"The current U.S. tax system does not allow for proportional distribution of tax burdens among different levels of income. While there are some parts of the system that reduce the progressivity of the tax burden distribution, the wealthiest taxpayers are, on average, disproportionately burdened by taxes in proportion to the income they earn." The research in this paper defends the thesis statement by demonstrating how high-income households are taxed at greater effective tax rates than low-income households.
Leighty, M. (2017). The U.S. Tax System: Too Taxing on the Wealthy? (Undergraduate honors thesis, University of Redlands). Retrieved from http://inspire.redlands.edu/cas_honors/149