The age-sex structure of a population refers to the number of females and males in different age groups. The two variables are represented in a population pyramid, the shape of which reflects the characteristics of the population

Based on the shape of the pyramid, there are three trends observable in any population, these are:

Expanding Population: represented in a triangular shaped pyramid with a wide base, is typical of less developed countries. These have larger populations in lower age groups due to high birth rates.

Eg: Nigeria

Constant Population: represented by a bell-shaped pyramid that tapers towards the top. Such populations have almost equal birth and death rates, leading to a near constant population.

Eg: Australia

Declining Population: represented by a narrow base and a tapered top, such pyramids show low birth and death rates. The growth rate for such a population is usually zero or negative.

Eg: Japan