The gap between rich and poor in access to higher education in the UK grew during the 1990s, a report says.
Reasons for different application rates may be complex
But the National Institute of Economic and Social Research report suggests cost was not the issue.
Rather, it argues the problem was that children from disadvantaged backgrounds did less well in their school exams.
This reinforces what universities say as they strive to encourage more applications from students in state schools and poorer communities.
The report says that when up-front tuition fees were introduced and grants abolished after 1998, many commentators predicted fewer poorer students would go into higher education - even though fees were means-tested.
But the authors say this did not happen.
Children from all socio-economic backgrounds were much more likely to go to university in 2001 than in 1994.
But the growth rate in poorer neighbourhoods was significantly less rapid, particularly in the early and mid-1990s.
Any income-driven inequality seemed to be part of a longer-term trend, perhaps related to the gradual reduction in financial support for students and the big expansion of the university sector.
After examining school attainment, the report concludes that much of the impact from social class on university attendance actually occurs well before people go into higher education.
In England the government has focused money and energy on disadvantaged communities, with exam results rising faster than average in the target areas.
And universities, under pressure from the government to widen participation, have said that - do what they might in the way of "outreach" programmes - the problem lies in schools.
There is a twist, however. The researchers say that the inequality they observed in attainment at earlier ages might still be related to problems in higher education.
"Students may look forward and anticipate barriers to participation in higher education and make less effort in school as a result."
They may not apply as much effort to their GCSE studies if they do not expect to stay on at school and take A-levels, and may not take many A-levels if they do not expect to go on to university.
This requires further research, the report says.
The widening socio-economic gap in UK higher education, by Fernando Galindo-Rueda, Oscar Marcenaro-Gutierrez and Anna Vignoles, is published in the National Institute Economic Review.