A large theoretical literature argues laws exert a causal effect on norms, but empirical evidence remains scant. Using a novel identification strategy, we provide a compelling empirical test of this proposition. We use incentivized vignette experiments to directly measure social norms relating to actions subject to legal thresholds. Our large-scale experiments (n = 7,000) run in the United Kingdom, United States, and China show that laws can causally influence social norms. Results are robust across different samples and methods of measuring norms, and are consistent with a model of social image concerns where individuals care about the inferences others make about their underlying prosociality.