Social development from a psychobiological perspective is part of evolutionary biology. From a functional standpoint two major interrelated themes can be discerned in the evolution of behavior: Wanting (referring collectively to the vital needs of an organism) and Knowing (referring collectively to the organism's knowledge or skill for meeting its vital needs). The social development of the immature primate involves the integration of these themes in two distinct but overlapping phases. In the initially most salient phase, the manifestations of wanting and knowing are focused on constructing an effective relationship with the mother (mother-directed). One of the most important achievements during this phase is the formation of an emotional attachment (probably based on a psychoneuroendocrine core) to a specific object in which elements of both wanting and knowing are intimately involved. The second phase becomes increasingly prominent as development proceeds. The salient manifestations of this phase are focused on relations with the world beyond the mother (other-directed), and involve a new integration of the motivational and emotional components of wanting and knowing, characterized by attraction to novelty, exploration, social interaction and acquisition of knowledge and skills in the contexts of foods, predators and other members of the species.