It’s comforting to think that our children can succeed whatever the odds, and we all want to believe that students who are bright and work hard can rise above the circumstances of their birth. This basic belief in the value of social mobility is shared across the political spectrum. It is something many of us see as crucial to a civilised and well-functioning society. But in most cases this simply isn’t happening anymore. It’s not impossible to move up, but it is extremely unlikely.