When fertility drops below replacement level (~2.1 births per woman), a population can still grow for decades — momentum from a young age structure keeps births outpacing deaths. Drag the slider to adjust fertility rate and watch natural increase respond.
In developed nations like Japan and Italy, fertility has fallen to 1.3–1.4 births per woman — well below replacement. Yet deaths still outnumber births only slightly, because these populations are old: decades of low fertility have aged the structure, and mortality rises slowly as large cohorts move into older age brackets. The gap between births and deaths narrows gradually, not overnight, even when fertility plummets.
Natural increase equals births minus deaths; replacement fertility (~2.1) keeps population stable long-term, but momentum from age structure means growth or decline lags behind fertility changes by decades.