Intimate social brain

“The evolution of unusually large brains in some groups of animals, notably primates, has long been a puzzle. Although early explanations tended to emphasize the brain's role in sensory or technical competence (foraging skills, innovations, and way-finding), the balance of evidence now clearly favors the suggestion that it was the computational demands of living in large, complex societies that selected for large brains. However, recent analyses suggest that it may have been the particular demands of the more intense forms of pairbonding that was the critical factor …” R. I. M. Dunbar* and Susanne Shultz, Evolution in the Social Brain, Science 7 September 2007:
Vol. 317. no. 5843, pp. 1344 – 1347.