Immigration, below-replacement fertility, and long-term national population trends

The long-term demographic effects of immigration on a population experiencing below-replacement fertility are studied by assuming that the size and age composition of the immigrant population do not change over time. The size of the first-generation immigrant population becomes stationary within a time period not greater than the human life span. Thereafter, the number dying equals the number entering over any given time interval. The stationarity of the native population, among which deaths exceed births, is maintained by the compensating number of births to the immigrant population. The limiting age distribution of the country's population, although stationary, may not decline monotonically with age and may look like a camel's back, with one or two humps.