This seems plausible, so I searched and found this on the Wikipedia:

Various studies have examined who gives more to charity. A study in the United States found that as income decreases, charitable giving increases as a percentage of income. For instance, the poorest fifth of Americans donated 4.3% of their income, while the wealthiest fifth donated 2.1%. In absolute terms, this translated to an average donation of $453 from an average income of $10,531, compared to $3,326 from an income of $158,388.