Presidential candidates in the United States are not required by law to release their income tax returns. However, many of them voluntarily release this information.

Ron Paul is an exception. Instead he releases information about investments. The Wall Street Journal reported  that "Rep. Paul’s portfolio is valued between $2.44 million and $5.46 million."

Meanwhile nonprofit organizations that file a form 990 are required to publicly disclose the income paid to their highest paid officers.

Source: Visual.ly