Age structure diagrams

Key points

• An age structure diagram, or population pyramid, shows the distribution of age and sex in a population. The y-axis of the diagram shows age groups from birth to old age. The x-axis shows the population percentage. The percentage of males and females in each age group are shown on the left and right sides of the diagram, respectively.
• The shape of an age structure diagram can give insight into the growth rate of a population. While every population is unique, most can be generalized into four categories based on their shapes:
  • A population experiencing rapid growth has a large proportion of young people and a small proportion of older people. This pattern is seen in populations with high birth and death rates. Rapid growth is common in low- and lower-middle-income countries, where families tend to have more children and individuals have shorter lifespans. An age structure diagram for a rapidly growing population is shaped like a pyramid with sides that curve inward. Other terms for this type of growth include expansive and expanding.
  • A population experiencing slow growth also has a higher proportion of young people compared to older people, but the difference is not as extreme as in a rapidly growing population. This pattern is seen in populations with a high birth rate and a declining death rate as more people are living into middle or old age. A variety of factors can lead to this age structure, including growth due to immigration or increasing lifespans. An age structure diagram for a slowly growing population is shaped like a pyramid. Like rapid growth, other terms for slow growth include expansive and expanding.
  • A population that is stable has relatively similar proportions of young, middle age, and older people. This pattern is seen in populations with a declining birth rate and a low death rate. Stable populations are often seen in high-income countries, in which families tend to have fewer children and individuals have longer lifespans. An age structure diagram for a stable population is shaped like a dome or a hill. Other terms for this type of growth include zero growth, stationary, and constant.
  • A population that is declining has relatively few younger individuals. This pattern is seen in populations with low birth and death rates. Declining populations are often seen in countries with long established economic development, which tend to have readily available education and health care. An age-structure diagram for a declining population is shaped like a dome that constricts at the bottom. Other terms for this type of growth include negative growth, constrictive, and contracting.
• The basic principles of these human population growth patterns hold true for many populations in nature. Populations with large fractions of young and reproductive-age individuals are likely to grow. Populations with large fractions of post-reproductive-age individuals are likely to shrink.