"The major sources of federal tax revenue are individual income taxes, Social Security and other payroll taxes, corporate income taxes, excise taxes, and estate and gift taxes. This report describes the federal tax structure, provides some statistics on the tax system as a whole, and presents analysis of selected tax concepts. The federal income tax is levied on an individual's taxable income, which is adjusted gross income (AGI) less deductions and exemptions. Tax rates, based on filing status (e.g., married filing jointly or single individual) determine the level of tax liability. Tax rates in the United States are progressive, such that higher levels of income are taxed at higher rates. Once tax liability is calculated, tax credits can be used to reduce tax liability. […] Analysis of tax statistics from the federal tax system as a whole leads to three conclusions: (1) federal revenue as a percentage of GDP is in line with historical trends; (2) the U.S. fiscal position is in line with the fiscal position of other industrialized nations (revenues and expenditures as a percentage of GDP are relatively low); and (3) over the past decade, average tax rates have fallen for individuals at all income levels, but have fallen more for lower-income individuals, reducing their share of overall tax liabilities. […] This report will be updated on enactment of major changes in the federal tax system."
CRS Report for Congress, RL32808