There may be more female graduates than in the past, but they are having fewer or no children. The traditional middle-class recruiting ground for students is shrinking, so, asks Alison Wolf, do academics face a future of empty lecture halls?

Without the modern woman, UK higher education would be a much less happy place. And, no, I am not thinking of complacent male professors burnishing their reputations on the backs of underpaid females on short-term research contracts. Universities have, for decades, been the leading beneficiaries of a revolution in women's lives. But there are storm clouds on the horizon.

For the past 50 years, higher education has been growing at a phenomenal and unprecedented rate. The only sectors that can boast even faster growth are the information and communication technology innovators.

The UK's higher education sector has gone from teaching less than 2 per cent of the age cohort in 1939 to well over 40 per cent today, but that is nothing unusual. Thailand, whose gross domestic product per head is about a quarter of this country's, enrols 40 per cent of its young people in tertiary education; Chinese universities have grown almost fivefold in the past ten years.

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We are now so used to higher education as the "normal" destination for any moderately successful sixthformer that we forget how much these figures incorporate not one but two social revolutions.

University was a highly unusual destination for young men in the 1900s, the 1930s or, indeed, the 1950s. For young women, it was hardly conceived of.

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On the eve of the First World War, the universities of England and Wales were producing about 650 female graduates a year; in the 1930s, less than a quarter of the tiny undergraduate population was female. When Cambridge appointed its first female professor in 1939, she could not even be a full member of the university. No woman could.

It is not long since the upper classes were sending their sons to academically demanding schools en route to Oxbridge and their daughters to institutions that considered it quite acceptable for them to leave with barely an O level to their name (think Diana, Princess of Wales). In the early 1960s, in the world's first mass university system, most US female students were citing "finding a husband" as a major reason for their attendance.

Today, women make up the majority of undergraduates across the developed world, and not because universities are marriage markets. There are now four women graduating with bachelors degrees in the US for every three men.

In the UK, almost 60 per cent of students, at undergraduate and postgraduate levels, are female. So the university explosion is the result of fast growth in male enrolments, but, even more, of extremely fast growth in women's.

Why are so many females now entering higher education? Why are so many parents spending huge sums to send their daughters to study abroad? Quite simply because the female labour market has been utterly transformed. A degree "pays" for women in the same way it does for men, and more and more women make it do so over a lifetime.

Since the 1950s, there has been a steady increase in the number of occupations that are not just open to women but open on equal terms. For women who work as men traditionally do - in comparable jobs without taking breaks for child-bearing and without going part time - being female has become irrelevant to career success. This is new. In the recent past, women's earnings over a lifetime were a small fraction of comparable men's earnings, especially if there were children, but even if there were not.

The gap for the educated but childless disappeared for the cohort that is now middle-aged. The gap for women with children is shrinking rapidly too.

Over a lifetime, educated younger women without children can expect to earn as much as men, and almost as much even if they have children.

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Growing opportunities make occupations that were once open only to the most determined and single-minded women, highly attractive. In the UK today, most trainee barristers and almost two thirds of medical students are female. At the same time, young women are aware that they may well never marry and, if they do, they are quite likely to divorce. Degrees thus look increasingly attractive. They are used by employers as a filter, so even though there are now far more graduates than there are genuinely "graduate"

jobs, it is still well worth taking a degree (and a loan). What might this mean for universities?

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Girls have traditionally been more inclined than boys to take arts degrees, even though these have lower returns in the labour market. If you were expecting to give up employment, future pay was not very important. Indeed, it still isn't, since loans get repaid only when you earn. But as women increasingly view careers from the same long-term perspective as men do, we can expect a further shift from conventional female enrolment patterns.

Law and quantitative degrees pay best. More than 40 per cent of practising solicitors and of undergraduates in science and technology faculties are already women. This may be bad news for subjects such as modern languages, literature and history but not for universities. But the knock-on effects of this revolution affect the sector overall.

We've all noticed that Western women are having fewer children. What has been less remarked on is that educated women are having many fewer.

Graduates are far less likely than average to marry or have children; or, if they do, more likely to have only one child. Native-born women with no formal qualifications are far more likely to have children and have them early.

Does this mean anything for universities? Or does it just open up new opportunities for social mobility? The recent vast expansion in higher education has largely benefited the children of the middle classes: so won't the children of the less educated take places left empty by previous graduates' failure to reproduce?

Not necessarily. We are making depressingly little headway in increasing the numbers of young people from deprived backgrounds who qualify to enter university. Back in the 1980s, a dip in teenage numbers led to predictions of unfilled places that look laughable in retrospect. But the gloomy projections took no account of how rapidly female enrolment rates would equalise with males. They also ignored the dramatic increase in the number of middle-class families. Parents who had benefited from postwar secondary education and the transformation of the job market were anxious for their children to enter university. Since then, the vast expansion in higher education has soaked up most of that market; so going to university is now a near-universal part of growing up middle class.

We can probably expect further expansion in the number of professional and technical jobs, but we already educate more graduates than any analysis suggests we "need". The fastest growing jobs are mostly low level - in leisure and "personal services", such as care assistants. So the apparently endless expansion we have become accustomed to can no longer count on pent-up home-grown demand from managerial families where parents' education stopped at 17 or 18.

Should we worry about recruitment? Not immediately - demand from overseas should hold up. At home, while the teenage cohort is due to shrink a good deal, "social class two" families have continued to bump up the numbers, having more children in the past 15 years than "class one" professionals.

Among women in their mid-forties, almost three quarters of those with A levels or equivalent have had more than one child, compared with half of graduates. In the short term, the drop in 18-year-old numbers has most to do with changes among the least educated native-born women, where family sizes, though still larger than among the middle classes, have been falling. Meanwhile, immigrant families, typically the hardest working and most driven, also tend to have more babies, at least in the first generation. Foreign-born mothers make up 8 per cent of the child-bearing population and 19 per cent of births. But in 20 years, with more and more female graduates in the child-bearing age group, the situation may look very different.

So much for the students - but how about the staff? We, too, are part of the new job market. Female academics are progressing rapidly as the generations feed through. The days must be numbered for short-term ill-paid research officer jobs that attract large numbers of highly qualified women.

These women have been subsidising research much as earlier generations subsidised girls' grammar schools, when school mistress was pretty much the only job for graduate women.

Academic life is, in fact, quite family-friendly. As job barriers crumble, I would bet on academe becoming the next feminised profession. But having more female professors will not resolve the tension between work and family that educated women face and society faces with us. If we do not find solutions, our successors may indeed be facing emptier lecture halls.

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Alison Wolf is Sir Roy Griffiths professor of public sector management, King's College London.