(pdf)  Introduction  Many left-leaning politicians have argued that the tax system is rigged to benefit those at the top, and that the wealthy are not paying their “fair share.” This is used to justify enormous tax increases, with some like Rep. Alexandria Ocasio-Cortez (D-NY) advocating for top income tax rates of 70 percent or more and presidential candidates Sen. Elizabeth Warren (D-MA) and Sen. Bernie Sanders (I-VT) introducing steep new wealth taxes as well. The income tax is only one component of tax burdens Americans face, but it generates by far the most revenue (over $1.7 trillion) and is changed more frequently than other sections of the tax code. That’s why the income tax is the centerpiece of many of the policy and political conversations about tax burdens and their impact on household budgets.  But new data from the Internal Revenue Service (IRS) shows that the income tax system has grown increasingly progressive since 1980, even as top marginal tax rates have been significantly reduced. The top…