Quote from jprad:
Anyone who thinks a percentage flat tax is fair hasn't taken into account that there are three dimensions to compensation.
The first is the socioeconomic ladder. It's disproportionately harder to move up when you're farther down at the start.
The second is income growth opportunity. Blue collar labors will see lower net growth in income over their career than a white collar worker will.
The third is lifetime output. Higher-compensated professions result in less annual hours worked and earlier retirement.
When you factor all three of those dimensions in a flat percentage tax becomes regressive for lower wage earners.
Someone who has to pick vegetables by day and clean floors by night for practically their entire life pays more tax as a percentage of their lifetime income then someone who enters the workforce in their 20's and is able to retire in their mid-50's, if not earlier.