Replacement-level fertility refers to an average level of completed fertility (this could be measured using cohort fertility rates) that, if maintained over the long run, would lead to an approximately constant population over time (assuming that there are no significant gains in longevity). The replacement-level fertility is typically slightly over 2, because women are about half the population. In populations that do not experience significant mortality or morbidity risks for people until their childbearing years, a reasonable ballpark for replacement-level fertility is 2.1.
Relation with population growth
A common misconception is that if the population is at replacement-level fertility, then the population would be stable. However, there are a number of complications:
- Population momentum arising from above-replacement fertility in previous generations means that the childbearing generation is larger in size than the oldest generation that is dying out. Therefore, even at replacement-level or sub-replacement-level fertility, population may grow due to population momentum. Population momentum could also work the other way: if previous generations had fertility that was considerably below-replacement, then population may decline even with above-replacement fertility rates.
- Continued increases in longevity leading to declining death rates. Note that high longevity per se is not the issue. Rather, the issue is increasing longevity.
- Tempo effects, i.e., changes in the age at which people are having children.