In 1960 only 15% of federal income tax returns had a zero tax liability, due mostly to low taxable income, a standard deduction, the personal exemption, the dependent exemption, and the Earned Income Tax Credit (a direct subsidy to people for being poor).

FORBES: Representation Without Taxation - So Where's the Outrage?

It begins with a 15% flat rate, but taxpayers who cross certain income thresholds owe more because of the 3.8% net investment income tax, the Pease limit and the Personal Exemption Phaseout, or PEP, a backdoor increase that limits personal exemptions.

WSJ: The New Capital-Gains Maze