Nearly a fifth of profitable U.S. corporations pay no federal income taxes.

That's according to a new analysis by the U.S. Government Accountability Office, which was asked to explore the issue of corporate taxes by Sen. Bernie Sanders. In 2012 (the most recent data available), 19.5 percent of larger companies -- defined as those with at least $10 million in assets -- that reported earnings paid no income tax that year, the GAO found.

Overall, including businesses that didn't report a profit, roughly two-thirds of all larger companies in the U.S. had no federal income tax between 2006 and 2012.

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"There is something profoundly wrong in America when one out of five profitable corporations pay nothing in federal income taxes," Sanders, a candidate for the Democratic presidential nomination, said in a statement. "Large corporations cannot continue to get more tax breaks when children in America go hungry. We need real tax reform to ensure that the most profitable corporations in America pay their fair share in taxes. That means closing corporate tax loopholes to raise the revenue necessary to rebuild America and create millions of jobs."

GAO

The GAO said there were several reasons why profitable businesses may not have owed taxes. Companies that lose money one year are allowed to "carry forward" the losses for accounting in future years, reducing their tax liability. A range of tax breaks, such as for R&D and depreciation of equipment, can also help lower the bill.

As a result, big companies, particularly multinationals that do business around the globe, often pay much lower taxes than the statutory rate of 35 percent. From 2008 to 2012, larger U.S. corporations that were profitable over that period paid an "effective" tax rate, as it is known, of 14 percent, according to the GAO.

Businesses must also pay state and local taxes, while companies that operate abroad are on the hook for taxes in those countries. Including those taxes, from 2008 to 2012 profitable corporations paid an average effective rate of 22 percent.

As a share of federal tax revenue in the U.S., corporate taxes have significantly declined over the years (see chart above). Average corporate income tax in the U.S. amounts to about 2.3 percent of gross domestic product, down from about 4 percent in the mid-1960s. Corporate taxes in countries in the Organization for Economic Cooperation and Development, which represents big market economies, average 2.7 percent of GDP.

In 2012, some four in 10 larger corporations in the U.S. had no federal income tax after accounting for credits, according to the GAO. The 58 percent of big businesses that did pay taxes in 2012 contributed a total of $268 billion.

Although many lawmakers favor corporate tax reform, the devil is in the details. As the presidential race intensifies, experts agree the issue is likely off the table until after the election in November.