„For a family on the middle rung of the income ladder, the fees rose between 1980 and 2015 from 20 to 50 per cent of their income, while even for highly affluent families on the 95th rung the fees doubled from 10 to 20 per cent of their incomes.“
Univ.-Prof. Dr. Francis Green u. a., Engines of Privilege (2019), S. 125.