"Our income tax system is designed to be “progressive.” That is, we generally agree that people with higher incomes should pay a greater percentage of their income in federal income taxes, because they can afford to pay more. But because capital gains are concentrated at the highest levels of income and taxed at favorable rates, many of the most affluent taxpayers pay a lower effective tax rate than those beneath them on the income scale."
http://www.americanprogress.org/issues/2011/02/te022311.html