Three separate reports published by the Institute for Fiscal Studies, Charities Aid Foundation and the London School of Economics say that the poorest donors to charity give away around 3% of their income in contrast to the wealthiest donors, whose gifts might be bigger in absolute terms but which cost a mere 1% of their available income.
(I guess that the more you have, the more you want to hang on to it. I wonder if they've ever read the story of Scrooge?)