Primates are, above all, social animals. This has inevitably led to the suggestion that such intense sociality is functionally related to the exceptional cognitive abilities of these animals, as reflected in their unusually large brains (Jolly 1969, Humphrey 1976, Kummer 1982, Byrne & Whiten 1988). This claim is supported by the finding that mean group size is directly related to relative neocortical volume in nonhuman primates (Sawaguchi & Kudo 1990, Dunbar 1992a). These analyses suggest that although the size of the group in which animals live in a given habitat is a function of habitat-specific ecologically-determined costs and benefits (see for example Dunbar 1988, 1992b), there is a species-specific upper limit to group size which is set by purely cognitive constraints: animals cannot maintain the cohesion and integrity of groups larger than a size set by the information- processing capacity of their neocortex.