Matt Ridley: From quantity to quality

This Halloween, the United Nations declared over the summer, a baby will be born somewhere on Earth who will tip the world's population over seven billion for the first time. Truly do international bureaucrats have the power of prophecy!

The precision is bunk, of course, or rather a public-relations gimmick. According to demographers, nobody knows the exact population of the world to within 100 million. (Incidentally, the record-setting baby will not be the seven billionth human being to have existed, as some press reports have implied—more like the 108 billionth.)

Nonetheless, the occasion will provide an excuse for yet another round of Malthusian gnashing of teeth about overpopulation. But we shouldn't let it obscure the real story of the past 50 years, which is not how much faster than expected, but how much slower, population has been growing.

In the 1960s, some experts feared an exponentially accelerating population explosion, and in 1969, the State Department envisaged 7.5 billion people by the year 2000. In 1994, the United Nations' medium estimate expected the seven-billion milestone to arrive around 2009. Compared with most population forecasts made in the past half century, the world keeps undershooting.

The growth rate of world population has halved since the '60s and is now expected to hit zero around 2070, with population around 10 billion, though some news outlets prefer to focus on the U.N.'s "high" estimate that it "could" reach 15 billion. The truth is, nobody can know, but if it's below 10 billion in 2100, we will have only increased in numbers by 1.5 times in the 21st century, compared with a fourfold increase in the 20th.

This "demographic transition" to lower birth rates began in Western Europe in the 19th century and later spread to North America, then Latin America, Asia and now Africa. In 1955, the birth rates per woman in Yemen, Iran, Bangladesh, Nigeria, Brazil and China were, respectively, 8.3, 7.0, 6.8, 6.5, 6.1 and 5.6. Today they are 5.1, 1.7, 2.7, 5.2, 1.8 and 1.7. Notice: The poorer a country has remained, the slower the fall.

The fall in the birth rate is a largely voluntary phenomenon. It has happened just as fast in countries with no coercive population policy as it has in China, with its Draconian two-child law. The demands for coercion that were common in the 1970s—"Why should the law not be able to prevent a person from having more than two children?" wrote Paul Ehrlich, Anne Ehrlich and John Holdren in 1977—seem embarrassing in retrospect.

Birth rates have gone down because of prosperity, not poverty. Everywhere it has occurred, it has followed a fall in child mortality and famine and an increase in income and education. The wider availability of contraception has been necessary, even vital, for this shift, but it has not been sufficient.

To a biologist, the demographic transition is both surprising and intriguing. No other species drops its birth rate when its food supply increases. Frankly, no expert has yet fully explained the phenomenon. It remains something of a demographic enigma.

The best guess is that modern society causes human beings to switch their reproductive strategy from quantity to quality. Thus, once child mortality drops and paid work becomes available to the children of subsistence farmers, parents become more interested in getting one or two children into education or jobs than in begetting lots of heirs and spares for the farm.

Whatever the explanation, history shows that top-down policies aimed directly at population control have generally proved less successful than bottom-up ones aimed at human welfare, which get population control as a bonus. The faster poor countries can grow their economies, the slower they will grow their populations.

Matt, an acclaimed author and former Science and Technology Editor for the Economist blogs at http://www.rationaloptimist.com.