IN 1980 Julian Simon, an economist, and Paul Ehrlich, a biologist, made a bet. Mr Ehrlich, author of a bestselling book, called “The Population Bomb”, picked five metals—copper, chromium, nickel, tin and tungsten—and said their prices would rise in real terms over the following ten years. Mr Simon bet that prices would fall. The wager symbolised the dispute between Malthusians who thought a rising population would create an age of scarcity (and high prices) and those “Cornucopians”, such as Mr Simon, who thought markets would ensure plenty. Mr Simon won easily. Prices of all five metals fell in real terms. As the world economy boomed and population growth began to ebb in the 1990s, Malthusian pessimism retreated. It is returning. On October 31st the UN will dub a newborn the world's 7 billionth living person. The 6 billionth, Adnan Nevic, born in October 1999, will be only two weeks past his 12th birthday. If Messrs Simon and Ehrlich had ended their bet today, instead of in 1990, Mr Ehrlich would have won. What with high food prices, environmental degradation and faltering green policies, people are again worrying that the world is overcrowded. Some want restrictions to cut population growth and forestall ecological catastrophe. Are they right? Lower fertility can be good for economic growth and society (see article). When the number of children a woman can expect to bear in her lifetime falls from high levels of three or more to a stable rate of two, a demographic change surges through the country for at least a generation. Children are scarcer, the elderly are not yet numerous, and the country has a bulge of working-age adults: the “demographic dividend”. If a country grabs this one-off chance for productivity gains and investment, economic growth can jump by as much as a third.

Less is more

However, the fall in fertility is already advanced in most of the world. Over 80% of humanity lives in countries where the fertility rate is either below three and falling, or already two or less. This is thanks not to government limits but to modernisation and individuals' desire for small families. Whenever the state has pushed fertility down, the result has been a blight. China's one-child policy is a violation of rights and a demographic disaster, upsetting the balance between the sexes and between generations. China has a bulge of working adults now, but will bear a heavy burden of retired people after 2050. It is a lurid example of the dangers of coercion.

Enthusiasts for population control say they do not want coercion. They think milder policies would help to save the environment and feed the world. As the World Bank points out, global food production will have to rise by about 70% between now and 2050 to feed 9 billion. But if the population stays flat, food production would have to rise by only a quarter.

When Mr Simon won his bet he was able to say that rising population was not a problem: increased demand attracts investment, producing more. But this process only applies to things with a price; not if they are free, as are some of the most important global goods—a healthy atmosphere, fresh water, non-acidic oceans, furry wild animals. Perhaps, then, slower population growth would reduce the pressure on fragile environments and conserve unpriced resources?

That idea is especially attractive when other forms of rationing—a carbon tax, water pricing—are struggling. Yet the populations that are rising fastest contribute very little to climate change. The poorest half of the world produces 7% of carbon emissions. The richest 7% produces half the carbon. So the problem lies in countries like China, America and Europe, which all have stable populations. Moderating fertility in Africa might boost the economy or help stressed local environments. But it would not solve global problems.

There remains one last reason for supporting family planning: on some estimates, 200m women round the world—including a quarter of African women—want contraceptives and cannot get them. A quarter of pregnancies are unplanned. In our view, parents ought to decide how many children to bring into the world and when—not the state, or a church, or pushy grandparents. Note, though, that this is not an argument about the global environment but individual well-being. Moreover, family planning appears to do little directly to control the size of families: some studies have shown no impact at all; others only a modest extra one. Encouraging smaller families in the highest-fertility places would still be worth doing. It might boost the economy and reduce the pressure of population in some fragile places. But the benefits would probably be modest. And they would be no substitute for other sensible environmental policies, such as a carbon tax.