Compliance with laws and regulations depends on the expected penalty facing violators. The expected penalty depends on both the probability of punishment and the severity of the punishment if caught. A key question in the economics of crime literature is whether increasing the probability of punishment is a more effective deterrent than an equivalent increase in the severity of punishment. This paper uses laboratory experiments to investigate this issue, and finds that increasing the severity of punishment is a more effective deterrent than an equivalent increase in the probability of punishment. This result contrasts with the findings of the empirical crime literature.