What is the age-sex structure?
The age-sex structure tells us how many people of each age, and how many males and how many females, live in a place. We draw it as a population pyramid.
How to read one: each row is an age group, for example 0 to 14 years. The youngest group is at the bottom. The bar on the left counts males. The bar on the right counts females. A longer bar means more people.
We use three big age groups. Children are 0 to 14. Working age is about 15 to 59 (some countries use 15 to 64). Old people are 60 or 65 and over. Children and old people are called dependants, because they usually need others to earn for them.
Three shapes of the pyramid
1. Wide base (expanding). Many babies are born, and fewer people reach old age. The pyramid looks like a triangle. The population grows fast. Schools, food and jobs for the young are the big needs. Many low-income countries show this shape.
2. Straight sides (stationary). Every group is about the same size. Births are close to deaths, so the total stays almost the same. Countries with small families and long lives are near this shape.
3. Narrow base (constricting). Few babies are born and people live long. The top is wider than the base. There are fewer workers for each old person. Pensions and health care cost more. Japan and several European countries show this shape.
India has been changing from the first shape towards the second: its base is getting narrower as families get smaller.
Sex ratio and why the sides are not equal
The sex ratio is the number of females for every 1000 males. Boys are born a little more often than girls (about 105 boys for 100 girls). But women usually live longer than men, so in the oldest rows there are more women. The ratio therefore changes as age rises.
If a lot of men move to another city or country for work, the working-age rows of that place can have fewer men. If girls are not allowed to be born or are neglected, the bottom rows show fewer girls. The pyramid is like an X-ray: it shows the history of births, deaths and migration.
What the shape tells a planner
Wide base: build schools, make child health care, plan jobs for the big young group that will soon be adults.
Fat middle: many workers. This can be a "gift" for the economy if they have jobs, because there are few dependants for each worker.
Wide top: build health care, pensions and care for old people, and think about raising the working age.
So reading the pyramid helps answer: how many children, how many workers and how many old people will this country have in 10 or 20 years? The people in the bottom rows today are the workers of tomorrow. This leads to the next lesson, on human capital and labour resources.
Try it: draw your own pyramid
Make a list of everyone in your street or your extended family. Put each person in one of five age groups: 0-14, 15-29, 30-44, 45-59, 60 and over. Count males and females in each group. Draw two bars for each row on squared paper: males to the left and females to the right, one square for one person.
Then predict before you check: is the base wider or narrower than the middle? Compare your shape with the three shapes in the 3D. Which one is it closest to?
Key formulas and definitions
- Sex ratio = (number of females ÷ number of males) × 1000
- Dependants = children (0 to 14) + old people (60 or 65 and over)
- Dependency ratio = (children + old people) ÷ working-age people × 100
- Working-age population: usually 15 to 59 (or 15 to 64)
- Pyramid shapes: wide base (expanding), straight sides (stationary), narrow base (constricting)
Worked examples
1. In a village, 400 are aged 0-14, 500 are aged 15-59 and 100 are 60+. Find the share of children.
Total = 400 + 500 + 100 = 1000. Children = 400 ÷ 1000 × 100 = 40%.
2. A town has 5000 men and 4800 women. Find the sex ratio.
Sex ratio = females ÷ males × 1000 = 4800 ÷ 5000 × 1000 = 960 females per 1000 males.
3. Same village as example 1: find the dependency ratio.
Dependants = 400 + 100 = 500. Workers = 500. Ratio = 500 ÷ 500 × 100 = 100. There are 100 dependants for 100 workers.
4. Country A: children 35%, workers 60%, old 5%. Country B: children 15%, workers 60%, old 25%. Which pyramid has the wider base, and which is ageing?
A has the wide base (35% children), so it is an expanding pyramid. B has a wide top (25% old) and a narrow base, so it is the ageing one. Both have 60% workers, but B has 40 dependants per 60 workers, the same as A: the ratio is equal, but the kind of dependants is different (schools for A, pensions for B).
5. In a school town 12% of the people are aged 60+ and 22% are aged 0-14. What percent are of working age?
Working age = 100 − 12 − 22 = 66%.
Common mistakes
- Reading the bar length as a number of people when the axis shows percentages. Always check what the axis says.
- Thinking a wide base means a rich country. It only tells you that many children are being born.
- Mixing up the sex ratio. In India it is females per 1000 males, not males per 1000 females.
- Thinking an ageing pyramid means the population is falling at once. It can still grow for some years.