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THERE is a fine line between a tease and a lie on a book cover. Take Paul Davies’s How to Build a Time Machine (Penguin, £6.99). He doesn’t tell us any such thing, of course. But his lucid discussion of wormholes edges us down the road to time travel. So we’ll let him off. But Caroline Bledsoe’s Contingent Lives: Fertility, time and aging in West Africa (University of Chicago Press, $22) simply does not fulfil its promise to tell us how women in Gambia “use contraceptives to have as many children as possible”. Stripped of some academic mumbo-jumbo, she shows how, like the rest of us,…