The number of students from low-income families at top-ranking US universities is in decline, a study has found.

Analysis by The Journal of Blacks in Higher Education identified both short and long-term decline at 30 prestigious universities between 1983 and 2006.

Students from low-income backgrounds fared only marginally better at the 30 leading liberal arts colleges examined by the journal.

The trend, which the journal describes as "disappointing and baffling", has persisted despite dramatic growth in top-tier university endowments and financial commitments at many leading universities to attract promising students from low-income families.

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The study is based on the proportion of undergraduates receiving federal Pell Grants, which are reserved for students from families typically with incomes below $40,000 (Pounds 20,000).

In the short term, between 2004 and 2006, the number of low-income students fell at nine of the ten wealthiest universities in the US - the exception being Harvard University, where the proportion of undergraduates from low-income backgrounds rose from 9.4 per cent to 11.9 per cent.

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In the longer term, the number of of those from economically disadvantaged families fell at eight of the ten wealthiest universities between 1983 and 2006, and at 20 of the 30 highest ranking universities.

The Journal of Blacks in Higher Education reports a similar picture at the 30 leading liberal arts colleges in the US, where the percentage of such students improved at only four of the top 30 between 2004 and 2006, and at only ten between 1983 and 2006.

This trend is set against a backdrop of soaring endowments at many of the universities and colleges involved. The total endowment of the ten wealthiest US institutions exceeds $140 billion, encouraging many to allocate more resources to student financial aid.

The latest figures from the US Department of Education show that Harvard spent $82 million on financial aid in 2006 - accounting for 0.3 per cent of its endowment value.

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Yale University also spent 0.3 per cent, Stanford University 0.4 per cent and Princeton University 0.5 per cent. The three universities' annual endowment yields are between 15 and 20 per cent.

However, the journal argues that throwing money at the problem is not the answer. Its editorial on the subject says: "Contrary to what one might expect, it appears that there is no strong correlation between the generous new fiscal measures and success in bringing low-income students to a campus. The only sure conclusion is that money alone will not do the job.

"The standard thesis that if you cut the price to zero the students you want will come does not hold true. Other measures such as aggressive recruiting are necessary. Particularly, it appears that university and college admissions officials need to plan more visits to public high schools in a wide range of working-class communities."

john.gill@tsleducation.com.