mortality conditions are poor.
A population with low mortality can still grow naturally if fertility is sufficiently high and its age structure contains many people in reproductive ages. Interpretation therefore requires attention to age structure, fertility and mortality together rather than treating natural increase or decrease as a direct health ranking.
Migration can substantially modify the consequences of natural change. Positive net migration can offset natural decrease, while negative net migration can offset natural increase. Migrants can also change the future natural balance because migration changes the age and sex composition of the population. If migrants are disproportionately young adults, their arrival can increase the number of potential parents and therefore future births. Over time, migrants also age and contribute to deaths. The relationship between migration and natural change is consequently dynamic rather than a simple one time adjustment. Population projections usually model births, deaths and migration separately and then combine their effects.
Natural population change is particularly useful for understanding whether a population is reproducing itself through births relative to deaths independently of migration. It allows analysts to distinguish a population whose growth is primarily internally generated from one whose growth is mainly associated with migration. It also helps identify demographic ageing and potential future population decline when natural decrease persists. Nevertheless, natural change should never be interpreted in isolation. Fertility, mortality, age structure and migration interact over time, and current natural change can reflect demographic decisions and conditions established decades earlier. It is therefore both a simple accounting measure and an important indicator of longer term demographic dynamics.
