Why are more and more school pupils passing exams? That question is being asked in many countries, and certainly has troubled policy- makers and leader-writers in England.

No one research project can answer it. But Scottish research - especially in educational sociology - has taken us quite a long way towards an explanation.

The changes in exam passes in Scotland in the past three decades have undoubtedly been as striking as anywhere in the UK. In 1965, just 4 per cent of school-leavers had five or more passes in the Higher grade exams (an excellent level of academic attainment); by 1997, the proportion was 18 per cent. In 1965, 12 per cent had three or more passes, then the official entry threshold for higher education; by 1997, the proportion was 30 per cent, and, by age 21, 47 per cent of young people had entered higher education. In 1962, by contrast, three-quarters of leavers had no nationally recognised qualifications; by 1997, the proportion was just 7 per cent.

One politically popular explanation has been tested repeatedly and found lacking - the claim that these changes are due to a fall in exam difficulty.

For example, a couple of years ago, an investigation by the Scottish Council for Research in Education (funded by the Scottish Office) found that assessment standards in Higher-grade exams had not declined - a similar investigation by the school curriculum authority in England also failed to find conclusive evidence of a decline.

The examinations had changed, of course. For example, a greater attention to project work and process skills was probably the reason why girls’ attainment was moving ahead even more rapidly than boys’. But standards had not dropped.

Scottish research, from numerous sources, has given five main explanations for these changes. The first and most important is the second-generation effect of rising educational capital. Put simply: the children of better-educated parents do better, and there are a lot more better-educated parents around now than there were 30 years ago.

That is because of the raising of the school-leaving age in 1973, and the rise thereafter in voluntary staying-on beyond age 16 and in the rate of entry to higher education. The children of these educated parents are now completing their schooling, and so appearing in the exam statistics.

This first explanation means that further expansion is inexorably built into the system. The same is almost certainly true in the rest of the UK, which has seen a similar raising of the school- leaving age and subsequent improvement in staying-on rates. But the explanation is circular - it explains expansion today by expansion yesterday.

If we ask why today’s parents started liking school two decades or so ago, Scottish research provides us with quite a specifically Scottish answer: comprehensive secondary schools. These were introduced more quickly and with less controversy than in England, and are still far more popular than in England.

The evidence from research is that such schools did indeed partly fulfil one of the intentions of their founders, of tapping into educational talent in social groups which had previously been wholly excluded. Their ideology of equal rights is probably also an explanation of why girls are now doing so well. However, growing social polarisation in the 1990s has posed new challenges to comprehensive education which have not yet been overcome.

Comprehensive inclusion of the working class was made easier to achieve politically by the fact that these same disadvantaged social groups were declining in size - the third point. The birthrate in the l950s and 1960s was falling more rapidly in those social classes which, on average, have lower attainment than in those which do better. This purely demographic trend would have tended to raise attainment overall even if nothing else had changed. It was intensified by social mobility - parents moving out of working-class occupations into middle-class ones, and their children then acquiring the educational characteristics of the middle class in general. This explanation would be relevant to most educational systems in Europe.

The fourth point is about how demand for education can be stimulated. Scottish research (in line with that in, for example, the USA, Italy and England) has shown that young people respond to what is on offer. There is no “fixed pool” of demand (as used to be thought in the 1980s and earlier). When the Government announces that higher education is to grow, young people rush forward to apply for the places. (On the other hand, some government policies can have the effect of restricting or depressing demand - as some fear will result from the Government’s new student loans and tuition fees scheme.) And the same can be said in the post-compulsory stages of schooling - the fifth point. Reforms to curriculum and assessment in the last four years of Scottish schooling have contributed to stimulating demand. The current reform of the Highers (Higher Still) will probably take this further, making the system more modular, with a greater variety of pathways corresponding to different aptitudes and inclinations.

So Scottish research has given us quite a consistent picture. Despite nearly two decades of Conservative government, there continued to exist a belief in public education as a public good. When governments have that faith, people respond.

The earlier social democratic reforms stimulated the first wave of demand - the comprehensives, and the 1960s Robbins expansion of higher education. The schools were able to channel the demand throughout the period because, in a general sense, their overall effectiveness rose steadily.

That then fed a popular appetite for more, which forced even a reluctant Conservative administration to respond. The new Scottish parliament which takes power next year will inherit all that - and will be goaded into action by the still unmet demand for yet more.

A final point is about the role of research in all this. Scottish educational research - especially of the sociological variety - has not only given us these answers to the question of why expansion has happened. It has been able to do so because it has kept its vision firmly on the big picture, avoiding the temptation of disappearing into a post-modernist black hole.

As a result, Scottish research has actually influenced Government action through the Scottish office, though keeping a critical distance from it. That basis ofpublicly-shared knowledge is one reason why faith in the education system remains strong.