Graduates can expect to earn £150,000 more over their lifetimes than those with just A-levels - £250,000 less than previously estimated - a report says.
More students are going to university
Economists at Swansea University said some subjects - such as the arts - could even mean losses, when fees and living costs are taken into account.
The report, based on the British Labour Survey, blames the increase in student numbers for narrowing the earnings gap.
But the government said university was still a "worthwhile investment".
Costs and benefits
During the passage of the Higher Education Bill - which raises maximum tuition fees in England to £3,000 a year from 2006 - the government said graduates could expect to earn £400,000 more over their lifetimes than non-graduates.
However, the Swansea study, led by Professor Peter Sloane and Dr Nigel O'Leary, found male graduates would earn £141,539 more than those who leave education with two or more A-levels.
For women the figure was £157,982.
Maths or computing degrees made the biggest difference to earnings, adding £222,419 for men and £227,939 for women.
However, arts subjects meant just £22,458 more over a lifetime for men, compared with A-level leavers.
Dr O'Leary said this could actually mean a loss of earnings, when tuition fees and living costs at university were taken into account.
He told the BBC News website: "It is quite stark. It shows how much earnings depend on the type of qualification.
"Some graduates can expect to do well, but for others the rate of return is much, much lower.
"For arts it could even have a negative effect."
Dr O'Leary added: "A lot of people who, 15 years ago, would have left after A-levels and got jobs are now staying on at university.
"They leave with bigger debts and go on to do largely the same jobs, which are now called 'graduate' jobs."
The government wants towards 50% of people in England under the age of 30 to enter higher education by 2010.
Dr O'Leary said this was reducing the scarcity value of graduates, further hitting their earning power.
A Department for Education and Skills spokeswoman said: "The £400,000 figure quoted during debate on tuition fees and the £140,000 estimates used in the Swansea study are calculated on a completely different basis - it is therefore misleading to compare them directly.
"Whilst the Swansea survey makes interesting reading, it would be unwise to comment further until we have the fuller picture that the DfES commissioned Moving On study will give when it reports in due course."
The research - The Changing Wage Return to an Undergraduate Education - was based on the British Labour Survey's findings from 1993 to 2003.