Ministers only half mean it when they say they want to improve social mobility. They do want upward mobility. They do want talented children from poor backgrounds to have the same opportunities available as those from wealthy backgrounds. But they don’t want anyone to be worse off as a result.

For most of the 20th century the dilemma was avoided easily. About 39 per cent of people born between 1950 and 1959 were upwardly mobile during their lives, compared with 25 per cent of those born in the decade before 1900.

This change was not the result of a social mobility White Paper. It was just that, in the earlier period, 18 per cent of all jobs were in social classes I and II. By 1950 that had risen to 42 per cent. The economy changed shape and there was more room at the top.

In the complex mathematics of this subject, that does not mean that social mobility today is especially good. Someone who starts out in the top social class today is 32 times more likely to end up there than someone who started at the bottom.

The argument about social mobility is always posed as the quest to ensure the passage of the bright child from the poor family. A far better test of social mobility, whether we have the stomach for it politically or not, is how large a cushion we allow for the fall of the stupid middle class.

The British bourgeoisie has proved to be highly adept at what the American sociologist Charles Tilly has called “opportunity hoarding”. In less academic language this is called looking after your children.

In education, for example, the middle classes prevent downward mobility by buying better training for their children.

Seven per cent of the country buys schooling privately. If families want their children to stay in the state system, they move into the catchment area of the best schools. This artificially inflates the local house prices and closes the opportunity for the less well-off.

This is very tricky terrain. It is no kind of political prospectus to demand that somebody’s life gets worse. Ministers are quite right not to say anything so brazenly silly.

But then they would be better off deleting their references to social mobility, which is just the way we measure something, and start talking about the fair society or the open society, which is what they really mean.

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