In 2010, the number of American workers for every recipient of Social Security fell below three for the first time. In Japan, the ratio has fallen to just two workers per retiree. Globally, the figure is expected to fall to four by 2050.

Why it matters: Societies typically leverage the labor and taxes of their working-age population to support their aged through pension and health programs. But as populations age and the number of workers shrink, governments will have to increase taxes, retirement ages, or both — or services will have to be cut.