The basic idea behind the lab is to rely on randomized trials  similar to the ones used in medical research  to study antipoverty programs. This helps avoid the classic problem with the evaluation of aid programs: it’s often impossible to separate cause and effect. If aid workers start supplying textbooks to schools in one town and the students there start doing better, it could be because of the textbooks. Or it could be that the town also happened to hire a new school administrator.

In a randomized trial, researchers would choose a set of schools and then separate into them two groups. The groups would be similar in every respect except for the fact that one would receive new textbooks and one wouldn’t. With a test like this, as Vinod Thomas, the head of independent evaluation at the World Bank, says, “You can be much more accurate and much more clear about the effect of a program.”

The approach can sound cruel, because researchers knowingly deny help to some of the people they’re studying. But what, really, is the alternative? It’s not as if someone has offered to buy new textbooks for every child in the world. With a randomized study, you at least learn whether your aid money is well spent.

Ms. Duflo, who’s 35, and Mr. Banerjee, 46, came to economics from opposite ends of the intellectual spectrum. She was studying history at the École Normale Supérieure, one of the most prestigious colleges in France, when she decided that the more scientific approach of economics offered a better way to address global poverty. He dropped out of the similarly prestigious Indian Statistical Institute after two and a half months of studying math; he found the subject too abstract.

By 2003, they were both working on development at M.I.T. At the time, randomized trials were becoming more popular in the United States, but they were still fairly rare in the developing world. So along with Sendhil Mullainathan, a colleague, Ms. Duflo and Mr. Banerjee founded the lab. (It’s named for the father of an M.I.T. alumnus, who owned the exclusive right to sell Toyotas in Saudi Arabia.) Day to day, the lab is now run by Rachel Glennerster, who came from the International Monetary Fund, and it has become a magnet for some of the world’s best development economists, including Marianne Bertrand, Michael Kremer and Edward Miguel.

Mr. Kremer and two other economists, in fact, did the textbook experiment  and found that textbooks didn’t improve test scores or graduation rates in rural western Kenya. (The students were probably too diverse, in terms of preparation and even language, to be helped by a single curriculum.) On the other hand, another randomized trial in the same part of Kenya found that treating children for intestinal worms did lift school performance. That study has led to an expansion of deworming programs and, as Alan Krueger of Princeton says, is “probably improving millions of lives.”

Mr. Banerjee estimates, very conservatively, that $11 billion a year  out of roughly $100 billion in annual development aid worldwide  could be spent on programs that have been proved to work. Unfortunately, nowhere near $11 billion is being spent on such programs. “Right now, we don’t have a lot of things that have been taken up by the policy world,” he said. “But the policy lag is usually substantial. Now that we have a lot more results, I expect that in the next 10 years we will have a lot more impact.”