For a quarter century, American education researchers have tended to favour qualitative and descriptive analyses over quantitative studies using random assignment or featuring credible quasi-experimental research designs. This has now changed. In 2002 and 2003, the US Department of Education funded a dozen randomized trials to evaluate the efficacy of pre-school programmes, up from one in 2000. In this essay, I explore the intellectual and legislative roots of this change, beginning with the story of how contemporary education research fell out of step with other social sciences. I then use a study in which low-achieving high-school students were randomly offered incentives to learn to show how recent developments in research methods answer ethical and practical objections to the use of random assignment for research on schools. Finally, I offer a few cautionary notes based on results from the recent effort to cut class size in California.