The Sixteenth Amendment (Amendment XVI) to the United States Constitution allows Congress to levy an income tax without apportioning it among the states on the basis of population. It was passed by Congress in 1909 in response to the 1895 Supreme Court case of Pollock v. Farmers' Loan & Trust Co. The Sixteenth Amendment was ratified by the requisite number of states on February 3, 1913, and effectively overruled the Supreme Court's ruling in Pollock.
Prior to the early 20th century, most federal revenue came from tariffs rather than taxes, although Congress had often imposed excise taxes on various goods. The Revenue Act of 1861 had introduced the first federal income tax, but that tax was repealed in 1872. During the late nineteenth century, various groups, including the Populist Party, favored the establishment of a progressive income tax at the federal level. These groups believed that tariffs unfairly taxed the poor, and they favored using the income tax to shift the tax burden onto wealthier individuals. The 1894 Wilson–Gorman Tariff Act contained an income tax provision, but the tax was struck down by the Supreme Court in the case of Pollock v. Farmers' Loan & Trust Co. In its ruling, the Supreme Court did not hold that all federal income taxes were unconstitutional, but rather held that income taxes on rents, dividends, and interest were direct taxes and thus had to be apportioned among the states on the basis of population.
For several years after Pollock, Congress did not attempt to implement another income tax, largely because of concerns that the Supreme Court would strike down any attempt to levy an income tax. In 1909, during the debate over the Payne–Aldrich Tariff Act, Congress proposed the Sixteenth Amendment to the states. Though conservative Republican leaders had initially expected that the amendment would not be ratified, a coalition of Democrats, progressive Republicans, and other groups ensured that the necessary number of states ratified the amendment. Shortly after the amendment was ratified, Congress imposed a federal income tax with the Revenue Act of 1913. The Supreme Court upheld that income tax in the 1916 case of Brushaber v. Union Pacific Railroad Co., and the federal government has continued to levy an income tax since 1913.
Critics of the Sixteenth Amendment have argued that it enables expansive federal government spending and facilitates central banking policies, with some, including Congressman Ron Paul, calling for its repeal on these and related grounds.
The Congress shall have power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration.
Other constitutional provisions regarding taxes
Article I, Section 2, Clause 3:
Representatives and direct taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers ...
Article I, Section 8, Clause 1:
The Congress shall have Power to lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States; but all Duties, Imposts and Excises shall be uniform throughout the United States.
Article I, Section 9, Clause 4:
No Capitation, or other direct, Tax shall be laid, unless in proportion to the Census or Enumeration herein before directed to be taken.
This clause basically refers to a tax on property, such as a tax based on the value of land, as well as a capitation.
Article I, Section 9, Clause 5:
No Tax or Duty shall be laid on Articles exported from any State.
Income taxes before the Pollock case
Until 1913, customs duties (tariffs) and excise taxes were the primary sources of federal revenue. During the War of 1812, Secretary of the Treasury Alexander J. Dallas made the first public proposal for an income tax, but it was never implemented. The Congress did introduce an income tax to fund the Civil War through the Revenue Act of 1861. It levied a flat tax of three percent on annual income above $800. This act was replaced the following year with the Revenue Act of 1862, which levied a graduated tax of three to five percent on income above $600 and specified a termination of income taxation in 1866. The Civil War income taxes, which expired in 1872, proved to be both highly lucrative and drawing mostly from the more industrialized states, with New York, Pennsylvania, and Massachusetts generating about 60 percent of the total revenue that was collected. During the two decades following the expiration of the Civil War income tax, the Greenback movement, the Labor Reform Party, the Populist Party, the Democratic Party and many others called for a graduated income tax.
The Socialist Labor Party advocated a graduated income tax in 1887. The Populist Party "demand[ed] a graduated income tax" in its 1892 platform. The Democratic Party, led by William Jennings Bryan, advocated the income tax law passed in 1894, and proposed an income tax in its 1908 platform. Proponents of the income tax generally believed that high tariff rates exacerbated income inequality, and wanted to use the income tax to shift the burden of funding the government away from working class consumers and to high-earning businessmen.