How accurately can behavioral scientists predict behavior? To answer this question, we analyzed data from five studies in which 640 professional behavioral scientists predicted the results of one or more behavioral science experiments. We compared the behavioral scientists' predictions to random chance, linear models, and simple heuristics like "behavioral interventions have no effect" and "all published psychology research is false." We find that behavioral scientists are consistently no better than - and often worse than - these simple heuristics and models. Behavioral scientists' predictions are not only noisy but also biased. They systematically overestimate how well behavioral science "works": overestimating the effectiveness of behavioral interventions, the impact of psychological phenomena like time discounting, and the replicability of published psychology research.