In fact, the 30 companies enjoyed a negative effective income tax rate over the three-year period, even as they earned pre-tax profits of $160 billion, according to two think tanks, Citizens for Tax Justice and the Institute on Taxation and Economic Policy. A negative tax rate represents a tax benefit like a credit, not necessarily direct funds from the government.

Pepco Holdings, a Washington, D.C.-area power company, had the lowest effective tax rate at negative 57.6 percent and 78 of the 280 Fortune 500 corporations studied enjoyed at least one year in which they paid an effective tax rate of zero or less.

In addition, the average effective tax rate for the entire group of corporations studied over three years was only 18.5 percent, compared to the statutory rate of 35 percent.