The more parents talk to their children, the faster those children's vocabularies grow and the better their intelligence develops. That might seem blindingly obvious, but it took until 1995 for science to show just how early in life the difference begins to matter. In that year Betty Hart and Todd Risley of the University of Kansas published the results of a decade-long study in which they had looked at how, and how much, 42 families in Kansas City conversed at home. Dr Hart and Dr Risley found a close correlation between the number of words a child's parents had spoken to him by the time he was three and his academic success at the age of nine. At three, children born into professional families had heard 30m more words than those from a poorer background.
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