Quantifying the Momentum of the Age Transition

The age transition is the single most important aspect of the overall demographic transition because the demographic changes to which a society must respond manifest themselves through changes in the age structure. It is also a difficult concept to quantify because it reflects so many changes taking place simultaneously. Professor Thomas Espenshade and some of his colleagues at Princeton's Office of Population Research have taken on this challenge by decomposing population momentum into that part which is due just to the difference between the actual and the stable age structures (what they call non-stable momentum), and that part which is due to the relative changes between fertility and mortality (which they call stable momentum). Their paper, which will appear in the journal Demography next year, is not necessarily an easy read, but here are some highlights:

Our estimates indicate that world population would grow by an additional 40% if global fertility rates had moved instantaneously to replacement in 2005. Nonstable and stable momentum contribute roughly equal shares to world population momentum. Taking natural logarithms shows that nonstable momentum accounts for about 53% of total world momentum, and stable momentum contributes roughly 47%.

The value for nonstable momentum reflects a country’s recent trend in fertility. In a stable population where fertility has been constant for a long time, nonstable momentum is nonexistent. But countries that have a history of fertility decline, especially a recent and sharp decline, will have larger values for nonstable momentum. The value for stable momentum is dictated by a population’s current level of fertility in relation to mortality. A high (low) net reproduction rate corresponds to a large (small) value for stable momentum. This decomposition gives us a new way of thinking about the determinants of overall population momentum. In addition, it allows us to integrate disparate strands of the population momentum literature and see how the various kinds of momentum that researchers have considered fit together into a single analytic and empirical framework.