The Dependency Ratio

A country needs its working-age population to support everyone else, through taxes, wages, and direct family care. The dependency ratio captures that burden in one number: it adds up the dependents, people under 15 and people over 65, and divides by the working-age population (15 to 64), then multiplies by 100 to express it as dependents per 100 workers. A high ratio means fewer workers are stretched to support more dependents, whether those dependents are babies or retirees. That is why a youthful country bursting with children and an aging country full of retirees can face the exact same squeeze: the math doesn't care which end of the age pyramid is heavy, only that the middle is thin relative to the edges.

To calculate it: add the under-15 population to the over-65 population, divide by the working-age population, and multiply by 100. The result tells you how many dependents each 100 working-age people must support. This number connects directly to the age pyramid shapes from earlier in the unit and to the demographic transition model: a country in stage 2, with a wide young base, has a high ratio driven by youth. A country deep in stage 4 or 5, like Japan, has a high ratio driven by the elderly instead. Watching how the dependency ratio shifts as a country moves through the stages shows why aging populations strain pensions and healthcare just as youth bulges strain schools and jobs.

The ratio also sets up a key extension: when a large generation, like a baby boom, ages out of the workforce, the dependency ratio rises sharply because that whole cohort flips from counted-as-worker to counted-as-dependent, even though the total population hasn't changed much.