AbstractCommon marmosets usually give birth to twins and form a social group consisting of a breeding couple and pairs of same‐aged siblings. The twins may engage in the first agonistic fights between them, twin‐fights (TFs), during adolescence. This study investigated the TFs based on records accumulated in our captive colony over 12 years to elucidate the proximate causations that trigger the TFs. We aimed to determine whether the TF onset mainly depended on internal events (such as the onset of puberty) as previously suggested or external events (such as the birth of the younger siblings and the behavioral change of the group members). Although both events usually occur simultaneously, the birth control method (i.e., manipulation of ovulation and interbirth‐intervals by prostaglandin administration to females) could temporally separate these events. A comparison of the onset day and occurrence rate with or without the birth control procedure revealed that TFs were triggered by a combination of internal and external events, that is, external events were the predominant triggers of TF, under the influence of internal events. The timing of TF onset was significantly delayed when the birth of the younger siblings was delayed and the twins grew older under the birth‐controlled condition, suggesting that the birth of younger siblings and related behavioral changes of group members, as well as twins' developmental maturation, could trigger TF. Higher TF rates between same‐sex twins were consistent with previous studies, reflecting the characteristics of same‐sex directed aggression in callitrichines.