Summary

The growth or decline of a population is determined by fertility and mortality. One way to think about growth is to consider the size of each generation in relation to the next. The Total Fertility Rate measures the number of children that would be born to a hypothetical cohort of women if they lived to the end of their reproductive years and experienced the current fertility rates throughout. The Gross Reproduction Rate is similar, except that it is limited to female children. The number of female children can be approximated using the sex ratio of birth. A GRR of one would mean that each woman bore on average one female child if she lived through to the end of her reproductive years. The GRR thereby gets closer to showing whether each generation is ‘reproducing’ itself.

In reality, not all women survive to the end of their reproductive years, and to understand better whether each generation is reproducing itself it is necessary to factor in mortality. This is achieved by adjusting the GRR using the _nL_x column of the life table. An NRR of one means that each cohort of women, subject to the current age-specific fertility and mortality rates throughout their lifetimes will, on average, be replaced by the same number of daughters. This is known as 'replacement level fertility'. An NRR of less than one would mean the population was declining and an NRR of more than one would indicate that each woman was replacing herself with more than one girl child, and the population therefore growing.

The NRR is affected by the shape and the level of the age-specific fertility and mortality schedules.

To translate the NRR into a growth rate (as will be described more fully in PAPP103_s07 , it is necessary to also know how long it will take each generation to reproduce itself. This is known as the average generation length. The measure can be estimated in various ways, either from the age-specific fertility schedule, or by factoring in mortality, or by using observed births in a real population.

The concept of reproductivity and measures such as the NRR and generation length which have been introduced in this session are used extensively in stable population models and in understanding population growth and momentum, as will be described in PAPP103_s07 and PAPP103_s08 .

The most common measures of reproductivity relate to female populations, but it is also possible to calculate male measures. Male TFRs and NRRs are usually higher than the female equivalents, and the male generation length is longer, because males tend to start reproducing later than females and their reproductive lives end later.