An age-sex population pyramid shows the percentage or number of males and females in each age group. Read the axes first, then use the base, sides, top and any irregularities to infer fertility, mortality, life expectancy and possible future population change.
This is core SL and HL content in Subtopic 8.1 Human populations. At HL, interpretation extends to dependency ratio and population momentum.
How to interpret the graph
- Check the horizontal axis: it shows population size or percentage. Do not compare bar length until you know which measure is used.
- Check the vertical axis: age cohorts usually increase upward, often in five-year intervals.
- Identify which side represents males and which represents females.
- Compare the main features:
| Feature | Likely interpretation |
|---|---|
| Wide base | High recent birth rate and a youthful population |
| Narrow base | Low or declining birth rate |
| Rapidly narrowing sides | Relatively high mortality and lower life expectancy |
| Nearly vertical sides | Similar-sized adult cohorts and slow population change |
| Broad top | High life expectancy and an ageing population |
| Bulge or indentation | Migration, conflict, disease, a baby boom or changing birth rates affecting particular cohorts |
A broad-based pyramid, such as that commonly associated with Niger, suggests rapid growth and future population momentum because many young people will soon enter reproductive ages. A narrower base and broad upper section, as seen in Japan, indicates low fertility, ageing and potentially natural population decline.
However, shape alone does not prove a cause. A missing working-age cohort could result from emigration, conflict or earlier low fertility, so conclusions should be supported by contextual data.
