Until 2002, the United Nations, when projecting future world population density, assumed that birth rates would never fall below 2.1 children per woman in most countries: that is the ‘replacement rate’, at which a woman produces enough babies to replace her and her husband, with 0.1 babies added in to cover childhood deaths and a slightly male-biased sex ratio. But in 2002, the UN changed this assumption as it became clear that in country after country the decline in baby-making went straight through the 2.1 level and kept on dropping. If anything, the decline may accelerate as the effect of
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