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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. As our briefing shows, 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.

Falling fertility in poor and middle-income societies is a boon in and of itself. It means that, for the first time, the majority of mothers are having the number of children they want, which seems to be—as best one can judge—two. (China is an exception: its fall in fertility has been coerced.)

It is also a boon in what it represents, which is greater security for billions of vulnerable people. Subsistence farmers, who live off their harvest and risk falling victim to rapine or drought, can depend only on themselves and their children. For them, a family of eight may be the only insurance against disaster. But for the new middle classes of China, India or Brazil, with factory jobs, cars and bank accounts, the problems of extreme insecurity lie in the past. For them, a child may be a joy, a liability or an accident—but not an insurance policy.

And falling fertility is a boon for what it makes possible, which is economic growth. Demography used to be thought of as neutral for growth. But that was because, until the 1990s, there were few developing countries with records of declining fertility and rising incomes. Now there are dozens and they show that as countries move from large families and poverty into wealth and ageing they pass through a Goldilocks period: a generation or two in which fertility is neither too high nor too low and in which there are few dependent children, few dependent grandparents—and a bulge of adults in the middle who, if conditions are right, make the factories hum. For countries in demographic transition, the fall to replacement fertility is a unique and precious opportunity.

Another inconvenient truth

Nonsense, say Malthus's heirs. All this misses the point: there are too many people for the Earth's fragile ecosystems. It is time to stop—and ideally reverse—the population increase. To celebrate falling fertility is like congratulating the captain of the Titanic on heading towards the iceberg more slowly.

The Malthusians are right that the world's population is still increasing and can do a lot more environmental damage before it peaks at just over 9 billion in 2050. That will certainly be the case if poor, fast-growing countries follow the economic trajectories of those in the rich world. The poorest Africans and Asians produce 0.1 tonnes of CO 2 each a year, compared with 20 tonnes for each American. Growth is helping hundreds of millions to escape grinding poverty. But if the poor copy the pattern of wealth creation that made Europe and America rich, they will eat up as many resources as the Americans do, with grim consequences for the planet. What's more, the parts of the world where populations are growing fastest are also those most vulnerable to climate change, and a rising population will exacerbate the consequences of global warming—water shortages, mass migration, declining food yields.

In principle, there are three ways of limiting human environmental impacts: through population policy, technology and governance. The first of those does not offer much scope. Population growth is already slowing almost as fast as it naturally could. Easier access to family planning, especially in Africa, could probably lower its expected peak from around 9 billion to perhaps 8.5 billion. Only Chinese-style coercion would bring it down much below that; and forcing poor people to have fewer children than they want because the rich consume too many of the world's resources would be immoral.

If population policy can do little more to alleviate environmental damage, then the human race will have to rely on technology and governance to shift the world's economy towards cleaner growth. Mankind needs to develop more and cheaper technologies that can enable people to enjoy the fruits of economic growth without destroying the planet's natural capital. That's not going to happen unless governments both use carbon pricing and other policies to encourage investment in those technologies and constrain the damage that economic development does to biodiversity.

Falling fertility may be making poor people's lives better, but it cannot save the Earth. That lies in our own hands.