Early social experience affects diverse neurophysiological functions, and social and emotional development. Brain serotonin has been implicated in regulating socio-emotional functioning. To understand how early social environment alters sociability and brain serotonin system at the different developmental stages, we use the common marmosets, a primate species which are known for high level of social ability. To study the influence of social environmental factors, we adopted a nursery rearing procedure after postnatal 1day, and solitary living after the age of 6 months for later social deprivation. In childhood less than 6months of age, nursery-reared (NR) animals were compared to age-matched parental-reared (PR) animals as controls. In adulthood after 2 years of age, NR animals were conducted as two kinds of group in terms of later social environment, NRLA who continuously lived alone and NRLP who lived in pairs, which were compared to age-matched PR animals who lived in pairs (PRLP). Sociability of the marmoset was tested by behavioral response to an unfamiliar age-matched animal. To investigate brain serotonin neurotransmission, we performed brain imaging by positron emission tomography (PET) with a specific tracer for serotonin transporters (SERT), [¹¹C]DASB, N,N-[¹¹C]dimethyl-2-(2-amino-4-cyanophenylthio) benzylamine. In childhood, NR animals displayed low sociability such as a social withdrawal in vocal and behavioral response to an unfamiliar conspecific, and a significant reduction in SERT binding in the thalamus, the midbrain, and the cerebellar vermis as compared to PR. In adulthood, NRLA showed a social withdrawal in behavioral response to an unfamiliar conspecific, while NRLP showed no less sociable than PRLP. NRLA showed tendency of lower SERT in the posterior and anterior cingulate, and medial prefrontal cortices where SERT binding activity was correlated with behavioral sociability, while NRLP showed tendency of higher SERT in the visual association cortex and amygdala. These results suggest that the early social deprivation causes long-lasting serotonergic dysfunction and social withdrawal, both of which can be restored by the late social contact with fellows.