Population pyramid shapes: expanding versus contracting population
A population pyramid (a graph of age and sex make-up) shows an expanding (growing) population by a wide base, a contracting (shrinking) one by a narrow base and broad top.
Remember
A wide base means high fertility (the average number of children born per woman) and a narrow base means low fertility, while rapid narrowing above a wide base means high childhood mortality. A wide top alone shows that many people are surviving into old age, but it says nothing about how many children are being born.
A population pyramid is a graph of the percentage of the population at each age, with males on one side and females on the other. The width of each band shows how many people are alive at that age. Read it from the bottom up: births show in the base, survival in the sides, and the number reaching old age in the top.
A very wide base means a high fertility rate (the average number of children born per woman). Rapid narrowing above the base shows that many children die before adulthood, so childhood mortality (the proportion of children who die before reaching adulthood) is high. A small top means few people reach old age. These give a rapidly expanding population, one that tends to keep growing. Afghanistan is an example, and the shape is typical of many least developed countries (those with the lowest levels of economic development). Where more children survive, as in Iraq, the sides narrow less severely and the shape is more triangular.
A narrow base means few babies are being born, and a broad top means longer lifespans. More people die than are born, so the population shrinks over time. That is a contracting population, as in Japan, where, on 2018 figures, the average number of children per woman was 1.4. A stable population, as in the USA, has a similar number of births and deaths and a more even pyramid.
A pyramid shows a population's tendency but does not directly measure future growth, because migration and changing fertility can alter what happens next.
Key facts
- Wide base
- a high fertility rate, so many children are born
- Rapid narrowing
- high childhood mortality, so many children do not reach adulthood
- Narrow base and broad top
- low fertility and longer lifespans, a contracting population
- Afghanistan and Iraq
- both are expanding populations, but Iraq has lower childhood mortality and so a less severe narrowing
- Stable population
- births and deaths are similar, giving a more even pyramid, as in the USA
- Wide top alone
- shows many people surviving into old age, but says nothing about how many children are being born
Exam traps
- A top-heavy shape (broad at the older ages) does not signal rapid expansion, because longer survival widens the top without showing how many children are being born.
- Low fertility does not fit a very wide base, because low fertility would give a narrow base, even though high childhood mortality does fit narrowing sides.
- High fertility with a wide base is not enough to show low childhood mortality: if most children survived, the sides would narrow less severely and more people would reach later life.
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