Why exactly has social mobility declined in the United States in the past thirty years, so that the probability has more than halved that a man born into the bottom 25 per cent of the income distribution will end his life in the top quartile?13 Once the United States was famed as a land of opportunity, where a family could leap from 'rags to riches' in a generation. But today, if you are born to parents in the bottom income quintile, you have just a 5 per cent chance of getting into the top quintile without a college degree.