Peter Orzsag calls the claim that the debt-to-GDP ratio can be lowered by providing a fiscal boost to a depressed economy the “Laffer curve of the left.” For those who have real lives and may not get the reference, the “Laffer curve” refers to the theoretical possibility that one can raise overall tax revenues by cutting tax rates. The intuition is that cutting tax rates provides incentives for working longer and saving more. In turn, this will boost economic growth sufficiently to bring in more revenue despite rates having been cut. The claim that it is relevant to the U.S. economy has been discredited empirically (and a long time ago).

In light of this, Orzsag’s claim that the “Laffer curve of the left seems to have as much empirical relevance as the original Laffer curve” is not only odd but also flat wrong.

Orzsag’s target is clearly a recent paper by DeLong and Summers that shows fiscal stimulus in a depressed economy has multiple salutary effects, not just on economic growth but even on long-run budget measures (like the debt-to-GDP ratio). The paper shows stimulus boosts near-term growth directly by relieving the constraint of insufficient demand; it boosts productive investments by giving firms an incentive (i.e., more customers coming in the door) to expand capacity; and it keeps chronic long-term unemployment from turning into a permanent erosion of workers’ skills (i.e., economic “scarring”). The assumptions about the strength of each of these effects that are needed to make fiscal stimulus debt-improving in a depressed economy are probably pretty close to real-life parameters.

Let’s do some simple math with widely-agreed upon parameters, even ignoring some of the supply-side measures DeLong and Summers examine. I’m going to round very aggressively here, but it doesn’t affect results much.

Today’s publicly-held debt is about 70 percent of GDP (call it $10.5 trillion on a base of GDP that is $15 trillion). Let’s say we decided to undertake fiscal stimulus in the form of $150 billion spent on high-multiplier activities like extending unemployment insurance, giving aid to states, or investing in infrastructure (we actually need more than this, but it’s a nice round 1 percent of overall GDP, so we’ll stick with it).

The “fiscal multipliers” on these activities are roughly 1.5, meaning they generate $1.50 in economic activity for every dollar spent on them (actually, it may be quite a bit higher, but we’ll take 1.5 as given).

So, (roughly) a year from now, this stimulus has increased the level of GDP by $225 billion (i.e., the $150 billion stimulus multiplied by 1.5). This extra GDP does indeed lower the budget deficit by bringing in more revenue. A reasonable estimate, based on CBO data, is that when the economy is operating below potential, each 1 percent increase in GDP growth yields a cyclical reduction in the budget deficit of about 0.35 percent of GDP. So, this $225 billion in additional output leads to a $79 billion improvement in the budget deficit, making the “net” fiscal cost of the stimulus just $71 billion ($150 billion minus the $79 billion offset from higher growth).

This $71 billion “net” cost of stimulus increases debt by roughly 0.7% ($71 billion divided by the current $10.5 trillion public debt). But GDP has increased by 1.5 percent. Given the current debt-to-GDP ratio of 70 percent, this means that this measure actually declines because the stimulus has increased debt by 0.7 percent but GDP by 1.5 percent.

None of these parameters, by the way, are particularly contested.1 And let’s say they’re slightly wrong, and that instead of outright improving the debt-to-GDP ratio, providing fiscal stimulus in today’s depressed economy actually makes it slightly worse – say it’s only 80 percent self-financing in terms of its impact on debt-to-GDP ratios. Would this really justify calling claims that providing fiscal stimulus in depressed economies does not damage public finances “the Laffer Curve of the left”? Not by my read of the evidence.