Rhesus macaques, like many other primates, live in stable, multi-male multi-female groups in which adult females typically outnumber adult males. The number of males in multimale]multi-female groups is most commonly discussed in terms of mate competition,where the sex ratio is a function of an adult males ability to monopolize a group of females.However, the relationship between sex ratio and group stability is unclear because the presence of many males may either reduce stability by increasing mate competition or may improve stability if adult males are key conflict managers. We investigated the relationship between sex ratio, male intervention behavior, and trauma in seven groups of captive rhesus macaques. Our results show that high-ranking adult males intervene twice as frequently as adult females (P< 0.0001 ) and are about twice as successful (P< 0.0001).Furthermore, the type of adult males present in the group affects the relationship between intervention behavior and rate of traumas: males must be unrelated to the highest-ranking matrilines. Groups with a lower ratio of females per male unrelated to the alpha and beta matrilines had better intervention success (P < 0.04) and fewer traumas requiring veterinary care (P= 0.003). We conclude that conflict management behavior by adult males, particularly those unrelated to the highest-ranking matrilines, is the mechanism by which sex ratio influences the frequency of traumas, and thus group stability. We therefore suggest that monitoring and managing the sex ratio of captive primate groups is one of many measurements to predict group stability.