We made behavioral observations in 37 social groups of the communal stripe-backed wren during 1990 and 1991 to investigate the recently-discovered shared paternity between dominant and subordinate males (or "SMs"). We found two distinct kinds of social groups that differed in terms of social behavior: "mother" groups, which contained only subordinate males that were sons of the dominant female (termed "DF-sons"), and "stepmother" groups, which contained at least one subordinate male unrelated to the dominant female (termed "DF-stepsons"). In mother groups only dominant males courted dominant females, mate-guarding was infrequent and aggression by dominant males toward other males was absent. On the other hand, stepmother groups were characterized by frequent association with and courtship of dominant females ("DFs") by both dominant males ("DMs") and DF-stepsons and relatively frequent aggression by dominant males toward DF-stepsons. DF-stepsons, moreover, sired 15% of all young in stepmother groups. Thus, incest avoidance dictated the behavior of subordinate males, and the mating system in wren groups was either monogamous or polyandrous, depending upon the relatedness between subordinate males and the dominant female.