The striking change in girls' examination results has been at age 16. Over the years, in the old O-level system, an equilibrium had been reached with about a quarter of the age group passing five subjects. Girls were very slightly ahead but nothing to write home about. Since the introduction of GCSE, as the proportion achieving five A-C grades has risen, a gap of about 8 per cent has opened up. In English, girls have gone 17 per cent in front.
Girls are also well ahead in the language tests at ages five (where there is baseline assessment), seven, 11 and 14, but this is something we could have expected from the old 11-plus, and is consistent with measured psychological differences. At A-level and degree level, girls' performance is improving, but the pattern is not yet much changed, with young men getting more A grades and firsts.
It has been the publication of schools' test and examination results which has focused attention on the differences, and people are increasingly asking what is happening. Radio phone-ins regularly discuss whether girls are basically more intelligent, whether boys are under-performing, and whether single-sex education would make a difference.
All good stuff, but the interesting more specific question is what is it about GCSE that enables the talents of girls to shine through. Three possible explanations come to mind - the level at which the exam is pitched, the nature of the exam, and society's changing script for the sexes.
We know from attempts to measure intelligence that the spread of boys' scores tends to be wider than that of girls. The girls' tend to bunch towards the middle. In an exam like O-level, aimed at the upper end of the ability range, the sexes would balance out. But GCSE, pitched as it is more towards the middle, would be within the compass of the greater number of girls there.
The different outcomes may also have something to do with the different exam formats. Boys tend to do well in unseen timed exams which O-levels were, and A-levels and degree exams can still be, whereas girls tend to do better in assessment methods, like those of GCSE, which reward consistent application. The changing pattern may however have little to do with the exams themselves. The cleverest person in my class at school was a girl called Stella but her dad absolutely insisted she leave at 16 because he firmly believed any more education would be wasted on a girl. Nowadays she would have been headed for Oxbridge and the top, but then the script was against her. Much of this unfairness has now been removed and today there is every incentive for girls to get qualifications.
At A-level boys are still ahead. In the 1960s when twice as many entries came from boys as girls, girls tended to get proportionally more A and B grades, presumably because they were more narrowly selected. During the 1970s and 1980s as girls' entries grew towards parity, the apparent advantage over boys disappeared.
During this period also the proportion of girls-only schools was decreasing and this has led to the suggestion that the seeming decline in girls' performance was due to co-education. In fact, there was no real decline, only a relative one. Even that has now reversed, while the number of single-sex schools continues to go down.
Nevertheless, since girls-only schools predominate in the league tables, particularly at GCSE, it seems obvious that there must be something educationally beneficial about single-sex education. What is obvious, however, is not necessarily true.
As we go about our everyday lives, it is obvious that the earth is flat. The evidence however compels us to accept that this is not so.
When it comes to educational performance, the evidence is that segregating the sexes has at most only a very small effect. The main players are ability, motivation and parental expectations (usually measured as social class). To a large extent, what a school can achieve in exam terms depends on who goes to it. Most of the single-sex schools which have survived are exceptional and are able to select pupils of very high ability, who work hard, and have strong parental support. Where it is possible to compare like with like, as for example with comprehensives, girls in co-educational schools and girls in single-sex schools do equally well.
None of this is a criticism of single-sex education. There may well be personal, social, cultural or religious reasons for preferring it, and perhaps we need more. But this does not mean that we should expect to improve the educational performance of girls, or boys for that matter, by separating the sexes. The excellence of some schools cannot be transposed to others merely through sexual segregation.
So what are we to make of the headlines? Girls' educational participation and performance have been going up throughout the period that the number of single-sex schools has been falling. While the improvement may not have been caused by co-education, it is difficult to see how the converse - that mixed schools are failing girls - can be claimed.
Alan Smithers is director of the centre for education and employment research at Brunel University.