[영문]When and how will third parties intervene in the conflicts of others? When will third parties further violence, and when will they foster peace? These are questions addressed by Black's theory of the third party. The authors conducted the first systematic test of the theory, drawing on quantitative and qualitative data from in-depth interviews with 100 men imprisoned for assault or homicide. Specifically, they tested Cooney's elaboration of Black's theory, which argues (1) that the social location of a third party, based on ties to the principals, predicts whether he or she will act as a partisan or settlement agent or remain uninvolved, and (2) that the third-party structure of a conflict, based on the general configuration of all the third parties present, predicts whether the conflict will escalate to violence. The study results confirm most of the theoretical predictions regarding both third-party behavior and violence. The discussion also extends the theoretical model, offering new concepts and suggesting how the theory can be applied to a range of subjects, such as international war and the long-term historical decline of interpersonal violence.