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The behavioral repertoire of male and female outbred CD‐1 mice was characterized during development (postnatal Days 23–47). To assess the effects of the rearing condition, half of the subjects were individually housed (IC) at weaning (Day 21), while the remaining mice were housed in pairs (PC) of same‐sex siblings. The occurrence of 22 behavioral items was scored during nine social encounters, 3 days apart, between pairs of unfamiliar same‐sex and same‐condition animals. Individually housed mice were much more involved than paired subjects in solitary play and social interactions (affilitive, investigative, and soliciting behaviors), while less interested in nonsocial behaviors (exploration, and escape‐oriented or maintenance activities). An over‐days increment for elements of social behavior was evident, with a peak of affiliative behavior at 23–32 days of age. For gender differences, in contrast with rat …