Population survival of developed countries needs more kids on average

(CN) — With global anxiety about falling birthrates, the world used to console itself with the idea that having an average of 2.1 children per woman could hold off humanity’s extinction. Researchers from Japan and the Philippines dispelled that notion in their study published Wednesday in PLOS One.

Takuya Okabe and his colleagues at Japan’s Shizuoka University looked at developed countries that met or tried to meet the 2.1 replacement level fertility — or the rate at which a population can replace itself generation to generation — and maintained low mortality rates but still had falling birth rates.

The team noticed that those same countries didn’t account for modernization and development increasing the opportunity cost of having children, thus decreasing the total fertility rate, or the number of children per woman. That is how G7 countries like the United States only have a rate of 1.66 children per woman.

Other problems with the 2.1 children per woman rate are that it assumes a nearly 1:1 gender ratio and the theory that a population can be so large that it guarantees stable long-term results when averaging some random events. To the contrary, the team pointed to ecological studies that showed stochasticity — or the property of randomness — can play an important role in the extinction risk. One example of randomness is when individuals below the reproductive age unexpectedly die, such as when researchers estimated that the replacement fertility level in Afghanistan, Burundi and Sierra Leone rose to 3.3 children per woman when survival rates dropped.

The team decided the time came to figure out the true fertility threshold number. To do that, the team said it considered a population that reproduced sexually instead of asexually and did not have overlapping generations, or a situation where a parent continued breeding by the time their original children began their own families. The team also calculated females giving birth to a random number of children, the probability of a single female’s lineage going extinct and the possibility that a minority of women remained childless.

That is how the team realized that a population needs at least 2.7 children per woman to replace itself. It also needs to have more female offspring, especially in smaller populations.

Humans may also naturally produce more girls in times of crisis. The team found that human parents exposed to various stressors like psychological distress, chemical exposure and even terrorism have more girls than boys, a trend that also occurs in mammal species like cattle, pigs, rats and mice.

Calculations aside, the team admitted some potential shortfalls in its work. After all, the assumption that generations don’t overlap is a simplification that humans don’t always meet, and overlapping generations can accelerate the speed of evolution by enhancing genetic diversity. Furthermore, the team said that the time scale of population size variation in the current study is too short compared to an evolutionary scale.

That is why they intend to perform further study on these factors and how migration, density dependence and environmental stochasticity affect humanity long-term. On a smaller scale, the team warned that current reproductive patterns put the family lineages of all individuals at risk of extinction, with very few exceptions. At this rate, at least 40% of over 6,700 spoken languages around the world will disappear in the next century, taking their culture, art, music and oral traditions with them.

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