The Congressional Budget Office has data on distribution of federal taxes. They plot the effective tax rates from 1979 to 2007.
I took the data from the CBO and plotted the graph below:
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*Source CBO. |
Looks mostly flat doesn't it? Note there is a pretty good drop in the effective federal income tax rates from 2000 to 2003 when it went from 11.8% to 8.4%. Thats about a 28% income tax decline for the average filer.
This is just the average effective rate for all tax payers. The effective rates for different income levels are going to be different.