The past two years Kansas reduced its state income tax rates. As a result, the top rate of income tax faced by Kansas residents (combined state and federal) rose from 41.45% in 2012 to 48.3% in 2013 and then fell a tad to 48.2% in 2014 (if they don't itemize.) That's a pretty tiny drop in the top marginal tax rate in 2014, and a much bigger rise in 2013.
I consider myself a moderate supply-sider, but I certainly wouldn't expect such a tiny tax cut to significantly affect behavior. And any effects that did occur would happen very gradually, over a period of many years. For instance, firms might be slightly more likely to move to Kansas. But even after the tax cut, the top rate is almost as high as in Massachusetts, so Kansas is certainly not a tax haven like Washington or Texas, which have no state income tax.
I can't imagine any serious economist predicting that the Kansas rate cut would boost Kansas GDP by 25% or more. Why did I pick that figure? Because the Kansas state income tax top rate fell from 6.45% in 2012 to 4.8% in 2014, which is roughly a 25% rate cut. In order for that rate cut to boost Kansas tax revenues, you'd have to see Kansas GDP rise by more than 25%. That's obviously absurd.
Why am I even discussing such crazy ideas? Because Paul Krugman seems to want to convince his readers that lots of supply-siders believe such nonsense:
Two years ago Kansas embarked on a remarkable fiscal experiment: It sharply slashed income taxes without any clear idea of what would replace the lost revenue. Sam Brownback, the governor, proposed the legislation -- in percentage terms, the largest tax cut in one year any state has ever enacted -- in close consultation with the economist Arthur Laffer. And Mr. Brownback predicted that the cuts would jump-start an economic boom -- "Look out, Texas," he proclaimed.
But Kansas isn't booming -- in fact, its economy is lagging both neighboring states and America as a whole. Meanwhile, the state's budget has plunged deep into deficit, provoking a Moody's downgrade of its debt.
There's an important lesson here -- but it's not what you think. Yes, the Kansas debacle shows that tax cuts don't have magical powers, but we already knew that. The real lesson from Kansas is the enduring power of bad ideas, as long as those ideas serve the interests of the right people.
Why, after all, should anyone believe at this late date in supply-side economics, which claims that tax cuts boost the economy so much that they largely if not entirely pay for themselves?
Actually, supply-siders do not claim that tax cuts pay for themselves, except in very unusual cases. Kansas is not one of those cases. The Laffer curve effect is typically applied to cases of extremely high marginal tax rates.