The United States Supreme Court last decided a federal income tax case on constitutional grounds in 1920a century ago. The case was Eisner v. Macomber , and the issue was whether Congress had the power under the Sixteenth Amendment to include stock dividends in the tax base. The Court answered “no” because “income” in the Sixteenth Amendment meant “the gain derived from capital, from labor, or from both combined.” A stock dividend was not “income” because it did not increase the wealth of the shareholder.

Macomber was never formally overruled, and it is sometimes still cited by academics and practitioners for the proposition that the Constitution requires that income be “realized” to be subject to tax. However, in Glenshaw Glass , the Court held in the context of treble antitrust damages that the Macomber definition of income for constitutional purposes “was not meant to provide a touchstone to all future gross income questions” and that a better definition encompassed all “instances of undeniable accessions to wealth, clearly realized, and over which the taxpayers have complete dominion.”

In the century that has passed since Macomber , the Court has never held that a federal income tax statute was unconstitutional. This behavior of the Court constitutes a remarkable example of American tax exceptionalism, because in most other countries income tax laws are subject to constitutional review and are frequently ruled unconstitutional.

In what follows, we will first examine three examples of how income tax law is constitutionalized in other countries (Part 1). We will then look at some of the larger tax expenditures in the U.S. and ask how they would fare under constitutional scrutiny (Part 2). Finally, we will attempt to answer the question whether US income tax law should be constitutionalized, and answer in a reluctant negative (Part 3). But we will also urge Congress, which is equally charged with upholding constitutional values, to take horizontal equity more into consideration when evaluating tax expenditures.

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