Members of captive colonies of common marmosets (Callithrix jacchus), a small New World primate, can occasionally be victims of repeated, and potentially fatal, attacks by a family-mate. This study examined the records of a colony, looking for past instances of such aggressions. The aim was to better understand the possible causes underlying this phenomenon and to identify variables that could minimize their occurrence. The results showed that both males and females behaved as aggressors at the same rate, but females attacked just females, whereas males attacked both males and females. Most aggressions involved siblings and occurred at a higher rate when the cages were occupied by more than 5 nonhuman animals. The influence of group size was significant only when females behaved as aggressors. This study suggests that several factors can determine the occurrence of these aggressions and that their relative importance depends on the sex of the aggressor. The results of this study highlight the importance of collecting further data to verify whether crowding in a confined space, rather than group size per se, affects the incidence of these aggressions.