The current interpretation of the Sixteenth Amendment taxation power are found in Commissioner v. Glenshaw Glass Co. (1955).
In general, tax statutes passed in 1913 after the ratification of the Sixteenth Amendment are referred to as the "modern" tax statutes. Hundreds of Congressional acts have been passed on the Sixteenth Amendment since 1913, as well as several codifications or reorganizations.
The federal income tax rates in the United States have been wide-ranging since 1913. In 1954, for example, Congress imposed a federal income tax on individuals, with the tax imposed in 24 income brackets at tax rates ranging from 20 percent to 91 percent.
Income tax is also collected by individual U.S. states and are in addition to the federal income tax. Some states allow individual cities to require an additional income tax. Nevertheless, state and local income taxes are deductible from federal taxes. This deduction effectively subsidizes a portion of an individual's state income tax if the taxpayer itemizes deductions for the federal government. Puerto Rico is considered a separate taxing entity from the rest of the United States. Puerto Rico's income tax rates are set separately. Only some residents there pay federal income taxes, although everyone must pay other federal taxes. Unincorporated Territories such as Guam, American Samoa, and the Virgin Islands all have income tax rates equal to the U.S. federal tax. Therefore, taxes in these territories are like the other states not having a local income tax to their taxpayers.
Many maintain that the current income tax system - the government's largest revenue source - is too progressive and redistributive. In 2007, the top 5 percent of income earners paid more than half of the federal income tax revenue and the top 1 percent of income earners paid 25% of the total income tax revenue. At the same time, forty percent of Americans pay no federal income tax. This raises concerns over wealth redistribution and the economics of controlling the size and spending of government
Tax protesters argue that the federal income tax is unconstitutional. They maintain that the Sixteenth Amendment was not properly ratified, and that since its ratification in 1913, all presidents, members of Congress, and judges who have served are part of a conspiracy to enforce unconstitutional or illegal laws.