The American wealthy are the most generous in the world, with U.S. donors giving $316 billion last year.

But some studies—and most charities—suggest they could give more. One analysis from The Chronicle of Philanthropy showed that people making $200,000 or more give 2.8 percent of their income. A recent study of estate tax returns from the Internal Revenue Service shows that American millionaires give an average of 7.5 percent of their estate.