The only approaches currently available to reduce transmission of the novel coronavirus severe acute respiratory syndrome–coronavirus 2 (SARS-CoV-2) are behavioral: handwashing, cough and sneeze etiquette, and above all, social distancing. Johannes Haushofer and C. Jessica E. Metcalf write in Science that policy-makers have a variety of tools to enable these “nonpharmaceutical interventions” (NPIs), ranging from simple encouragement and recommendations to full-on regulation and sanctions. However, these interventions are often used without rigorous empirical evidence: They make sense in theory, and mathematical models can be used to predict their likely impact, but with different policies being tried in different places—often in complicated combinations and without systematic, built-in evaluation—we cannot confidently attribute any given reduction in transmission to a specific policy.