Congress should establish a policy-evaluation office, modeled after the JPAL or IPA, to run randomized, controlled trials on social policies. The office should have broad authority to do test-runs of proposals of its choosing, operating under the same rules of informed consent used in medical studies.

Members of Congress should have the power to request studies, particularly when they bear on current debates and can be done relatively quickly. For example, the Obama administration is likely to push for immigration reform next year. In concert with the state involved, an evaluation office could randomly select one town and grant its illegal immigrants permanent residency, and randomly select another town and leave its undocumented residents in a legal gray zone. Within a few months or a year, researchers should be able to see whether bringing the immigrants out of the shadows hurts native-born workers’ wages, reduces employer abuse or has any number of other consequences. That’s a reasonable time for Congress to wait before adopting huge changes to the immigration code.

Other questions wouldn’t be answerable that quickly. By definition, a study that seeks to find out whether access to preschool increases children’s earnings as adults would take decades to complete. The office should engage in such projects — known as longitudinal studies — even without congressional prodding, providing updates along the way. The questions at stake are too important, and the amount of effort needed too great, to leave to the whims of Congress.