In demography we like to predict what the population will look like in the futur
In demography we like to predict what the population will look like in the future. Since almost anything can happen to birth, death, and immigration rates, there is no way to know this for sure. We like to assume that “present trends continue.” We can do this by studying a hypothetical “stable population” based on today’s fertility and mortality rates.
A stable population is one where:
- There is no immigration or emigration
- age-specific fertility rates do not change
- age-and-sex-specific mortality rates do not change
- enough time has passed for the age structure of the population to stabilize.
Why does it take time for a population to stabilize? Well, just because fertility rates stay the same this year and next doesn’t mean that the birth rate won’t change. The birth rate depends not only on fertility rates but also on the number of women of childbearing age, which could be different next year than it is this year.
Similarly, as we learned when standardizing for age, death rates depend not only on age-specific mortality rates but also on the age distribution of the population. So death rates won’t stabilize until the age structure stabilizes.
Consider a population where fertility has recently fallen. The birth rate will not necessarily immediately fall. Fertility may have dropped, but there may be a larger number of women of childbearing age this year than there was last year due to higher fertility rates in the past.
Between 2004 and 2009, Canada experienced a small baby boom. The crude birth rate rose from 10.5 births per 1000 to 11.3 per 1000. Some of this baby boom was likely due to the fact that, during the late 1940s and the 1950s and the early 1960s, many Canadians were born – our Baby Boom Generation. The grandchildren of these boomers were reaching child-bearing age between 2004 and 2009. Also, many of the children of the baby boom generation had delayed having children and were having them at this time.
The birth rate and death rate are not going to stabilize until fertility rates and mortality rates have been constant for some time, long enough for the age structure to stabilize. Once the age structure stabilizes, birth and death rates can stabilize too, and the overall rate of population growth or shrinkage will stabilize.
Later in this chapter we will learn how to calculate the age structure of a stable population. We will also learn how to calculate the rate of growth (or shrinkage) of a stable population. The rate of growth of a stable population is called the intrinsic rate of natural increase. It’s “natural” because it comes from the fertility and mortality of the population, and not because of immigration or emigration. It’s “intrinsic” because it derives from the fertility and mortality rates and will eventually emerge no matter what the initial population looks like.
A stationary population is a type of stable population. As with every stable population, the fertility and mortality rates are constant, and there is no mig