The Earth isn't Flat, and Neither is Illinois' - or any other state's - Income Tax

In her book, Flat Earth: The history of an infamous idea, 1 Christine Garwood explains that from medieval times, long predating Christopher Columbus’ famous voyages, educated people knew that the earth was not flat. Nevertheless, the flat earth myth persisted for centuries and even has some, possibly (non)ironic, modern adherents (see R. Brazil 2020).2 Simplistic and wrong-head ed ideas often persist despite unassailable evidence to the contrary and a consensus among those who have careful ly considered the question. The exaggerated dichotomy between “flat tax rate” and “graduated tax rate” states is a current manifestation of this phenomenon. Many members of the public, and some people on the fringes of the policymaking world, overstate the differences between these cases and conclude that only graduated rate states can deliver “progressive” tax policy that results in tax liabilities rising with incomes. Tax policy professionals realize that tax liabilities depend not only on the structure of tax rates but also on many other facets of the tax system including the definition of the tax base and myriad rules about tax credits, exemptions, and interactions between different tax systems (e.g. state and federal tax systems).