Stationary populations with below-replacement fertility

BACKGROUND A population with sustained below-replacement fertility and constant immigration eventually becomes stationary. Stationary-through-immigration (SI) populations have unusual age structures that depend on the distribution of immigrants' arrival ages. OBJECTIVE I summarize known formal relationships between the distribution of immigrants' entry ages and the long-run size and structure of SI populations. I clarify a previously published result about SI dependency ratios. RESULTS The long-run size and age structure of an SI population depend on the remaining life expectancies of arriving immigrants, but are also sensitive to the expected numbers of native children born after arrival. Numerical calculations with contemporary Austrian data show (1) contrary to previously published results, immigration flows need not be concentrated in early working ages in order to ensure low overall dependency, and (2) the SI dependency ratio is minimized when all immigrants are in their mid-30s.