Scientific American – Claudia Wallis 
Children from the poorer strata of society begin life not only with material disadvantages but cognitive ones. Decades of research have confirmed this, including a famous 1995 finding by psychologists Betty Hart and Todd Risley: By age four children reared in poverty have heard 30 million fewer words, on average, than their peers from wealthier families. That gap has been linked to shakier language skills at the start of school, which, in turn, predicts weaker academic performance. (more)