The Uneasy Case for a Comprehensive Tax Base

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In discussing tax simplification, commentators often advocate a comprehensive income tax system that has a broad base and low rates over income tax systems that have a narrow base and higher marginal rates (such as the one in the US). Advocates assert that a comprehensive tax base with low marginal rates will reduce tax evasion and tax avoidance and increase economic efficiencies. This paper shows that two of the benefits often attributed to a comprehensive tax base, decreased tax evasion and increased economic efficiency, are not as certain as may first appear. With respect to tax evasion, economic theory cannot predict whether reduced tax rates will result in less tax evasion. Depending upon assumptions about how risk aversion changes in response to income, some models predict that tax evasion will actually increase as tax rates decrease. Since there is significant uncertainty about how risk aversion actually functions in the real world, there is no basis for preferring any particular set of assumptions regarding risk aversion. Thus, the effect of tax rates on evasion has to be viewed as an empirical question. The empirical studies, however, have produced contradictory results. As to the other claimed benefit, increased efficiency, economic theory also cannot predict whether broadening the tax base and reducing the marginal rates will increase economic efficiency as compared to a narrower tax base with higher rates. Increasing the tax base will cause items that had previously not been taxed to be taxed and, therefore, increase the excess burden with respect to those items. At the same time, the lower tax rate will decrease the excess burden of items that had already been taxed in the narrow base. Whether the increases will exceed the decreases cannot be predicted by theory. Instead, the question is again an empirical one that is very difficult to resolve. With respect to the benefits of a comprehensive income tax that are discussed herein, the only certain one is that it should reduce the opportunity for tax avoidance since it reduces differential treatment of items.