s40.
Robin Dunbar, “Coevolution of Neocortical Size, Group Size and Language in Humans”, Behavioural and Brain Science, 16, 1993, pp 681–735. See also Robin Dunbar, “The Social Brain Hypothesis”, Evolutionary Anthropology, 6, 1998, pp 178–190; and Robin Dunbar (1996), Grooming, Gossip and the Evolution of Language.