Will humanity become extinct? 2.1 children per woman is not enough to survive

In order for the human population not to die out in the long term, there should be at least 2.7 children per woman globally. This is a much higher fertility rate than previously assumed, according to new analyses published in "PLOS One."

According to the definition, the total fertility rate means the number of children that an average woman would give birth to during her entire reproductive period (15–49 years), assuming that in the individual phases of this period she would give birth with the intensity observed in the year under consideration, i.e. assuming that the partial fertility rates from this period are constant.

A fertility rate of 2.1 per woman is often assumed to provide the replacement level needed to maintain the population. However, this figure does not take into account random variations in the number of children, as well as mortality rates, sex ratios and the probability that some adults will never have children, researchers from Japan and the Philippines note in PLOS One.

Researchers point out that within small populations such variability may contribute to the disappearance of entire families.

In the new study, scientists used mathematical models to investigate how such demographic variability affects population survival over multiple generations.

The authors of the study concluded that, due to sudden fluctuations in the birth rate, a fertility rate of at least 2.7 children per woman is necessary to ensure the survival (or, to put it differently: to avoid extinction) of especially small populations. In other words, to ensure the long-term survival of human populations, women should give birth to at least 2.7 children in their lifetime.

The risk of a population going extinct decreases when the sex ratio of newborns changes in a certain way: when more girls than boys are born. Such a change helps the larger lineage survive in the long run.

The conclusions from the analyses in PLOS One may be helpful in explaining a long-observed evolutionary phenomenon: the fact that in difficult conditions – such as war, famine or environmental disturbances – more girls are usually born than boys.

It also suggests that while the extinction of large, well-developed populations is not inevitable, most lineages will eventually disappear.

The authors of the publication conclude that true "sustainability" of populations (as well as languages, cultural traditions or diverse family lines) requires rethinking traditionally understood fertility goals . These findings also have implications for attempts to protect endangered species, for which target fertility rates are established. (PAP)

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