I found this article from Cell Press's Trends in Cognitive Science very interesting - although, yes, it is also high in geek factor. The authors take a look at the impact of socio-cognitive capacities on brain size, a model that generally predicts larger brain size in species (often mammals) that have greater levels of complexities in their social organization. The article seeks to integrate the social brain hypothesis with their own broader framework they call cultural intelligence.
Full Citation:Van Schaik, CP, Isler, K, and Burkart, JM. (2012, May). Explaining brain size variation: From social to cultural brain. Trends in Cognitive Sciences, Vol. 16, No. 5. doi: 10.1016/j.tics.2012.04.004
Carel P. van Schaik, Karin Isler and Judith M. BurkartAnthropological Institute & Museum, University of Zurich
ABSTRACT:Read the whole article.
Although the social brain hypothesis has found near universal acceptance as the best explanation for the evolution of extensive variation in brain size among mammals, it faces two problems. First, it cannot account for grade shifts, where species or complete lineages have a very different brain size than expected based on their social organization. Second, it cannot account for the observation that species with high socio-cognitive abilities also excel in general cognition. These problems may be related. For birds and mammals, we propose to integrate the social brain hypothesis into a broader framework we call cultural intelligence, which stresses the importance of the high costs of brain tissue, general behavioral flexibility and the role of social learning in acquiring cognitive skills.
The Social Brain
It has long been known that various mammalian and bird lineages differ in brain size, relative to body size [1,2]. The most widely accepted explanation is known as the Machiavellian intelligence  or social brain hypothesis . Developed to explain variation order in relative brain size among mammalian orders and within primates, the hypothesis argues that large brains are adaptations for dealing with the complexities of social life. Several studies have documented the remarkably sophisticated understanding of the social world in primates , as well as some birds . Although the originally used adjective ‘Machiavellian’ seemed to stress the competitive aspects of social life,
the hypothesis equally applies to cooperative aspects, such as social learning, cooperation and coordination [7,8].
The social brain hypothesis is also consistent with the unusually large brain size of humans. In contrast to all other nonhuman primates, humans have evolved novel socio-cognitive abilities that build on a fully-fledged theory of mind and specialized social learning skills . They arise early in ontogeny  and precede the development of physical and spatial cognitive abilities . The most important among these abilities is language, which critically relies on shared intentionality, cooperative communicative intent and the attribution of mental states to others .
Broad comparative analyses support a prominent role of social interactions in the evolution of cognition in primates, other mammals and birds. The relative size of a species’ brain or neocortex is correlated with the size of its social network  or the presence of coalitions and strong social bonds, including intrasexual pair bonds [8,14]. Indeed, being able to establish and maintain close social bonds is adaptive [15–17]. Consequently, mammalian lineages with stable social groups have shown far steeper increases in brain size over time than solitary ones [14,18].
Despite this compelling support for the social brain hypothesis, two significant problems remain. First, it does not explain the existence of grade shifts, in that some lineages exhibit equally high socio-cognitive abilities with much smaller brains, both within primates  and other mammals , and most spectacularly, fish . Second, recent studies suggest that primates and some other organisms show evidence for general intelligence, and it is not immediately obvious how selection on modular cognitive abilities (i.e., abilities limited to a specific domain) could have produced more general cognitive abilities. We will discuss these problems below and suggest that they are related. We then consider a broader hypothesis for birds and primates that encompasses the social brain hypothesis.