Moreover, despite the obvious role of contraception as a proximate determinant of fertility, the additional effect of contraceptive availability or family planning on fertility is quantitatively small and explains very little cross country variation.

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In Kenya, for example, as a result of diminished financial support for family planning and increasing fertility, the country's projected population in 2050 is set to reach 83m, from 54m.

ECONOMIST: Population booms, and busts

And in Iran, public policy and investment in family-planning services made the difference between rising fertility between 1979 and 1984 and falling fertility thereafter, not the rise in national incomes, nor female education.

ECONOMIST: Population booms, and busts

An international group of fertility experts, led by Italian obstetrician Severino Antinori, is planning to start work on what they hope will be the first cloned human baby in October, despite an outcry from the Vatican that the venture is unethical and immoral.

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The rich countries, on their part, could do themselves and the world a huge favour by putting their efforts into reducing their greenhouse-gas emissions and following through on promised aid, including for universal access to family-planning services, rather than on raising their own fertility rates.

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That reflects reduced aid for family planning in the past 15 years and political ambivalence about cutting fertility in Africa itself.

ECONOMIST: Africa��s population

Take Bangladesh, a country mired in poverty yet with a fertility rate only slightly higher than America thanks to a progressive family-planning policy.

ECONOMIST: Population booms, and busts