AsianScientist (Jul. 29, 2011) – Researchers have developed a new statistical technique to forecast changes in fertility rates around the world.
The study, involving researchers from Singapore, the United States, and South Africa, was published online on 12 July in the journal Demography.
The new method is based on the hypothesis that all countries follow a similar pattern in the transition from high to low fertility rates. Thus, the method predicts future fertility rates in the country of interest using not only historical fertility rates for that country, but also by using data from all 195 countries in the world.
The researchers note that, 200 years ago, fertility rates were high throughout the world, with each woman giving birth to five or more children in her lifetime. After the Industrial Revolution, this rate started to fall in Europe. This trend spread to other regions of the world in the 20th and 21st centuries.
The authors argue that the transition from high to low fertility rates generally takes place in three phases, beginning with stable, high fertility in the first phase (typically six to seven children per woman).
The second phase marks the transition from high to low fertility, beginning when the fertility rate falls to five children per woman and ending when it reaches or falls below the replacement level rate of 2.1 children.
In the third phase, a country’s fertility rate recovers to slightly above 2.1 children.
According to the authors, Singapore, the United States, and 18 European countries have entered this final phase in the last five years. Singapore’s fertility rate fell to a record low of 1.16 births per woman in 2010.
The new estimates predict an unexpected trend among countries where fertility rates have fallen below the replacement level: that fertility rates will continue falling below 2.1 births, reaching as low as 1.5 births, and then begin to increase slowly.
This is contrary to expectations that fertility rates will remain stable once they have fallen below the replacement level.
This trend has already been observed in the United States, the only country in which the lifetime fertility rate has fallen below replacement level and then later recovered.
However, the researchers also note that rates of migration and mortality are also major factors that influence population growth. Therefore, methods that take these factors into account will have to be developed for more accurate population forecasting.
The article can be found at: Alkema L et al. (2011) Probabilistic Projections of the Total Fertility Rate for All Countries .
Sources: Eunice Kennedy Shriver National Institute of Child Health and Human Development.
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