Regressive Taxation:
The U.S. income tax system is actually regressive in several ways, because earned labor income:

Income caps also make the taxes more regressive (i.e. no Social Security tax on earned income above a specified amount; e.g. $137,700 for year 2020).
Sales taxes are ALWAYS regressive taxes, which always hit the people of lesser income hardest.
For example, if you buy a car for $20,000 and the sales tax is 6%:
(a) and your annual income is $40,000,  the 6% tax of $1,200  is  3.00% of your annual income (i.e. 100 x (0.06 x $20,000)/$40,000 = 3.00%)
(b) and your annual income is $90,000,  the 6% tax of $1,200  is  1.33% of your annual income (i.e. 100 x (0.06 x $20,000)/$90,000 = 1.33%)
Therefore, a national FLAT sales tax (such as the system laughably called the FAIRTAX proposed and promoted by many Republicans) would also be a regressive tax, because ALL sales taxes are regressive.
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Different Types of Income are taxed at different rates:
Also, capital gains income is not subject to Social Security and Medicare1 taxes (unless investment income exceeds $200K for single, $250K for married).
Apparenty, some types of income are superior to others.
This is how Mitt Romney paid 15% on $42 Million in year 2012.
Yet, IF you make $80K per year in ordinary income, you paid 35.3% on your income for Social Security, Medicare, and federal income taxes.
However, IF you make $452,400K per year in capital gains, you paid 15% on your income for only federal income taxes.
Is that fair?

Ordinary Income Tax Rates (Social Security cap in 2020 = $137,700):


Capital Gains Tax Rates (2019-to-2025) for Single:


Capital Gains Tax Rates (2019-to-2025) for married, filing separately:


Capital Gains Tax Rates (2019-to-2025) for married, filing jointly:


Capital Gains Tax Rates (2019-to-2025) for head-of-household:


1NOTE:  There is a 3.8% tax only on investment income above $200K for single, or $250K for married;