Out of the mouths of babes spurts a rush of words, at least once they reach their second year. Now mathematics may finally explain why. A sudden explosion in a child’s vocabulary usually strikes at around 18 months, with usage expanding dramatically to include more complex words, but scientists have previously failed to provide a convincing explanation. Writing in the US journal Science, psychologist Bob McMurray, at the University of Iowa, shows that the rapid improvement is an inevitable consequence of the way languages are structured. Between birth and adulthood, children learn around 60,000 words, an average of eight-10 a day. Studies of word usage by infants show that while a smattering of easy words, such as ‘hi’ and ‘bye’ are grasped by 12 months, just four months later, their lexicon has often broadened substantially to balls, dogs, birds and bottles, babies, books and shoes. According to Dr McMurray, the reason lies in statistics. In almost every language studied, there are relatively few very easy words and far more moderate and harder words to learn. Easy words tend to be nouns that can be quickly linked to an object, such as a cup or a cat. […]