1) (Dunbar, 1992, 1998 Barton, 1996 Barton & Dunbar, 1997 Dunbar & Shultz, 2007 Dunbar, 2011a), although the relationship improves as the measure of brain size is focused more toward the frontal lobes (Joffe, 1997 Dunbar, 2011a). The primary evidence in support of the social brain hypothesis comes from the fact that, across primates, there is a correlation between mean social group size and more or less any measure of brain size one cares to use (Fig.
The original idea for the social brain dates back to the 1970s, when a number of primatologists suggested that primate intelligence might be related to the demands of their more complex social world (Jolly, 1969 Humphrey, 1976 Kummer, 1982), and the name itself was later coined by the neuroscientist Lesley Brothers ( 1990). To borrow an analogy, social neuroscience has devoted its time to examining the bricks and mortar in great detail but has so far overlooked the complexity of the building that lies at the real heart of primate (and human) sociality. In exploring the nature of this unique kind of primate sociality, this article shall argue that, so far, social neuroscience has barely scratched the surface of what is actually involved in what it means to be social. This does not mean that they live in larger social groups than other species of animals (in fact, they don’t), but rather that their groups have a more complex structure. The conventional explanation for this is known as the “social brain hypothesis,” which argues that primates need large brains because their form of sociality is much more complex than that of other species (Byrne & Whiten, 1988). Primates have unusually large brains for body size compared to all other vertebrates. This article considers the implications of these findings for the evolution of human cognition over the course of hominin evolution. In addition, primate sociality involves a dual-process mechanism whereby the endorphin system provides a psychopharmacological platform off which the cognitive component is then built. Neuropsychologically, these are all associated with the size of units within the theory of mind network (linking prefrontal cortex and temporal lobe units). The relationship between brain size and group size is mediated, in humans at least, by mentalizing skills. This is reflected in a correlation between social group size and neocortex size in primates (but not other species of animals), commonly known as the social brain hypothesis, although this relationship itself is the outcome of an underlying relationship between brain size and behavioral complexity.
Primate sociality is based on bonded relationships that underpin coalitions, which in turn are designed to buffer individuals against the social stresses of living in large, stable groups. Primate societies are unusually complex compared to those of other animals, and the need to manage such complexity is the main explanation for the fact that primates have unusually large brains.