The social brain hypothesis (SBH) offers a framework for integrating evolutionary and social psychological perspectives on human social complexity. SBH offers an explanation for the evolution of unusually large brains among primates and we outline briefly the background theory and evidence. SBH predicts a natural community size of ∼150 for modern humans, and there is now considerable evidence confirming that this is the typical size of both personal social networks and key types of human community. Human communities are highly structured, with a distinct signature of grouping levels that scale with a ratio of ∼3 (i.e., layers at 5, 15, 50, 150, etc.). We argue that the layering arises from a trade-off between the costs of maintaining relationships (a linear function of time spent interacting) and the benefits that accrue from a particular level of investment (an asymptotic function of time). We suggest that trust is a particularly important mechanism in the stability and functionality of relationships.