The former head of Green Templeton College, Oxford, did an analysis of the final results achieved by people from different backgrounds who had got into Oxford, and the conclusion was that those who came from fancier backgrounds did less well in their degrees than their poorer colleagues with similar entry scores. The hypothesis was that the private schools were very good at hothousing borderline students, but that this obviously stopped happening once they were admitted and got the same environment as everyone else.

This is one of the reasons that Oxford is beginning to seriously concern itself with background - it turns out that if you're already expecting top entry grades it's a good way of getting better students.

That, and its long-standing gross overrepresentation of children from posh schools is getting embarrassing.