Population Composition
A population pyramid packs a country's entire age and sex structure into one shape — and that shape alone can tell you whether a country is growing fast, holding steady, or about to shrink.
A population pyramid stacks horizontal bars for each age group, split into males (usually on the left) and females (usually on the right). The overall shape tells the story: a wide base narrowing sharply toward the top means a high birth rate and rapid growth (Stage 2 of the Demographic Transition Model); a rectangular shape with roughly equal bars all the way up means low, stable birth and death rates (Stage 4); and a shape that’s narrower at the base than in the middle — constrictive — signals a declining birth rate and a population that may shrink or age significantly in the coming decades.
Two other numbers come straight out of that same age-sex structure. The sex ratio compares males to females, usually expressed as males per 100 females — a war or a policy that skews births toward one sex will show up as a visible notch in the pyramid at exactly that age group. The dependency ratio compares dependents (under 15 and over 64) to the working-age population (15–64) — it’s a measure of how much economic weight each working-age person is, on average, carrying.
The same overall dependency ratio can hide very different pressures. A country with mostly child dependents needs to invest in schools and child care; a country with mostly elderly dependents needs to invest in healthcare and pensions — the number alone doesn’t tell you which.