There are many global problems in which the United States is painted as prime villain; there are not many where Iran is also hailed as the solution. But finding ways to make the world’s population grow more slowly is one of those rare cases because of the recurrent coyness of the US about promoting contraception, and the ayatollahs’ sure-footedness in doing just that.

Suddenly, population control is back in the spotlight, after 30 years in which it has been taboo. This year’s State of the World’s Population report, by the United Nations Population Fund, does not help as much as it might.

Every year the authors of this technically sophisticated survey seem to feel that they must find a new angle. This year they have picked an odd one: women’s greater suffering (they assert) because of climate change.

This distorts, and understates, the case that springs from the figures: a global population of 6.5 billion, rising towards 9 billion or more within several decades. But there is also a new hope, based on recent dramatic falls in birthrates, that the numbers could be coaxed lower.

There is no mystery why, since the UN’s population conference in Cairo in 1994, it became so difficult to talk about family planning within overseas aid. Religious anger found a politically confident voice at that conference. Richer countries were already nervous of being accused of telling poorer ones how many babies to have, while the past coercion of China and India had left revulsion.

The distaste of the US, under President George W. Bush, for promoting contraception, and its horror at even a tangential connection with abortion services in other countries, then cut US funding.

That the subject is back is thanks largely to climate change, which has licensed many to say what they recently might have found unsayable. In February Jonathon Porritt, chairman of the Government’s Sustainable Development Commission, said that curbing population growth through contraception and abortion must be at the heart of efforts to combat global warming, and that couples who had more than two children were irresponsible.

The argument is backed up by the despair of development economists at getting economies to grow faster than populations. Pakistan is in danger of losing that battle, and the consequences for unemployment and militancy do not need spelling out.

But success also lies behind the new interest. Iran’s ayatollahs, despite their other failings, have proved numerate, and able to appreciate the impact of a soaring population on living standards and support for the regime. After initial enthusiasm during the 1979 revolution for a huge population, they promoted contraception through the mosques.

The size of families fell from seven children to an average 1.9 in three decades. That record is not alone. In more than 70 countries, including Brazil, China, and Southern India, families now have fewer children than would maintain numbers.

The Economist magazine, in a cover story last month , said that fertility rates were falling so fast that the problem would fix itself. That is too dismissive of the value of cutting, say, an extra half billion off the peak world population, or of the social cost to those countries that fail. The time is right, instead, for a vigorous attempt to tackle the issue, supported, for once, by the US President.

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