Population momentum is the potential for population change that is inherent in a population’s age structure. Many conceptualize population change as the result of fertility, mortality, and migration—but this is only partially correct. Fertility, mortality, and migration rates determine the age structure of any particular population. Once created, however, a population’s age structure embodies certain intrinsic properties of change. This brief demonstrates how population momentum works with simulated examples of positive and negative momentum. The brief then examines population momentum in India and Japan.

• This technical note is available in English and Arabic.