Anthony Seldon had a nice letter the other day. In fact the headmaster of the independent Brighton College often gets them, from parents of pupils who have transferred their children to him from state schools.

The writers have a common theme, Mr Seldon says. Even though the local comprehensives are“not at all bad”, parents want him to know that their offspring are happier and more fulfilled in his school, with its small classes and excellent facilities.

These days, about half the pupils at Brighton College have been in state schools at some time, as have three-quarters of their parents. And its head is not alone in feeling that a growing number of parents are choosing private schools, for the Government made much the same assertion in its recent Green Paper on schools.

In a passage bordering on the apocalyptic, the paper warned that there was a danger of more and more parents turning to private education, particularly at secondary level.

“If this were to occur on a large scale, growing numbers of people would become less willing to pay taxes to fund public education which would then decline in quality and provide only for the disadvantaged. It is hard to imagine under those circumstances how social cohesion could be achieved,” it warned.

But are Mr Seldon and the ministers right? Are a growing number of parents choosing private schools? If so, is the Government right to suggest the phenomenon springs from the state sector’s failure to challenge pupils or prepare them for life in the“new economy”?

The answer to the first question is “yes.” Government statistics show there were 560,000 pupils in independent schools in 1999 compared with 523,000 in 1987, a rise of 7 per cent.

Moreover, the figure remained fixed at around 520,000 throughout the 1970s, starting to rise only in the late 1980s. Annual censuses by the Independent Schools Information Service, whose members represent about eight-tenths of the privately-educated population, back this up and suggest there was a further rise of between three and four thousand last year.

But what of the proportion of pupils in independent education? Here, a quite different picture emerges. In real terms, the independent school population rose in the late 1980s from 6.5 per cent of the total to 7.5 per cent before dropping back sharply in the early 1990s to 7 per cent. Since then, it has remained steady.

And a further look at the statistics inevitably reveals that more than half the increase in numbers is the result of a boom in nursery or pre-prep schools. Back in 1985 the proportion of nursery pupils who were in independent schools was just 6 per cent, but now it has risen to 7.6 per cent. Put more starkly, there were 25,000 pupils in private nursery schools in 1985, but in 1999 there were 44,000 - an increase of 76 per cent.

At the other end of the scale, the latest government figures show that the proportion of sixth-formers in independent schools has actually gone down from 21.5 per cent to 19 per cent over the same period.

So do scares about state school standards have an effect? Not if you look at these “macro” level figures. There was no drop in 1991, for example, after the great literacy debate of 1990 and no great exodus from state comprehensives in 1997after the discipline crisis which followed melt-down at The Ridings comprehensive in Halifax in 1996.

There is one factor, though, which explains the trend. To coin a phrase:

“It’s the economy, stupid.”

Dick Davison, spokesman for the Independent Schools’ Information Service, says what happens to the private sector overall as very little to do with what is going on in the state sector.

“If you look at the figures over the past 20 years the principal determinant of what is going to happen to numbers has been economic. It has very little to do with what the Government is doing,” he says.

But that does not fully explain how parents make decisions. Few people sit down with their children and ask themselves how well the economy is doing before they choose a school. They ask what their child’s needs are, and where they may best be met. Then, if they want a fee-paying school, they ask themselves if they can afford it.

In recent years, ISIS has commissioned an annual MORI poll on parents’ views about independent schools. Respondents consistently mention high standards, small classes, good discipline and specialist facilities as reasons for choosing to pay.

When asked whether they would choose an independent school if they could afford it, about half of all parents say they would. And that means that more than 40 per cent of parents whose children go to state schools might make a different choice if they were richer. So when the economy hots up, inevitably some of those who benefit financially will choose to pay fees.

Of course, things are never quite so simple. If state schools’ classes were as small as independent schools’, for example, some well-off parents would be bound to stick with them and use the fees for some other purpose. The proportion of nursery pupils increased sharply throughout the lean years of the early nineties. Even here though the economic factor still comes into play, for parents can currently expect the state to pay at least a proportion of the cost while their child is at private nursery school. This must have helped to swell numbers.

Having tasted private education, will those parents find it hard to go back to the state sector? Will those pupils now go on to swell the numbers in private primary and secondary schools?

That will depend on the economy, of course. But it may also depend on what happens to state schools. Since 1997, the economy has fared well but the influx of pupils into independent schools has not matched that of the late Eighties. Could it be that parents are happier than politicians with state schools? Most research shows that nine out of 10 parents are content with their children’s state schools.

Parents often tell ISIS, in response to its MORI polls, that they would have chosen a state school but were unable to find one they liked in their area.

For the truth is that between those parents who would never consider a state school and those who would never pay is a much larger group who make decisions on a case-by-case basis, even educating one child at a state school and another privately.

So if the Government really can bring about a sea-change in state schools, leading to much smaller classes and better facilities, that might make some difference.

Surprisingly, Mr Seldon hopes he will not see those new pre-prep children coming through the doors of his own school in a few years’ time. He even argues that better-off parents should be means-tested and made to pay for state education so the money can be used to make their schools better. And that would make headteachers like him redundant. But he is certain it will happen, and he looks forward to the day.

“I think it’s got to come. It is utterly wrong that such large sums of money are tied up educating the middle classes who can afford to pay. It is wrong that the children who are the most privileged in society have the best education. All children need good, all-round development and you cannot provide that on the kind of money they are collecting through taxes at the moment,” he says.