A paper entitled "The evolution of the social brain: Anthropoid primates contrast with other vertebrates" just came out in Proceedings of the Royal Society, B (a great journal, I would add). The article is open access, so the link will take you directly to the paper. The authors, Susanne Shultz and Robin Dunbar (of “social intelligence” or “social brain” hypothesis fame), examine the relationship between brain size and pairbonding/enduring social bonds. Previous research has attempted to correlate big brains with group size and amount of deception, for example. Basically the theory goes: Animals have large brains to deal with living in groups - it takes a big brain to be social.
The interesting thing that they find in this paper is that, in non-primate taxa, pairbonded species have larger brain sizes than would be predicted for group size. So there’s something about monogamy or related to monogamy that requires more brain power. In primates this effect does not happen. For them, those species that live in the largest groups have the larger brains. The authors give a short explanation as to why the pattern does not hold in primates, namely because in primates “these bonded relationships have been generalized to all social partners”. The authors also do a pretty good job of explaining many of the potential confounders (phylogeny, ecology etc…).
In the case of humans, it is hard to disentangle our ecological niche in terms of food type from our social systems. Some argue that we need large brains to obtain hard-to-get foods (meat, roots, nuts etc…) while others claim is that it’s our social complexity that requires a big brain. The two are not mutually exclusive since you need to be cooperative and have a social network to get food.
There’s got to be some way to separate these two, or some kind of comparative analysis that would separate the two variables (feeding ecology and sociality). And then, let’s not forget the sexual selection hypothesis that says that a large brain, in humans mainly, is the result of sexual selection.