The bottom 50% of income earners, while paying 3.9% of federal taxes, earn 13 % of the income. This number comes from the IRS, and does not include things like the earned income tax credit, child tax credits... etc... that 3.9% is a lower in reality.
The Top 50% earn 87 % of the money, and pay 96.1 % (again, more than 96.1 % when taking into account the EITC and child credits the "bottom 50%" gets) of the taxes.
So... If you think about it, the US Government (meaning the taxpayers) cannot do any more to alleviate the tax burden on the lowest 50% of income earners. They have no tax burden.
Do I mind that ? As someone who spent the first 1/2 of his life there in the lower 50%, no.
Another stat ... The top 1 % pays 38 % of all taxes on 20 % of income. In other words, for every $1000 that top guy makes, he pays 7 times as much in taxes as the average bottom 50% guy.... Note, that is for EVERY $1000 ...
