A few years ago, economist Esther Duflo, PhD '99, found a problem that threatened to stump her. In the rural villages of Udaipur, a district in northern India with one of the worst child mortality rates in the world, parents were spurning health clinics' offer of free immunizations against deadly diseases such as measles and tuberculosis. Only 2 percent of local children were being immunized by age two.

Duflo, MIT's Abdul Latif Jameel Professor of Poverty Alleviation and Development Economics, specializes in finding unorthodox ways to help the world's poor. So she concocted an experiment with MIT-based collaborators Abhijit Banerjee and Rachel Glennerster, along with officials from Seva Mandir, a local nongovernmental organization. In some villages, they offered parents about two pounds of free lentils when they brought their children in for shots. Before long, families started streaming into these clinics. About four in 10 children got immunized where free lentils were available.

According to mainstream economic thinking, the success of the lentil giveaway made no sense. The shots were already free. The lentils, a cheap staple of the Indian diet, added little value. "The standard theory of human capital accumulation cannot explain why you go from a few percent to 38 percent," says Duflo. "The fact that there is huge responsiveness to such a small thing is contrary to theory."

But that is precisely why she likes to perform experiments. Duflo, 37, a native of France, has gained renown for using the world as a laboratory to see why aid programs succeed or fail. In so doing, she has not just tweaked conventional wisdom but helped revitalize global antipoverty efforts. For these efforts, she was given a 2009 MacArthur Foundation "genius" award in September.

Duflo's field needed rejuvenation. For decades, governments and aid groups have sunk hundreds of billions of dollars into programs intended to improve global welfare, while economists have toiled to identify a formula that would put poor nations on a path to economic self-sufficiency. But the impact of many aid programs remains hard to measure and subject to intense debate, even as the need grows: from 1970 through 2000, the billion people in the world's poorest countries got slightly poorer, while the rest of the global population realized annual gains in wealth between 2 and 5 percent in each decade.

Duflo, Banerjee, and their collaborators do not focus on sweeping theories. Instead, they run rigorous field experiments to find single factors that make aid programs work--factors like the lentils in Udaipur. In Kenya, Duflo and colleagues got farmers to use more fertilizer by providing free delivery right after harvest. In India, she figured out how to improve instructor attendance in rural one-teacher schools; when teachers' salaries were tied to their attendance, monitored by having them take time- and date-stamped photos of themselves, the absence rate fell by half, improving student performance significantly.

Not all these experiments work so decisively. But when they do, Duflo aims to expand their scope. In 2003, she, Banerjee, and Sendhil Mullainathan (now at Harvard) cofounded MIT's Abdul Latif Jameel Poverty Action Lab (J-PAL), of which Duflo is a director, to fund and popularize successful fieldwork. After J-PAL-affiliated researchers Michael Kremer, an economist at Harvard, and Edward Miguel of the University of California at Berkeley demonstrated that ridding children of intestinal worms is a spectacularly cost-effective way of improving school attendance, the lab worked to publicize the results and promote school-based deworming programs; Duflo, Kremer, and others at J-PAL helped start Deworm the World, a nonprofit organization that provided technical assistance to help the Kenyan government treat 3.6 million children in 2009.