Social-dilemma research has shown that imposing sanctions on defection may increase cooperation, a principle behind attempts to solve real-world social dilemmas. Yet sanctioning systems are often difficult to implement: They are unpopular and often have large surveillance and enforcement costs. A new sanctioning system, intentionally punishing defection intermittently for some but not all group members, is shown to increase cooperation among those not punished, a finding labeled the spillover effect. This study suggests that the effect cannot be attributed simply to cooperative tendencies, as factors affecting cooperation do not affect the effect's size. The benefits of such a sanctioning system, which preserves the characteristics of social dilemmas, could include minimization of surveillance and enforcement costs, and greater public acceptability.