Every two years, the United Nations issues its latest estimate of future population trends. Its 2019 projection reveals a stark divide. Across all of Asia, Europe and the Americas, population stability has already been achieved or soon will be, with the median projection suggesting an increase from 6.4 billion today to 6.5 billion in 2100, a rise of just 2 percent. By contrast, the U.N. projects that Africa's population will soar from 1.34 billion to 4.28 billion.

Beyond a few decades, population trends depend crucially on forecasts of future fertility rates, which are inherently uncertain. But across all of the world's developed economies, the current fertility pattern has held for so long that it seems likely to remain a stable feature of human society.

In all economically advanced countries, fertility rates fell fast between the late 19th century and the 1920s, as contraception became increasingly available, and women were increasingly liberated from the domestic sphere by education and greater participation in the formal workforce. But after falling below 2 in many countries between the world wars, fertility rates rose again in the immediate postwar era, reaching around 2.4 in Northern and Western Europe, and just over 3 in North America.