It is not surprising to find that among living primate species there is a strong positive relationship between group size and brain size—species that tend to have a terrestrial lifestyle in large groups also tend to have bigger brains. They need the brain processing power to keep track of the increased number of social relationships that arise as groups increase in size. This was discovered by the anthropologist Robin Dunbar, who consequently argued that among living primates brain size is a direct measure of social intelligence. Dick Byrne concurs with this result by finding a strong positive relationship between brain size and the frequency of deception in social strategies—the more complex the social scene, the more devious you are going to need to be to win more friends without winning more enemies.

From Stephen Mithen’s The Prehistory of the Mind: A Search for the Origins of Art, Religion, and Science, London: Phoenix, 1998 (first published in 1996), p. 118.