The outcomes of interaction in online communities depend to a large extent on finding solutions to typical problems of interaction, such as free-riding and lack of trust. This article presents a theory which argues that a member's online behaviour sends signals about how (s)he regards the relationship to other members and to the group. Under specific conditions, members take the signal sending into account when they decide whether to contribute to group discussions and to participate in trust-demanding online activities. Community administrators can use the insights to influence members' behaviour by using social control. Three forms of social control are distinguished. Group conditions influence which form is more adequate for diminishing free-riding and lack of trust. A theory-guided typology of online groups and communities clarifies what type of community is more likely to suffer from problems of interaction and the effects of each kind of social control.