Even if the world population does stabilise soon and starts to glide downwards, that won't solve the world's environmental problems. The real issue is not overpopulation but overconsumption - mostly in rich countries that have long since given up adding substantial numbers to their population.

Take one measure: carbon dioxide emissions. Stephen Pacala, director of the Princeton Environmental Institute, calculates that the world's richest half billion people - that's about 7 per cent of the global population - are responsible for 50 per cent of the world's emissions. Meanwhile, the poorest 50 per cent are responsible for just 7 per cent of emissions. One American or European is more often than not responsible for more emissions than an entire village of Africans.

Every time those of us in the rich world talk about too many babies in Africa or India, we are denying our own culpability. It is the world's consumption patterns we need to fix, not its reproductive habits.

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