The history of public policy is littered with failures to solve large-scale social problems using interventions derived from behavioral science theories. In contrast, numerous small-scale problems have been solved through applications of behavioral science. This difference in outcomes reflects a mismatch between methods and aspirations. Behavioral science research relies on small world studies to develop theories of behavior, but such studies will rarely capture the many important forces that combine to produce and sustain big world problems. However, small world studies can provide solutions to specific problems that arise repeatedly in particular contexts by identifying features of the person and environment that must be controlled to solve the problem. Using this bottom-up approach, organizations and professions have learned to reduce costly errors by structuring informational and decision processes, holding people accountable for following prescribed procedures, and providing feedback when errors occur. By turning decision situations into small worlds that focus actors on the right information and the right considerations, public and private organizations can promote accuracy, efficiency, and fairness.