3. The claim of the Laffer Curve is theoretical. The claim is that there is an “optimal” income tax rate, one which maximizes government tax revenue. It doesn’t make an empirical prediction about what the “optimum” tax rate is and so, it can’t be tested by the empirical results of tax rate changes. It’s true that Laffer himself thought the 1980s income tax rates were above the “optimal” rate, but that was just his guess.