Immigrant's ages and the structure of stationary populations with below-replacement fertility.

A sustained regime of low fertility plus immigration yields an unusual kind of stationary population. The author demonstrates that all stationary populations have a common structure, and that the familiar replacement-level fertility population is the youngest among the many stationary populations corresponding to a particular life table. This finding has important consequences for policy because although fertility increase and immigration are equally effective at halting population decline, immigration is inferior as a means of rejuvenating low-fertility populations. In fact, an immigration-based policy could make a low-fertility population older rather than younger. The paper includes examples using U.S. and West German vital rates.