Romney paid 14 percent effective tax rate in 2011

His effective tax rate was up slightly from the 13.9 percent rate he paid in 2010.

The majority of the candidate's income last year came from his investments, said Brad Malt, the trustee of Romney's blind trust, in a blog post. Full documents related to Romney's 2011 tax return will be released at 3 p.m ET.

The couple gave just over $4 million to charity.

In addition, the Romney campaign said Romney's tax filings from 1990 to 2009 show that the couple paid 100 percent of the federal and state income taxes they owed and that their overall average annual effective federal tax rate was 20.2 percent. Annually they never paid an effective rate below 13.66 percent.

Romney has been criticized by both Democrats and even some Republicans for not releasing more than two years' worth of tax returns. And he has often gotten flak for having paid a low effective tax rate given his outsized income.

But contrary to popular perception, Romney's effective federal income tax rate is still higher than that of most Americans -- 80 percent of whom have an effective rate below 15 percent. That number, however, does not include other federal taxes such as the payroll tax.

Romney's running mate, Paul Ryan, released his final 2011 tax return this summer. He paid $65,000 on $323,416 in income, giving him an effective tax rate of 20 percent.

The reason Romney's rate is so low -- despite having one of the highest incomes in the country -- is because his income was derived almost entirely from capital gains and dividends from his extensive portfolio of investments. And that form of investment income is typically taxed at just 15 percent, well below the 35 percent top tax rate for high earners.