Marmosets, tamarins and Goeldi's monkeys Callimico goeldii (Callitrichidae) are well represented in zoos. Owing to their small size, their attractive appearance and their social organization in family groups along with extensive alloparental care, these clawed New World monkeys make fine contributions to any primate collection. In the wild, callitrichids became famous for their so-called ‘social flexibility’, whereas in captivity they can only be kept in heterosexual pairs or grown family groups. This contradiction is addressed in this article. Based on proximate aspects of behaviour, it is concluded that monogamy is the modal social grouping of any callitrichid taxon. In captive family groups of callitrichids, the underlying behavioural mechanisms that ensure the social and sexual integrity of that taxon-specific pattern give rise to the kind of social dynamics that zoo personnel have to cope with at regular intervals; that is, marked intra-group aggression and expulsion of group members. In summary, the overall behavioural theme of naturally grown groups of marmosets, tamarins and Goeldi's monkeys seems to be that their members are torn between cooperation and competition, resulting in occasional periods of social instability.