Fertility and Population Age Structure

Fertility is the actual childbearing of a population and is commonly summarised by the total fertility rate, the number of children a woman would bear if prevailing age-specific fertility rates continued through her reproductive years. Age structure is the distribution of people among different age groups. Fertility shapes it by determining the size of successive birth cohorts.

How fertility changes the population pyramid

• Sustained high fertility produces large successive birth cohorts, giving the population pyramid a broad base and a high proportion of children.
• A fertility decline reduces births relative to the previous trajectory, so younger cohorts gradually form a smaller share of the population.
• These changes operate through the cohort effect: large or small birth cohorts retain their relative size as they move through childhood, working ages and old age.
• The effect is gradual because today's age structure reflects fertility patterns accumulated over several decades.

Changing age structure during population growth

A population can continue growing even after fertility falls because its earlier high fertility has created a large generation entering reproductive ages. This is called population momentum.

• In an early phase of growth, falling mortality combined with high fertility produces a youthful population.
• As fertility declines, the child share falls and the working-age share generally rises, creating the potential for a demographic dividend.
• As large cohorts grow older and low fertility persists, the median age and elderly share rise, producing population ageing even while total population is still increasing.

Feedbacks and wider determinants

Age structure also influences future births: even moderate fertility can produce many births when a population contains numerous women of reproductive age.

• A larger working-age share creates an economic opportunity only when supported by health, education and employment.
• Fertility does not act alone; mortality and migration also alter the survival, movement and relative size of age groups.
• Consequently, population growth rates and total population size do not by themselves reveal whether a population is becoming younger or older.