Many scientists and colony managers assume that social housing is a beneficial living condition for all captive primates. Several older studies of primate development question the generality of this assumption. We recently tested this assumption by comparing the social development of pigtailed macaque infants raised in pairs and those that were raised in individual cages. All animals received 30 min of daily socialization in a playroom. Infants paired from postnatal week 3 through month 4 developed a playroom behavioral repertoire consisting largely of mutual clinging, fear, and social withdrawal. This was especially true of females. Unlike the singly caged infants, pair-reared monkeys did not successfully adapt to living in a large social group at 8–10 months of age. In this situation, pair-reared infants were subordinate and spent almost all of their time huddling on the pen floor. It was concluded that rearing macaque infants in pairs produces a behavioral repertoire that is maladaptive with respect to social development.