Observers of deviant social behavior sometimes communicate disapproval directly or indirectly to the perpetrator of a deviant act. This reaction has been termed ‘social control’. Three field studies were conducted to explore the influence of the number of bystander-observers on the likelihood of social control. We predicted that the presence of others would inhibit people's tendency to communicate their disapproval to the deviant but only if personal implication was low. In the first study, we measured participants' perceptions of two fictive situations, one in which a deviant draws graffiti in an elevator of a shopping center and one in which a deviant litters in a small neighborhood park by throwing a plastic bottle in the bushes. As expected, participants considered both behaviors to be equally counternormative but felt personally more implicated by the littering behavior in the park. In Studies 2 and 3, the two situations were enacted with confederates of the experimenter. Naïve bystanders served as participants, and social control was the primary dependent variable. Consistent with our hypothesis, we found evidence for a bystander effect in the low personal implication situation (‘graffiti in the elevator’) but not in the high personal implication situation (‘littering in park’). These results make clear that perceived personal implication moderates the extent to which people are inhibited by the presence of others when they decide whether they should exert social control or not. Copyright © 2002 John Wiley & Sons, Ltd.