Future Populations - Not an explosion but an implosion

Laurence Siegel is the director of research at the CFA Institute Research Foundation. 

"High death rates are the cause of high birth rates," explains Siegel. World population grew very slowly in the Malthusian past because, although people had lots of babies, more than half of them died before reaching adulthood. Modern sanitation and medicine and greater supplies of food meant falling death rates; that combined with still-high birth rates to produce a population explosion, with the number of people in the world rising from 1 billion in 1800 to 7.7 billion now.

The global total fertility rate—that is, the number of children each woman is likely to bear over her lifetime—has fallen from around 5 in 1960 to 2.42 now. The United Nations forecasts that world's total fertility rate will eventually fall below the conventionally defined replacement rate of 2.1 children per woman; the U.N. says population will then stabilize around 11 billion, and Siegel basically agrees.

So humanity is demographically transitioning from its natural state of high birth and high death rates to a more recent stage of high birth and low death rates to the low birth and low death rates seen in much of the world now. About half of the world's population currently lives in countries with below replacement fertility. The U.S.'s total fertility rate, for example, has dropped to a record low of 1.73 children per woman.

Siegel's projections of future population growth may in fact be excessively high. In a 2018 study, demographer Wolfgang Lutz and his colleagues at the International Institute for Applied Systems Analysis offer an alternative scenario projecting rapid economic growth, rising levels of educational attainment for both sexes, and technological advancement—all factors that tend to lower fertility. They expect that world population could peak at about 8.9 billion by 2060 and then decline to 7.8 billion by the end of the century.

Why are more people around the world having fewer children? Incentives, explains Siegel. Rearing children in modern societies costs a lot, both in money and in foregone opportunities and pleasures. Given that about 99 percent of kids born in countries like the U.S. will make it to age 20, parents are choosing to spend more resources on fewer children, who will thereby be more likely to enjoy successful lives. "To put it just a little too crassly, in wealthy societies and increasingly in less wealthy ones, children have become a cost center (some would even say a luxury good), not a profit center," Siegel observes.

These trends mean that there will be many more old people in the future.

https://reason.com/2019/12/24/apocalyptic-thinking-is-wrong/