By offering comparative data from a range of species, Dunbar shows that, as groups of social mammals increase in size, there is also an increase in the degree of social complexity that an individual member of a group must understand. He then proposes that this increase in social complexity placed evolutionary pressure on primates and led to the development of larger brains – or more specifically, larger neocortexes in relation to overall brain size. Dunbar provides evidence suggesting that the relative size of the neocortex correlates neatly with mean group size in several species of social mammals, including primates, bats, and canines. Further, it appears that it is not the sheer quantity of the relationships involved in large groups of primates that led to increasing social complexity, but the quality of those relationships and the need to balance many competing sets of ideas. Dunbar suggests that the complex negotiations necessary to balance the various relationships in a group are carried out through mutual grooming.
During this phase of the discussion, Dunbar first begins to hypothesize about the correlations between primate and human behavior – and, importantly, human language. Using the neocortex ratio of 4:1, Dunbar predicts that human groups will consist of approximately 150 people; this is approximately three times the size of most primate social groups, but it is surprisingly prevalent at many different levels of human society. For instance, church congregations, living descendants of one couple, military companies, the number of acquaintances with whom the average person has a genuinely social relationship, and the size of horticultural villages, in many parts of the world and at many periods of time, all consist of approximately 150 people. As human group size and the relative size of the human neocortex increased, the biological stage was set for language to emerge.
Because human social groups are so large, the time needed to maintain them through grooming would be far too great (about 40 percent of our day). Thus
Dunbar suggests that language evolved as a way to maintain such large social groups. Language is more efficient than grooming because several people can be
addressed at the same time, and large amounts of information can be exchanged in a short period.
Robin Dunbar, Grooming, gossip, and the evolution of language. Cambridge, MA: Harvard University Press, 1996. Reviewed by Robin M. Queen