Replacement Level Fertility

Replacement level fertility is in essence how many children the average woman in a country has to have to keep the population stable. So in an ideal world, each woman would have one male and one female child, who would both grow up, marry (let's ignore incest for now) and then have one male and one female child. This would keep the population at an exact level population point over the years.

The issue is that not all children survive to adulthood. Let's say you're in a poor area where 50% of all children die before they reach age 18. So if you had just one boy and one girl, you would have a falling population over time because of those deaths. Half of the children would not reach age 18 to then have the next generation of kids.

I got into a discussion on a forum where a poster was *convinced* replacement ferility rates were affected by all sorts of bizarre other issues. Really, it is not. It is solely a judgement of how many infants a woman has to create in order for 2.0 of them to reach adulthood to begin the next cycle.

Here are some of the expert opinions I got which supported this view:

The "replacement" fertility rate is as you describe it, only that the mortality that has to be taken into account extends all the way from birth to approximately the mean age of childbearing. It takes infertility into account only in the sense that infertile women don't contribute to the birth rates in the respective age groups.

-- Joseph E. Potter, University of Texas




"In terms of your question ~ Basically, "replacement level fertility" refers to a population having a Total Fertility Rate (TFR) of 2.1, and the TFR is basically the age-adjusted birth rate of women aged 15-44 for a given population. Demographic transition theory says that as infant mortality decreased and child survival increased, women had to have fewer births to achieve the same number of children surviving to adulthood. Thus, because women were reducing their births because of decreasing mortality, the TFR fell. The other major cause of decreases in the TFR has more to do with contraceptive availability and use than with female infertility or subfertility (or subfecundity in demo-speak). This is only really an issue if you're focused on older women (40+)."

So that would imply that its not about fertility so much as infant mortality, demographic transition and contraceptive use.


-- Sukriti, Brown University

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