Esther Duflo, a thirty-seven-year-old professor of development economics—the economics of poor countries and poor people—runs field experiments that measure different ways to save the world. In February, she went to Long Beach, California, to give a presentation at TED, an annual conference devoted to “ideas worth spreading.” Her demeanor was brisk and unsmiling. “I’m short, I’m French, I have a pretty strong French accent,” she began. Then she showed a slide of a neighborhood in Port-au-Prince that had been knocked down in the recent earthquake. “There’s something like a Haiti earthquake every eight days,” she said. That is, twenty-five thousand children around the world die of preventable causes every day. How should the developed world respond? With more aid? She showed a graph that plotted the billions in development aid given to Africa in recent decades, along with the continent’s G.D.P. per capita over the same period. Aid had risen sharply; G.D.P. had not. But the statistics revealed nothing about cause and effect, Duflo said. Without this money, Africa might have turned out better, or worse, or the same. “We have no idea,” she said. “We’re not any better than the medieval doctors and their leeches.” A slide of a leech.

Eight days later, Duflo was back in her office at M.I.T., in Cambridge. There was a view of the Charles River, and the central heating made plaintive, human sounds. She fiddled ceaselessly with a binder clip; though she was sitting on a narrow office chair, she managed to pull her legs up under her. Her manner mixed intellectual assurance with slight social impatience. (A favorite English phrase: “Give me a break.”) Pinned to the door was a poster, in Bengali, related to work she had done in India on local elections; she translated the text as “Together we can talk and solve our problems.”

She wondered if her references to Haiti at TED had been “a bit cheap.” But, she said, “I’m already understated, and not very funny. I have to be a little in-your-face.” She added, “I had two dinners with Bill Gates in two days. It was efficient.” She laughed, to acknowledge her fondness for statistical discipline, and for reliable data in great bulk—a preference for the measurable that can approach self-parody. Her response to almost any question about the future is a frown of bemusement and the words “How would I know?”

But Duflo also shows hints of idealism. She is a left-of-center French intellectual with faith in redistribution, and she subscribes to the optimistic notion (which, perhaps, runs ahead of firm data) that tomorrow might turn out better than today. Along with a thing for rock climbing, and for music (classical, with a pop-music exception made for Madness, the British group), this seems to be what fills Duflo’s head: the thought of doing good science, and the thought of doing good. And she is in large measure responsible for the emergence of a new, and fashionable, strand of academic study that combines these instincts. She and her colleagues in the Abdul Latif Jameel Poverty Action Lab (J-PAL), which she co-founded at M.I.T., in 2003, follow a line of pragmatic idealism, where you must first believe that there’s something to be done about poverty—not all economists would agree—and then you try to do it.

Within economics, Duflo and her colleagues are sometimes referred to as the randomistas. They have borrowed, from medicine, what Duflo calls a “very robust and very simple tool”: they subject social-policy ideas to randomized control trials, as one would use in testing a drug. This approach filters out statistical noise; it connects cause and effect. The policy question might be: Does microfinance work? Or: Can you incentivize teachers to turn up to class? Or: When trying to prevent very poor people from contracting malaria, is it more effective to give them protective bed nets, or to sell the nets at a low price, on the presumption that people are more likely to use something that they’ve paid for? (A colleague of Duflo’s did this study, in Kenya.) As in medicine, a J-PAL trial, at its simplest, will randomly divide a population into two groups, and administer a “treatment”—a textbook, access to a microfinance loan—to one group but not to the other. Because of the randomness, both groups, if large enough, will have the same complexion: the same mixture of old and young, happy and sad, and every other possible source of experimental confusion. If, at the end of the study, one group turns out to have changed—become wealthier, say—then you can be certain that the change is a result of the treatment. A researcher needs to ask the right question in the right way, and this is not easy, but then the trial takes over and a number drops into view. There are other statistical ways to connect cause and effect, but none so transparent, in Duflo’s view, or so adept at upsetting expectations. Randomization “takes the guesswork, the wizardry, the technical prowess, the intuition, out of finding out whether something makes a difference,” she told me. And so: in the Kenya trial, the best price for bed nets was free.

For work based on such experiments, Duflo won this year’s John Bates Clark Medal, for the best economist in America under forty—a Nobel-in-waiting. Last year, she received a MacArthur “genius” fellowship, addressed a committee of the General Assembly at the United Nations, and presented lectures at the Collège de France, in Paris, the youngest person to have done so. (A British newspaper headline: “STEP ASIDE, SARTRE: THIS IS THE NEW FACE OF FRENCH INTELLECTUALISM.”) In France, two books based on those lectures became best-sellers; in America, Duflo was included in Foreign Policy’s survey of the Top 100 Global Thinkers. In large part thanks to J-PAL, randomized evaluations are becoming popular at aid agencies and at the World Bank. And J-PAL itself has expanded to the point where it is generating counter-revolutionary grumbles from scholars: Is this really economics?

Duflo recalled one of her TED-conference dinners with Bill Gates: twenty people were present, among them Jeff Bezos, of Amazon; Nathan Myhrvold, the former Microsoft executive; Vinod Khosla, formerly of Sun Microsystems; and “the Facebook guy.” She spent some time that evening trying to calculate the per-capita income of the group but lost track. “I’m not good with money,” she said. Duflo found the socializing at TED to be “a bit high-pressure. I’m not very social, in the sense of cocktail-party social. I don’t like to talk to people I don’t know.” But she is comfortable when discussing her work, and her scientific approach clearly resonates with the philanthropists of the Internet age. Gates later pressed M.I.T. to make Duflo’s undergraduate course on poverty available online, and told her, “We need to fund you.”

