Internal Revenue Code Part I

Part 1 of the Internal Revenue Code (26 U.S.C. § 1-5 or simply IRC Part 1), titled "Tax on Individuals" is the part of the Internal Revenue Code that contains the sections imposing a federal income tax on the taxable income of individuals in the United States of America, and sets forth the amount of the tax to be paid. The Part contains five sections, most significantly section 1, titled "Tax Imposed", which "imposes an income tax on the income of every individual who is a citizen or resident of the United States", and also provides for the taxation some non-resident aliens. The section historically contained tables from which the exact tax owed could be computed based on the marital status and filing options of the taxpayer. While these tables remain in the statute as examples, the actual amounts are provided by the Internal Revenue Service and regularly adjusted for cost of living changes. Section 1 also imposes an income tax on income earned by certain estates and trusts. A similar tax on corporations is set forth in IRC §11, but "the primary target for the income tax is the individual taxpayer", as set forth in section 1, and some businesses, such as sole proprietorships and single-member LLCs that do not elect to be taxed as a corporation are disregarded entities, so that if the owner of such an entity is an individual, then its activities will be reflected on the owner's tax return under section 1.

Although the tax imposed by section 1 applies to all taxpayers regardless of age, special provisions are made in section, 1(g) (previously section 1.1(i)1T), for "unearned income of certain minor children", also known as the "kiddie tax". Another highly controverted provision in the section, 1(h), provides the rules for taxation of capital gains. Other points of dispute arising from the section have included the so-called "marriage penalty" (a circumstance in which married households with multiple income earners tend to pay a higher rate than households of unmarried people), and the so-called "dividend tax" (a circumstance where corporate payouts to shareholders are taxed upon distribution despite previously having been taxed when the income was received by the corporation).

Since the consolidation of individual tax rates into section 1 in 1954, the section has been amended dozens of times through at least twenty-nine separate acts of Congress, reflecting repeated revisions to federal income tax rates, filing statuses, and related provisions. Amendments have frequently focused on the number of tax brackets and the rate of taxation in each bracket, as well as attempts to address the aforementioned points of dispute. Other sections of this Part have been amended less frequently. Section 2 of this Part provides definitions for some of the terms used in section 1, particularly for the meanings of "head of household" and "surviving spouse" within the context of the IRC. Section 3 originally contained an optional tax structure for persons with an income of less than $5,000 per year, but was later changed by amendment to a tax table for lower income individuals, while section 4, originally containing additional rules for this optional tax structure, was repealed on October 4, 1976. Finally, section 5 contains "Cross references relating to tax on individuals" noting other sections of the tax code relevant to persons such as nonresident aliens, and members of the United States Armed Forces.