Social Brain Hypothesis

In this review (see abstract below), Dunbar and Shultz reviews some of the explanations for large brain size and discuss in greater detail the social brain hypothesis (SBH), suggesting that "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". They discuss some of the recent findings on the relationships between monogamy/enduring social bonds and large brain size.
Evolution in the Social Brain

R. I. M. Dunbar and Susanne Shultz

Science 7 September 2007: 1344-1347.

Abstract: 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 that triggered this evolutionary development. This may explain why primate sociality seems to be so different from that found in most other birds and mammals: Primate sociality is based on bonded relationships of a kind that are found only in pairbonds in other taxa.