The fertility rate of 2.1 with which populations are expected to eventually stabilize is known as the long-term replacement rate. In other words, if a nation maintains a fertility rate of 2.1 over a long period of time the population of that nation will eventually stabilize and it will neither grow nor shrink. Immigration allows the people to grow faster than the fertility rate would suggest. Also because of past growth, the population is younger than it would otherwise be. This means that there are more potential mothers and fathers. Only after its residents age can the population of a country stabilize.