When the top personal tax rates are above the corporate rate, high income individuals have an incentive to reclassify their earnings as corporate rather than personal income for tax purposes. At least U.S. tax law imposes strict limits on the extent to which employees in publicly traded corporations can engage in such income shifting. However, entrepreneurs setting up new firms can easily reclassify their income for tax purposes. This tax incentive therefore favors entrepreneurial activity. In the United States, these tax incentives were huge during the 1950s and 1960s, though they have been much smaller since then.
The sensitivity (i.e., elasticity and built-in flexibility) of the U. S. individual income tax to changes in national income is of great interest to researchers and policymakers. However, the direct measurement of this sensitivity—that is, the measurement obtained from time-series observations of the relevant variables—has always been difficult, and even at times impossible, because changes in the legal structure of the tax have been too frequent to provide enough observations that relate to the same legal structure to allow statistically significant coefficients to be determined. This was particularly true in the United States before 1954, when the rates were changed frequently; it has also been true since 1963, when important changes occurred in rates, personal exemptions, deductions, and other features. In contrast, during the period between 1954 and 1963, hardly any significant statutory changes occurred in the tax.