Our "tax system" is virtually FLAT when one considers all taxes and the hodgepodge of charges and levies we struggle with. Suffice it to say that we have a big "regressive employment tax" that have enlarged over the decades.
http://www.statesman.com/business/personal-finance/how-much-can-the-99-percent-squeeze-out-1928809.html
http://assetbuilder.com/blogs/scott_burns/archive/2011/10/21/how-much-can-the-99-squeeze-out-of-the-1.aspx
A regressive tax is a tax imposed in such a manner that the tax rate decreases as the amount subject to taxation increases.
http://www.ask.com/wiki/Regressive_tax