In a blog post responding to David Brooks’s column on the German recovery, Jonathan Chait writes:

The giveaway line in Brooks’ column is “natural experiment.” There’s no such thing as a natural experiment in macroeconomics. You can have a natural experiment in micro-economics — treat one group of workers one way, and treat a control group a different way. But whole economies are too large to run experiments with — there are always innumerable other factors at play.

It is true that macroeconomics can’t be an experimental science. But I get the sense that Chait is misinterpreting the meaning of the phrase “natural experiment.” His definition corresponds to a “controlled experiment,” which is of course a different thing. Here is an entry for “natural experiment” from The New Palgrave Dictionary of Economics Online:

Natural experiments or quasi-natural experiments in economics are serendipitous situations in which persons are assigned randomly to a treatment (or multiple treatments) and a control group, and outcomes are analysed for the purposes of putting a hypothesis to a severe test; they are also serendipitous situations where assignment to treatment ‘approximates’ randomized design or a well-controlled experiment.

This is how the term is used in common parlance, as in Greg Mankiw’s Principles of Macroeconomics textbook:

Friedman and Phelps had made a bold prediction in 1968: If policymakers try to take advantage of the Phillips curve by choosing higher inflation to reduce unemployment, they will succeed at reducing unemployment only temporarily. This view — that unemployment eventually returns to its natural rate, regardless of the rate of inflation — is called the natural-rate hypothesis. A few years later, Friedman and Phelps proposed this hypothesis, monetary and fiscal policymakers inadvertently created a natural experiment to test it. Their laboratory was the U.S. economy.

Again, note that natural experiments referred to serendipitous or “natural” situations.

The rest of Chait’s post is an excerpt from a characteristically interesting post by Paul Krugman on the German recovery. Krugman was not responding to David, as he wrote the post a few days before David published his column, as Chait makes clear before sharing the excerpt:

Basically, here’s the German story: it’s an economy that didn’t have a housing bubble, so it wasn’t caught up directly in the bust. But it’s very export-oriented, with a focus on durable manufactured goods. Demand for these goods plunged in the early stages of the crisis — so that Germany, remarkably, had a bigger GDP decline than the bubble economies — but has bounced back since summer 2009. This has pulled Germany back up; exports to China have done especially well.
If there’s a slam-dunk argument for austerity in there, it’s remarkably well hidden.

My impression is that David would agree. In his column, he was pretty humble about the claim he was advancing in the column:

Results from one quarter do not settle the stimulus/austerity debate. Many other factors are in play. For example, Germany is surging, in part, because America is borrowing. Essentially, we Americans borrowed from our kids, spent some of that money on German machinery, and ended up employing German workers.
But the results do underline one essential truth: Stimulus size is not the key factor in determining how quickly a country emerges from recession. The U.S. tried big, but is emerging slowly. The Germans tried small, and are recovering nicely.
The economy can’t be played like a piano — press a fiscal key here and the right job creation notes come out over there. Instead, economic management is more like parenting. If you instill good values and create a secure climate then, through some mysterious process you will never understand, things will probably end well.

That is, David was explicitly telling his readers not to overinterpret the German recovery. Why? As someone who knows David and his work, my sense is that he was trying to shed light on an issue many readers find confusing. He was not trying to settle the stimulus/austerity debate, not least because he thinks that there might be room for fiscal stimulus at some point going forward. David avoids theological approaches to economic questions.

My colleagues at Economics 21 have written a follow-up editorial on the German economy that you might find useful:

The Germans enacted a relatively large stimulus program (the proportion of tax cuts was twice as large as for the U.S.), but were extremely sensitive about not reaching the inflection point where a marginal dollar of stimulus would decline in effectiveness (or turn negative). The focus since the early part of the year has been on reversing stimulus, enacting credible government spending cuts, and encouraging the rest of the developed world to do the same. The result (coincidental or not), has been a return to pre-recession levels of business confidence, a 2.4% annualized increase in households consumption expenditures in the second quarter, and more than a 20% annualized increase in business investment. Although growth in net exports did boost GDP in the quarter, it accounted for only 36% of German second quarter GDP growth, less than the 50% contribution from private sector consumption and investment growth (excluding inventories). Were this simply about trade flows and demand for manufactured goods, the German economy would probably resemble that of Japan, where exports were up by more than 25% (annualized) in the second quarter, but the economy remained stagnant due to contractions in consumer spending and private sector investment.
As German business confidence rose, U.S. small business confidence exhibited large declines. Data released August 30 by the Bureau of Economic Analysis (BEA) show that the average personal savings rate for the three months ending in July was up 11% from the average savings rate in the first quarter. Overall, average annualized personal savings is up 13% over this same period, suggesting modest income gains have been saved. Again, the decline in business confidence and uptick in savings could be coincidental and have nothing to do with concerns about increases in future tax rates or the expansion of government’s involvement in the economy, but certainly these data points fit a pattern uncovered in previous empirical research. [Emphasis added.]

The Economics 21 team acknowledges that the results of a natural experiment can never be as dispositive as the results of a controlled experiment. But they argue that the limited evidence we have from the German recovery is suggestive.