Richard J. Wood (Capital) has published Supreme Court Jurisprudence of Tax Fairness, 36 Seton Hall L. Rev. 421 (2006).  Here is part of the Introduction:

These two norms, horizontal and vertical equity, stated as one principle with two important divisions, have long been used to evaluate the fairness of tax legislation. Most published work on tax fairness has focused on these two norms and the four views of tax justice in the political (legislative) context rather than the judicial. My concern is that the tax structure of horizontal and vertical equity, within which the principles mentioned above and others may compete for acceptance, ought to be reflected in the United States Supreme Court’s tax decisions.

My thesis is that the Court uses systemic, but not substantive, tax fairness norms to classify taxpayers as similarly situated to each other and the government.  Further, a trend is developing in the Court’s Commerce Clause and tax fairness jurisprudence: the Court has become more willing to consider distributive justice principles in the context of vertical tax equity.  My research shows that the Court has refined horizontal equity into what I call systemic and substantive branches. Furthermore, the Court’s use of vertical equity suggests a starting point for future application of distributive justice principles in Supreme Court tax fairness jurisprudence.

I have organized this Article by focusing on three areas in which the Court has used tax fairness norms in resolving tax disputes: tax administration, statutory construction, and state taxation tested against the Equal Protection and Commerce Clauses of the United States Constitution. In reviewing the materials, I found that the Court employs both vertical and horizontal equity norms as well as subordinate equity principles such as the benefit principle and the ability to pay principle in resolving tax disputes.  I start by explaining the tax fairness norms observed in the cases reviewed.  I then show how the least controversial norm, horizontal equity, was developed early by the Court and expanded over time.  I next show that the Court divided horizontal equity into two branches.  One branch, systemic horizontal equity, evaluates similarity without resort to content.  In other words, the Court does not consider ability to pay measured by income, wealth, or consumption when using this branch of horizontal equity. Instead, analysis under systemic horizontal equity is concerned primarily with consistency and, to a lesser degree, regularity and certainty of application.  The second branch of horizontal equity, substantive horizontal equity, uses ability to pay as the primary criterion for grouping similarly situated taxpayers.  The evolution of vertical tax equity and the related points of the benefit principle and the ability to pay principle are also addressed in this Article.