A current rough scientific consensus is that the brain’s neocortex evolved for advantage in dealing with social complexity. Invocations of a “Dunbar number“, an upper limit on group size, reflect a crude popularization of this scientific understanding. Among vertebrae, primates have unusually large brain size relative to body size and are unusually sociable. Moreover, across primates, as well as across other mammalian groups, larger neocortex size relative to total brain size is associated with greater social complexity. The social brain hypothesis explains these facts of comparative biology.
Recent work indicates that female primate sociality was central to neocortex evolution. Most primate groups include more females than males. Ecological resources and predator risks tend to drive female grouping, while seeking sexual access to females tends to drive male mobility. Female sociality is more affiliative and less competitive than male sociality. Female and male species-typical behaviors characterize different biological routes to evolutionary success.
Across primates, female group size is positively correlated with neocortex size relative to total brain size. Male group size isn’t. Female social communication thus seems to have been central to neocortex evolution. Put differently, the social brain hypothesis primarily describes females’ role in developing primate species’ neocortex.
Across primates, male group size is correlated with subcortical brain structures more closely associated with motor centers and emotional response, including aggression. Female group size isn’t. Male group size thus seems to indicate the extent of sexual competition among men. It correlates with brain structures that would provide advantage in such competition.
The evolution of primate brains indicates the importance of sex to social communication. Vocal communication evolved with socialty. Consistent with the sex-informed social brain hypothesis, significant human sex differences exist in the use of telephones and other communication tools, in reading and writing, and in online gaming. Good analysis of human communication should not ignore sex differences.
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- differences between men and women in social communication
- cultural cognition in the intellectual elite’s social self-construction
- sex differences in human behavior
 Silk (2007), Shultz and Dunbar (2007), and Dunbar and Shultz (2007) discuss the social brain hypothesis, evaluate it across various taxa, and relate it to ecology and life-history. Dunbar and Shultz (2007) notes, “the social brain hypothesis is not about the relationship between brain/neocortex size and group size per se; rather, it is about social complexity….” Other measures of social complexity that correlate with relative neocortex size include size of grooming cliques and frequency of tactical deception. For analysis across the largest possible set of primate species, group size is the best currently available indicator for social complexity.
 See Lindenfors, Fröberg, and Nunn (2003).
 For this and the subsequent paragraph, see Lindenfors (2005) and Lindenfors, Nunn, and Barton (2007). For commentary on this work, see Dunbar (2007).
Dunbar, Robin IM (2007), “Male and female brain evolution is subject to contrasting selection pressures in primates,” BMC Biology, 5:21, doi:10.1186/1741-7007-5-21
Dunbar, R.I.M and Susanne Shultz (2007), “Understanding primate brain evolution,” Philosophical Transactions of the Royal Society B, 362, 649-658, doi: a0.1098/rstb.2006.2001
Lindenfors, Patrik (2005), “Neocortex evolution in primates: the ‘social brain’ is for females,” Biology Letters 1, 407-410, doi:10.1098/rsbl.2005.0362
Lindenfors, Patrik, Laila Fröberg, and Charles L. Nunn (2003), “Females drive primate social evolution,” Proceedings of the Royal Society of London B (Suppl.) 271, S101-S103, doi: 10.1098/rsbl.2003.0114
Lindenfors, Patrik, Charles L Nunn, and Robert A Barton (2007), “Primate brain architecture and selection in relation to sex,” BMC Biology 5:20, doi:10.1186/1741-7007-5-20
Shultz, Susanne and R.I.M Dunbar (2007), “The evolution of the social brain: anthropoid primates contrast with other vertebrates,” Proceedings of the Royal Society of London B 274(1624): 2429–2436, doi: 10.1098/rspb.2007.0693
Silk, Joan B. (2007), “Social Components of Fitness in Primate Groups,” Science 317(5843):1347-51, doi:10.1126/science.1140734