The federal tax code is complex. This complexity generally arises from two factors: the use of the tax code for purposes other than raising revenue, and the feedback process of amending the code.

While its main intent is to provide revenue for the federal government, the tax code is frequently used to direct the behavior of businesses and individuals in an attempt to achieve social, economic, and political goals.

For example, the tax law provides a deduction for mortgage interest in order to encourage home ownership. A theoretically pure income tax would not allow this deduction, which is not an expense incurred for the production of income. The allowance of the mortgage interest deduction is seen by some as discrimination against taxpayers who rent, rather than own, their home: the payment of rent for one's home is not deductible. Of course in theory, landlords generate tax savings on their mortgage interest payments, and pass these savings on to renters.

Because the government uses the tax code as an instrument of social policy, the code as a whole appears to lack a coherent organizing principle. This lack of a coherent organizing principle has become magnified over time, due to the interplay between successive legislative amendments and regulatory changes to the law and the private sector responses to those amendments and changes. For instance, suppose that Congress enacts a tax credit to encourage a particular type of activity. In response, a group of taxpayers who are not the intended beneficiaries of the credit re-order their affairs, or the superficial aspects of their affairs, to qualify for the credit.

Congress responds by amending the code to add restrictions and target the credit more effectively. Certain taxpayers manage to use this change to claim additional benefits, so Congress acts again, and so on. The result is a feedback loop of enactment and response, which, over an extended period of time, produces significant complexity.

In general, the U.S. income tax is highly progressive, at least with respect to individuals that earn wage income. As of 2001, the top 1 percent of individual taxpayers paid approximately 23 percent of all federal taxes. The top 5 percent paid approximately 39 percent, and the top 10 percent paid 50 percent of all federal taxes. The bottom 20 percent of taxpayers paid a little over 1 percent of all federal taxes.

Moreover, the progressivity of the U.S. tax system has gradually increased over recent decades. The top 20 percent of taxpayers paid approximately 56 percent of all taxes in 1980, and this figure gradually has risen to 65 percent, as of 2001. In recent years, however, a reduction in the tax rates applicable to capital gains has significantly reduced the income tax burden on non-wage income. In this regard, the general structure of the U.S. tax system has begun to resemble a partial consumption-tax regime.