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The idea that tax cuts pay for themselves repeatedly has proven to be nonsense, perhaps most spectacularly when President George W. Bush’s own Treasury Department concluded that his enormous tax cuts did not produce anywhere near enough economic growth to recoup their costs. Yet this repeatedly disproven supply-side economics theory pushed by fringe economist Arthur Laffer and others is alive and well and was most recently promoted at a Ways and Means subcommittee hearing on July 30.

Supply-side economics suggests that by allowing people to keep more of their income, tax cuts encourage people to supply more capital and labor. This supposedly generates such increases in income and profits that the resulting boost in tax revenue will partly cancel out or even exceed revenue loss from the tax cut.

Proponents of tax cuts and supply-side economics have for years called on the Joint Committee on Taxation (JCT), the official revenue-estimators for Congress, to use a method called dynamic scoring to take into account these supply-side effects that allegedly reduce the cost of tax cuts or even result in a revenue increase.

But given the utter uncertainty about these macroeconomic impacts, it is entirely reasonable that they are left out of official revenue scores that Congress and the public must rely on to understand the effects of tax legislation.

Nonetheless, supply-siders and their elected allies twisted JCT’s arm into providing dynamic scoring for the tax reform plan introduced in February by House Ways and Means Chairman David Camp, and this analysis was the focus of last week’s hearing.

JCT found that the macroeconomic growth effects of Camp’s plan would increase revenue “by $50 to $700 billion, depending on which modeling assumptions are used,” over a decade. (CTJ found that while the Camp plan would be revenue-neutral in the first decade, it would lose $1.7 trillion in the following decade, a hole that no dynamic analysis can fix.)

Scott Hodge of the Tax Foundation, a hearing witness, argued that the macroeconomic benefits would have been greater if the Camp plan included more tax breaks. For example, he argued that revenue would actually be higher under the Camp plan if it made permanent the recently expired 50 percent expensing for investment (often called bonus depreciation), as the House of Representatives voted to do with a stand-alone bill in July. CTJ has explained why this tax break, which was projected by JCT to cost $276 billion over a decade, is unlikely to have any economic benefit at all.

The Problem with Dynamic Scoring

Supply-side economists sometimes claim that JCT provides only “static” analysis that ignores behavioral effects entirely, which is not actually true. For example, when JCT estimates the effects of a higher income tax rate on capital gains (profits from selling assets for more than they cost to purchase), it does account for behavioral effects by assuming that some people will want to avoid this tax increase by selling fewer assets. This will reduce the revenue increase that would otherwise result. (A CTJ report goes into great detail about the debate over these assumptions.)

What JCT usually does not take into account are impacts that tax legislation might have on the whole economy (macroeconomic impacts) because these are usually small and always impossible to predict. In fact, economists can’t even agree on the direction of such impacts. For example, a lower tax rate could in theory encourage people to work more because they’re able to keep more of what they earn, but it could also encourage people to work less because they don’t have to work as much to reach whatever earnings goals they’ve set for themselves. In other words, a tax cut could cause the economy to expand or contract.

Yet another problem with dynamic scoring is that its proponents never want to apply the same logic to spending. If tax cuts boost the economy enough to offset part of their costs, then surely the same could be true for public investments such as education and infrastructure, which everyone agrees boost the economy. But don’t expect Arthur Laffer or Dave Camp to be making that argument any time soon.