The question “What is Conservative Party policy towards grammar schools?” is now too complicated to be asked at A level, never mind the 11-plus. The answer, as I understand it, is that David Cameron does not want any more of them – except in Buckinghamshire, a county that appears to have acquired a relationship rather like that of the Vatican City with the rest of modern Italy. The Tory position is less incoherent than anarchic.

The question “How did the Conservatives ever get into this mess?” is much more straightforward and reflects less badly on them. David Willetts, the Shadow Education Secretary, is an extremely clever man who has long been nicknamed “Two Brains”. It is his misfortune that he possesses at least one and a half brains too many to be understood by many of his fellow Tory MPs. This row started, and should still be rooted, in that Mr Willetts’s belief that social mobility in Britain is on the decline and, with it, the ideal of meritocracy.

Meritocracy is a slippery concept. The word achieved popular currency almost half a century ago with the publication of Michael Young’s The Rise of Meritocracy. Young detested the country as it was in the 1950s, arguing that “nobody should be born with a silver spoon in his mouth or, if he is, it should choke him”.

At the same time he was appalled at the thought of a society divided into new classes on the basis of academic intelligence alone and feared that it could result in a new elite that would have no sense of compassion for, or shared responsibility with, those “below” them.

Despite Young’s misgivings, meritocracy is regarded as worthy in the modern world (on the whole, rightly) – although he was wise to to give warning against a pure Brave New World approach to brainpower. The reality, however, as Mr Willetts has conceded, is that Sad Old World is more plausible. The evidence that Britain has become less socially mobile is clear, even if it is not widely understood. This is partly because, while sociologists are wonderful people, their language is rarely less than opaque.

Social mobility is measured by “transitional matrices”, in which absolute social mobility is taken to mean that a person’s ultimate income and social class is unrelated to that of their parents. If someone is born into a family in the bottom quarter of the population by income, for instance, if they had total mobility they would have an exactly equal chance of ending up in any of the four quartersof relative wealth.

The evidence suggests otherwise and it is pretty disturbing. Two huge sub-sections of the population were identified – those born in 1958 and 1970 – and their lives closely followed. Of those born in 1958, those in the poorest quarter had a 31 per cent chance of being in the same place at the age of 33 and a 17 per cent chance of having moved up to the highest quarter. By 1970 the same sort of people had a 35 per cent chance of being stuck where they were and a 16 per cent chance of jumping up.

The results for the middle class are much more dramatic. In 1958 those born to the wealthiest quarter had a 35 per cent chance of staying where they began and a 17 per cent chance of slipping down to the lowest income section by the age of 33. But those who were born in 1970 had a 42 per cent chance of carrying on in comparative luxury and a meagre 11 per cent chance of descending to the lowest income quarter.

Put brutally, the story of the past 50 years is that the middle class has got bigger and has also become highly effective at defending its own – including the least impressive of its offspring.

How and why has this happened? The conventional explanation, with which Mr Willetts agrees, is that the massive expansion of higher education and degree qualifications in the past few decades is a rave to which the working class has not been invited. There were 220,000 students attending universities in 1974. Thirty years later there were more than 1.6 million. Yet it is those who were born into comfortable households who secured these new places (especially the girls) and benefited from the economic rewards that flow from them.

If so, this is one hell of a dilemma. It implies that the principal challenge of social mobility today is less how to raise up the bright but poor (though this is a real challenge) than how to force down the dim but rich.

That is really, really hard. No one objects to the State intervening to assist the smart but disadvantaged. A massive middle class would go nuts at the sort of measures that government would need to introduce to ensure that Tristan and Jemima were exposed to serious penalties for being, well, thick. This is, to use a technical phrase honed by experts, a bummer of a public policy problem.

Where I part company with Mr Willetts is on cause and effect. The distinction between those born in 1958 and 1970 is not solely a surge in university places. The former group was largely educated under the grammar school system, while the latter (like me) had the dubious privilege of being conscripted into the comprehensive experiment. The Cameronites seem blind to the thesis, which to my mind is credible, that the destruction of the old system was a social and intellectual catastrophe. The grammar schools have, to all intents and purposes, been replaced by independent schools where it is not only the ability to think but the capacity to sign a chequebook that is crucial.

I agree with Mr Willetts that Britain has a crisis of social mobility. I am far from convinced that academic selection is the reason for this rather than, reintroduced with sophistication, the possible solution.

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