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What exactly is going on when we're tetchy, spotty, fearful, tearful or just plain awful? Hormones rule our internal world: they control our growth, our metabolism, weight, waterbalance, body clocks, fertility, muscle bulk, mood, speed of ageing, whether we want sex or not (and whether we enjoy it) and even whom we fall in love with. Their effects may occur in seconds and be over in a flash, or emerge over months and last for years. In The Truth About Hormones, Vivienne Parry explains how exactly these mysteriously powerful chemicals affect our bodies and our behaviour.