It is critical that one understand both the causes of variation in the quality and quantity of care provided to offspring in a family environment, and the consequences of this variation for later biobehavioral profiles in adolescents and adults. This chapter examines both issues in marmosets, who are mostly monogamous, biparental, and family-living primates from the eastern coastal forests of Brazil. This chapter highlights both endocrine and genetic influences on variation in maternal and paternal care, and documents the preliminary evidence of the pervasive and persistent effects of variation in early care for later somatic, physiological, and behavioral functioning in marmosets. Understanding of social contributions to epigenesis is growing rapidly, particularly those aspects of development that are systematically influenced by variation in early maternal care. In this sense, variation in the nature of dyadic interactions between mothers and offspring can alter developmental trajectories in important ways. The data presented in the chapter provides a compelling argument that family contexts, with the possibility of triadic and higher order social interactions, also constitute an important component of early epigenesis. The marmoset is an ideal animal model for evaluating these influences, since there are multiple classes of individuals providing care for offspring. Further, the ability to identify important sources of variation in care, and to track the consequences of these differences longitudinally, makes the marmoset particularly useful for providing insights into the ways in which family social environments shape offspring.