Forget what you know about the replacement rate—new research suggests the world needs more children than previously thought to secure its future. A new study published Wednesday in PLOS One finds that human populations need at least 2.7 children per woman to "reliably avoid long-term extinction," as a press release puts it. This is higher than the commonly cited replacement rate of 2.1 children per woman. But as Earth.com explains, that replacement fertility rate "assumes a stable world," one where child mortality is low and equal numbers of boys and girls are born, for instance. The "reality is messier," it observes
The research, led by Takuya Okabe of Shizuoka University, Japan, used mathematical models that took that messy reality into account; by including random demographic fluctuations, mortality rates, sex ratios, and the likelihood that some adults never have children, researchers landed on the 2.7 the figure. The study also found that a female-biased birth ratio, with more girls than boys born, reduces the risk of extinction as it increases the population of would-be mothers. The researchers write that may "explain the observed phenomena of female-biased births under severe conditions" like war and famine; it may be an evolutionary way to avoid extinction.
The authors note that while extinction is not imminent in large, developed populations, "since fertility rates are below this threshold in developed countries, family lineages of almost all individuals are destined to go extinct eventually." As for where things stand now, Earth.com reports two-thirds of the global population live in countries where the fertility rate is below the 2.1 mark; in the US, it's around 1.6. (This content was created with the help of AI. Read our AI policy.)
,
uBlock 
,
and
Ghostery 
)
