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One of my great assets of being in this business, or maybe I’ve developed it over time, is I don’t have many opinions to start with,” she told me. “I have one opinion — one should evaluate things — which is strongly held.

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Not everyone is persuaded. Among the critics of j-pal’s work, the best known is Angus Deaton, the Princeton economist. Despite his admiration for Duflo, Deaton has become increasingly wary of the fashion for randomization—to the point where, at the end of 2008, he found himself giving a high-profile lecture in London that began with two photographs on a PowerPoint slide. One showed someone floating beneath an open parachute; in the other, a person appeared to be jumping out of a plane without a parachute. The first photo was marked “Esther Duflo,” the second “Abhijit Banerjee.” The point was this: Do we need to test the idea that parachutes are useful to people who jump out of planes? Deaton’s tone came as a surprise to some. Joshua Angrist, Duflo’s former adviser, recently wondered if “someone put sand in Angus’s toothpaste.”

Deaton’s lecture, and a subsequent paper, made two main arguments. One: Randomized trials are more difficult than they look, and “experiments are frequently subject to practical problems that undermine any claims to statistical or epistemic superiority.” Two: Even if your data are perfect, how can you generalize from the information? Does the policy that works in India work in Brazil? Does the approach that works for a hundred people work when applied to ten million people? This is the problem of “external validity.” As soon as you leave behind the particular context of the experiment, then any wider economic point you might like to make derives from inference, which is only as good as the thinking—the theory—behind it.

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As soon as you leave behind the particular context of the experiment, then any wider economic point you might like to make derives from inference, which is only as good as the thinking—the theory—behind it.