👪 Population Pyramid

It is the standard demographic technique for projecting a population forward: each age-sex cohort is aged forward by the time step, losing people to age-specific mortality (survival), while new births are added at the bottom from women of childbearing age and net migration is added or removed across the working-age bands. Repeating this step by step traces out how the whole age-sex structure — not just the total population — changes over decades.

Deaths come from an annual survival probability per band, built from a baseline mortality curve (about 4% in the first year of life, roughly 0.15–0.35% through childhood and working ages, then rising exponentially after 50) and adjusted by (72 ÷ life expectancy)^1.8, so a higher life-expectancy slider genuinely lowers mortality at every age. Births equal (total fertility rate ÷ 35) × women aged 15–49 × the time step in years, added to the youngest band split 51.2% male and 48.8% female, the natural sex ratio at birth.

Expansive means a wide base and progressively narrower older bands — a young, fast-growing population typical of high-fertility countries. Stationary means roughly straight sides, indicating births and deaths are close to balance. Constrictive means a narrower base than middle, showing below-replacement fertility and an ageing population. Boom echo starts with a bulge partway up the pyramid, mimicking a country that had a fertility spike a generation ago whose children are now entering the population themselves.

Median age is found by walking up the age bands until the cumulative population reaches half the total, then interpolating within that band. The dependency ratio is the population under 15 plus the population 65 and over, divided by the working-age population aged 15–64, expressed as a percentage — a standard measure economists use to gauge how many dependents each working-age person effectively supports.

A country's pyramid shape is a direct record of its fertility and mortality history: a sustained high total fertility rate produces a wide base and an expansive pyramid (much of sub-Saharan Africa today), a fertility rate near 2.1 that has held steady for decades produces a stationary pyramid, and fertility that has fallen well below 2.1 for a generation or more — as in Japan, Italy, and South Korea — produces a constrictive pyramid with a shrinking base and a growing elderly population. Migration and past baby booms or wars can also leave permanent bulges or dents in the shape.