Prior research suggests that negligent harms are punished because of the resulting negative outcomes. Under this account, negligent but completely harmless acts should not be punished. An alternative possibility is that negligence is punished as a way of modifying future thought and behavior. Across three studies we find support for this second proposal. Study 1 demonstrates that punishment is assigned to negligent agents, irrespective of whether or not a harm actually occurs. Study 2 demonstrates that non-negligent agents who cause harm are punished less than negligent agents who do not cause harm. Study 3 shows that the punishment of harmful negligent actions is only judged to be successful when it results in the agent ceasing to act negligently, and not when it results in the harm ceasing to occur. Together, these results suggest that a primary function of punishment in cases of negligence is modify future thought.