On a misty afternoon several weeks earlier, on a narrow dirt road running through a village in Rajasthan, in northwestern India, Duflo had been surrounded by dozens of schoolgirls trying to learn how to say hello in French. “One, two, three: bonjour!” Duflo said. “One, two, three: bonjour!” the girls said back, laughing. None of them had heard of France. Duflo was dressed in a shalwar kameez, with a pale-orange scarf over her shoulders. The light was strange, giving every view—small single-story homes with corrugated-iron roofs, flat farmland behind, cows and carts—the texture of a nineteenth-century landscape painting. Earlier, Duflo and a number of others, all holders of advanced degrees, had debated whether the spectral disk of light above our heads was the moon or the sun.

Duflo was midway through a fast-paced trip to India and Rwanda—a series of cramped car rides and short flights on budget airlines, during which she always took from her bag an English translation of “2666,” the sprawling novel by Roberto Bolaño, and opened it, with relish, but then found herself pulled into the activities of a professor in demand: judging a paper submitted to a scholarly journal that she edits; responding to e-mail requests for a guest lecture. She was also working on a book, as yet untitled, about the economic lives of the poor. It will be pitched at an alert general reader, she said: “We’re not talking ‘Freakonomics,’ but there won’t be equations.”

Her co-author is Abhijit Banerjee, a professor at M.I.T., a co-founder of J-PAL, and now, like Duflo, one of its directors. (A third founder, Sendhil Mullainathan, has moved to Harvard but remains affiliated with J-PAL.) In January, Banerjee was one of a number of colleagues whose travels coincided with Duflo’s for a few days. A decade older than Duflo, Banerjee has a loping, liquid walk and an ironic manner; he grew up in Calcutta, and he was Duflo’s guide on her first trip to India, in 1997. “She’s very comfortable in India,” he told me. “She doesn’t get information overload.” Her affection for the country is evident: she returns at least once a year, and in Cambridge she is surrounded by Indian friends and colleagues. In her apartment, in Beacon Hill, where she lives alone, she cooks Indian food in a kitchen remodelled in a spiky modern spirit by an Indian architect. And it is largely in India that Duflo has done the work on which her reputation stands.

In 2003, for example, she helped devise a randomized trial that addressed the problem of absentee teachers in schools run by an Indian nonprofit group. The trial involved a hundred and twenty schools. In sixty of them, teachers were asked to have a photograph taken of themselves, with their students, at the start and the end of each school day, using a tamper-proof camera that time-stamped each image. Pay was then adjusted to attendance. Compared with a control group of the same size, the photographed teachers were half as likely to be absent. They did not resent the cameras, but it wouldn’t have troubled Duflo if they had: “Who do you care about? Lazy teachers who show up sixty per cent of the time, or the kids? O.K., I care about the kids.” Because the teachers turned up more, they taught more, and their students performed better on tests. Banerjee has described watching Duflo work on a follow-up experiment, and thinking that he was witnessing “a new economics being born.”

In the village, Duflo and her colleagues were waiting for a play to begin. We were an hour or so from the city of Bharatpur; judging by Duflo’s studies of other districts in Rajasthan, local families were probably living on no more than a few dollars a day, and had no cell phones, no TV, and limited literacy. Across the road, a van with a loudspeaker tied to its roof was parked next to a raised patch of ground. A red mat had been laid out, creating a stage, and three musicians—playing drum, cymbals, and hand-pumped harmonium—sat cross-legged on it. A hundred or so villagers gathered in the road to watch, men on the left, women and children on the right. Crows squawked. In the van’s passenger seat, a young man struggled, like Superman, to change into women’s clothes.

The musicians began to play, and the loudspeaker let out a shocking jolt of sound. Two actors walked onstage: the man dressed as a woman—rural Rajasthan shares with Elizabethan England an unease about females onstage—and a round, middle-aged man, a master of ceremonies, wearing a maroon jacket, a loose maroon turban, and white pants. He puffed out his chest and, in a good-humored performance, sang and danced, directed a puppet show, mimed a motorbike crash, and delivered a good amount of talk—every line shouted into a microphone—about the wisdom of voting for a village leader who is not corrupt, and who might be a woman. The audience was attentive, at least for the first hour. At one point, we all stepped out of the way to allow a tractor to pass through the village.

It was odd to recall that the script had recently been assessed by a human-subjects ethics committee at M.I.T. (A relevant form asks, “Will radiation or radioactive materials be used?”) The play was one small part of a labor-intensive J-PAL experiment that Duflo had helped design. In conjunction with a local organization, J-PAL had set this morality play in motion, hiring ten troupes of actors, each one identically equipped: ten vans, ten maroon turbans. A local election was imminent, and in recent weeks these performers had been visiting village after village; among them, they performed the same show, or some close variant of it, four hundred and sixty times.

This is how Duflo asks a question about women’s empowerment. It is also one way that she responds to those who tell her that although they can appreciate experimental evaluations of bed nets or textbooks, they struggle to understand the idea in the messier context of human behavior and politics. (Duflo recalled an emblematic exchange, during a meeting at the World Bank in New Delhi. After Abhijit Banerjee made a passing comment—“Randomized evaluation can be taught, it is not nuclear physics”—a UNICEF official stood up and said, “Studying human beings is much more complicated than nuclear physics.”)