On "Weak" and "Strong" Population Momentum

Thomas J. Espenshade, Princeton University
Analia S. Olgiati, Princeton University

This paper decomposes total population momentum into two multiplicative parts: “weak” momentum and “strong” momentum. Weak momentum depends on deviations between a population’s observed age distribution and its stable age distribution. Strong momentum is a function of deviations between a population’s stable and stationary age distributions. This paper provides numerical illustrations and then shows analytically that an initial population with arbitrary age composition, fertility and mortality is asymptotically equivalent (or very nearly so) to its stable equivalent population with respect to replacement fertility and mortality. We calculate weak, strong and total population momentum for 176 countries, the world and its major regions. The paper brings together seemingly disparate strands of the population momentum literature and shows how the various kinds of momentum fit together into a unified framework.