Thomas Malthus first published his “Essay on the Principle of Population”, in which he forecast that population growth would outstrip the world’s food supply, in 1798. His timing was unfortunate, for something started happening around then which made nonsense of his ideas. As industrialisation swept through what is now the developed world, fertility fell sharply, first in France, then in Britain, then throughout Europe and America. When people got richer, families got smaller; and as families got smaller, people got richer.

Now, something similar is happening in developing countries. Fertility is falling and families are shrinking in places— such as Brazil, Indonesia, and even parts of India—that people think of as teeming with children. The fertility rate of half the world is now 2.1 or less—the magic number that is consistent with a stable population and is usually called “the replacement rate of fertility”. Sometime between 2020 and 2050 the world’s fertility rate will fall below the global replacement rate.

At a time when Malthusian worries are resurgent and people fear the consequences for an overcrowded planet, the decline in fertility is surprising and somewhat reassuring. It means that worries about a population explosion are themselves being exploded—and it carries a lesson about how to solve the problems of climate change.

Worth a bundle

Today’s fall in fertility is both very large and very fast. Poor countries are racing through the same demographic transition as rich ones, starting at an earlier stage of development and moving more quickly. The transition from a rate of five to that of two, which took 130 years to happen in Britain—from 1800 to 1930—took just 20 years—from 1965 to 1985—in South Korea. Mothers in developing countries today can expect to have three children. Their mothers had six. In some countries the speed of decline in the fertility rate has been astonishing. In Iran, it dropped from seven in 1984 to 1.9 in 2006—and to just 1.5 in Tehran. That is about as fast as social change can happen.

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