The world is entering an era of "mass access" to higher education, with "immense" implications for societies and university systems.
The era's advent was heralded by Philip Altbach, director of the Centre for International Higher Education at Boston College in the US, in the wake of a report published last week by the Organisation for Economic Co-operation and Development (OECD).
The analysis, Education at a Glance, says that the number of people with degrees or equivalent qualifications in member nations rose by 4.5 per cent every year since 1998, a phenomenon led by "massive" public investment in universities. In some nations, including Spain and Turkey, the figure was 7 per cent a year or more.
Across all OECD countries, one in three people aged between 25 and 34 now have a tertiary level qualification, the annual report says. In Canada, Japan and South Korea, the figure is one in two.
In an interview with Times Higher Education, Professor Altbach said that the world is moving towards a mass-access model, with many countries, including most in the OECD, achieving access of more than 30 per cent.
The implications, he said, were huge, and posed major political questions, including the crucial issue of who should pay for higher education.
He said: "More money is being spent on post-secondary education, and increasingly the funds are coming from students and their families because societies are unable, or unwilling, to pay for increased access."
This has led to unprecedented growth in private provision and the privatisation of public higher education, and has driven a global trend towards higher tuition fees, he said.
"Only a few OECD nations, especially in northern Europe, still resist significant tuition fees," Professor Altbach added.
He predicted that the era of mass access would put the greatest pressure on less prestigious institutions, which are absorbing students from families who have not previously sent their children to university.
But he added that elite universities catering for the upper-middle classes would also face challenges, not least greater competition from mass-access institutions for state funding and support.
The OECD report shows significant variations in access between nations and major shifts over time.
On average, across countries where information is available, university-level graduate rates doubled from 18 per cent in 1995 to 36 per cent in 2007.
"As the pace of change has differed widely ... the relative standing of countries on this measure has changed dramatically since 1995," the report says.
Finland, for example, moved from tenth in 1995 to third in 2007, whereas the US dropped from second to 14th over the same period.
Professor Altbach said: "The reason that the US has fallen behind is that it continues to do an excellent job of access to higher education, but a poor job of ensuring that students complete their degrees. Indeed, this is the hidden crisis of American higher education."
He said that most countries needed to do a better job in several areas, including ensuring that students who enter post-secondary education are able to complete their degrees in a timely fashion.
"This means attention must be paid to cost issues, the academic preparation of students and study conditions once they matriculate," he said.
"Countries will need to ensure that their top research-intensive universities are not short-changed when catering for higher enrolment at the bottom of the system."
The OECD report says there has been "massive" public investment in higher education.
From 2000 to 2006, expenditure on educational institutions per student increased by 11 percentage points on average in member countries, having remained stable from 1995 to 2000.
"This shows governments' efforts to deal with the expansion of tertiary education through massive investment," it says. But it also notes more "cost sharing" between public and private - usually household - funds.
Among OECD countries for which trend data are available, the share of public funding in tertiary institutions fell from 78 per cent in 1995 to 72 per cent in 2006.
The report also illustrates higher education's impressive returns to individuals and nations.
Across member countries, it states that a male student, for example, who completes a university degree can look forward to a lifetime gross earnings premium of more than $186,000 (£112,000) on average, compared with someone who completes only secondary education. The net public return from investment in tertiary education can be almost twice the amount of public investment.
Professor Altbach said: "The fact is that higher education is a good investment for individuals and society. One would hope that these facts would have implications for public spending on higher education; such spending is good public policy."