Christian Smith, a sociologist at Notre Dame, has been reporting the results of a large Science of Generosity project. Sociology is particularly useful for giving us a sense of proportion of how phenomena are distributed in a large population.
Americans as a whole are generous people. We give away huge amounts of money for good causes.
However, nearly half of Americans - 45% - give nothing.
The high standard of tithing (giving at least 10%) is met by only 3%.
Poorer people give away a higher proportion of their income than richer people do.
Looked at another way, 57% of all the charitable dollars in America are contributed by the 5% of Americans who are most generous.
And I should note that the main point of The Paradox of Generosity, by Christian Smith and Hilary Davidson, is that people who give more away lead happier and healthier lives.