Introduction

Following Harlow's work (e.g., Harlow & Harlow, 1965), numerous studies have confirmed the fundamental importance of the peer affectional system in the social development of humans and nonhuman primates (e.g., Mason, 1963; Lewis & Rosenblum, 1975). Peer-oriented activities predominate during youth in most social primate species, including humans, and the formation of stable peer relationships early in life appears essential to the development of normal adult social behavior (Argyle, 1969; Harlow & Harlow, 1965; Lewis & Rosenblum, 1975; Loizos, 1967; Mason, 1963; Suomi & Harlow, 1975; van Lawick-Goodall, 1968). As a result of extensive research since the 1970s, we now recognize that peer contact can affect social development even during infancy. Distinctly structured peer relationships of considerable complexity can begin to develop as early as 1–2 months of age in both human and nonhuman primates. In hand-reared nonhuman primates, infant peer contact is able to offset loss of the mother (Fritz, 1984).

Although by now the published literature on early peer social development is substantial, it is deficient in two aspects that have guided the design of the current project. First, prior research on infant peer social development had focused either on humans or on a narrow range of monkey species, primarily rhesus macaques. I found anecdotal evidence but no published systematic studies on infant peer social development in the great apes. This is a serious deficit, as the great apes are the closest living genealogical relatives to humans.