At one level, trying to have a serious discussion of the economic impacts of the Cut Cut Cut Act – sorry, the Tax Cuts and Jobs Act – is arguably a waste of time. Republicans who believe, or pretend to believe, that tax cuts will produce an economic miracle, who didn’t change their minds after the Clinton boom, the Bush debacle, the Kansas disaster, and the strength of the economy after 2013 aren’t going to be persuaded by further analytical discussion.

But some of us have spent our lives trying to understand such things, and there are some intellectually interesting aspects of the current tax debate even though the would-be reformers aren’t interested in a real discussion. So let me indulge myself.

The thing is, while Republicans always claim that tax cuts will produce miraculous growth, both the proposed tax cuts and the supposed sources of the miracle are a bit different this time. Instead of focusing on individual tax rates – aside from the estate tax – this time it’s mostly about corporate taxes. And instead of claiming huge increases in work effort from lower marginal rates, they’re mostly claiming that lower corporate taxes will bring huge capital inflows, raising wages and GDP.

There are multiple reasons to be skeptical about these claims; the actual magnitude of any positive effect on GDP is likely to be far smaller than anything Republicans say. The Penn-Wharton model says that GDP in 2027 would be between 0.3% and 0.8% higher with the tax cuts than without, i.e., basically an invisible effect against background noise; and this doesn’t even take into account the longer-run negative effects of discouraging higher education, slashing nutrition programs, and all the other things that will probably happen due to higher deficits.

But let me make a different point: GDP is actually the wrong measure. If you’re going to be pulling in foreign capital, you’re going to be paying more investment income to foreigners; so gross national income – income accruing to domestic residents – is going to go up by less. And surely that’s the measure we care about.

In fact, when you bear in mind the reduced taxes collected on foreign investors who are already here, GNI could actually go down, not up.

One way to say all of this is that Cut Cut Cut would be an attempt to bring a bit of leprechaun economics to the United States. Ireland, famously, is a country where GDP vastly exceeds national income, by a growing margin:

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The reason is a low corporate tax rate, which attracts both real foreign investment in capital-intensive sectors – investment that raises GDP but does little for workers – and also creates an incentive to use transfer pricing to make profits appear in Ireland even though they have nothing much to do with Irish activity. Not incidentally, Kevin Hassett appears to be confused about the economics here, imagining that a paper reduction in the US trade deficit due to changes in transfer pricing would bring in real jobs. It wouldn’t.

There are really two bottom lines here. One is that the true growth impacts of Cut Cut Cut would be even more pathetic than the numbers you’ve been hearing. The other is that if you’re going to make international capital flows central to your arguments, you really need to think about the implications for future investment income.