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The conventional replacement level fertility (RLF) of 2.1 children per woman is simply not enough in many parts of the world, especially developed countries, to prevent human extinction, a new study has suggested.
Instead, the fertility rate should be at least 2.7 children per woman in these areas, according to the research paper.
The researchers noted that the world experienced its peak growth rate in the 1960s and the global population has continued to increase and is projected to reach 8.5 billion in 2030.
“Thus, studies and policies focused more on the carrying capacity of the human population and curbing population growth,” they observed.
“However, at present, two-thirds of the world’s population lives in areas where the total fertility rate is below the replacement level fertility (RLF),” according to the paper.
The experts made use of mathematical models to examine how the demographic variability of 2.1 children per woman affects the survival of populations over many generations.
They found that, due to random fluctuations in birth numbers, a fertility rate of at least 2.7 children per woman is needed to reliably avoid eventual extinction—especially in small populations.
“It should be remarked that this condition has already been met in developed countries. Extinction is not an immediate issue owing to the large population size in these countries. However, the present results have a profound implication from an individual perspective: The family lineages of almost all individuals are destined to go extinct, whereas very few exceptions may survive for many generations,” according to the paper.
Languages also face the risk of extinction, with at least 40 per cent of more than 6,700 spoken languages in the world threatened to disappear within the next 100 years. “The extinction of a language results in the disappearance of a culture, art, music and oral traditions,” the experts said.
They added, however, that a female-biased birth ratio, with more females than males born, reduces the extinction risk, helping more lineages survive over time.
Conventional fertility targets should thus be rethought, according to the researchers.
Threshold fertility for the avoidance of extinction under critical conditions has been published in the journal PLOS One. The authors are Diane Carmeliza N. Cuaresma, Hiromu Ito, Hiroaki Arima, Jin Yoshimura, Satoru Morita and Takuya Okabe.