|Left scale for top US marginal income tax rates. Right scale for GDP growth in chained 2005 dollars. A 10 year moving average for each figure is also included. GDP data from the BEA. Tax rate data from TruthAndPolitics.org|
Overall, the past 80 years show us a thorough lack of clear correlation between the top marginal tax rate and GDP growth. The data's closest hint of a relationship derives from the slightly more robust average GDP growth back when the top rates were higher, but that closest hint isn't close enough to be sure of an ideal rate. The notion that lowering the top tax rates improves the economy just doesn't hold water. Indeed, these 8 most recent decades show us that increasing the top tax would not necessarily have any impact on the economy, let alone slow it at all.
Update 02/03/2011: see Chained to Real: Tax Rates, Inflation-adjusted GDP Measures, and the Difference for a version of the above chart using another inflation-adjusted measure of real GDP growth (with roughly the same result).