Gerald Baral

Replacement fertility in one chart: Even if 90% of women have children and average 2.2 each, we still fall short. Why? The fertility rate of a population equals the product of the proportion of women who have children and the average number of children per mother. That is, if 90% of women have children and the average number of children per mother is 2.2, the fertility rate of this population is 1.98. This simple formula gives us the relationship between the proportion of mothers in a population and the average number of children per mother required to reach the replacement rate. As I explained two days ago (check my feed if you missed it), this replacement rate is 2.1 in Western countries, where sex selection and infant mortality are low. The figure plots the result (if you are technical, this is called the iso-replacement curve). Obviously, if 100% of women become mothers, the average number of children per mother required to reach replacement is 2.1. If we move to 90%, this average rises to 2.33. Notice that if we fall to 80%, the average increases substantially to 2.6. I selected 80% because it implies that one in five women never becomes a mother, close to what we now see in Japan and parts of Southern Europe. The current young cohorts in advanced economies seem to be on track to be well below 80%, but we will not know for sure for another 20 years or so. Having an average of 2.6 children per mother requires many very large families. And modern societies are not organized for this to happen.