Most of President Obama¡¯s income came from royalties for his best-selling books about his life. As a large share of those sales were overseas, Mr. Obama benefitted from a foreign tax credit of more than $22,000.

Mr. Romney¡¯s income came almost entirely from investments, which are taxed at a much lower rate than wages. His tax bill did go up by about $230,000 because he had to pay the alternative minimum tax.

The bulk of the income for Mr. Gingrich and his wife, Callista, came from profit distributions from an S-corporation that oversaw their businesses. Since profits are not subject to payroll taxes, the Gingriches saved tens of thousands of dollars.

Most income for the median taxpayer is from wages and the largest portion of the federal tax bill is for Social Security and Medicare.