At http://rspb.royalsocietypublishing.org/content/280/1765/20131151.full.pdf+htmlT. David-Barrett, R. I. M. Dunbar Published 26 June 2013.DOI: 10.1098/rspb.2013.1151 Abstract Sociality is primarily a coordination problem. However, the social (or communication) complexity hypothesis suggests that the kinds of information that can be acquired and processed may limit the size and/or complexity of social groups that a species can maintain. We use an agent-based model to test the hypothesis that the complexity of information processed influences the computational demands involved. We show that successive increases in the kinds of information processed allow organisms to break through the glass ceilings that otherwise limit the size of social groups: larger groups can only be achieved at the cost of more sophisticated kinds of information processing that are disadvantageous when optimal group size is small. These results simultaneously support both the social brain and the social complexity hypotheses. 1. Introduction Social living can take one of two forms: animals can form loose aggregations (exemplified by insect swarms and antelope herds, which are typically based on short-term advantages and whose persistence depends on immediate costs and benefits) or they can form congregations (exemplified by the bonded social groups of primates and some other mammals, including cetaceans, elephants and equids among others) whose advantages derive from long-term association, and whose persistence mainly depends on the trade-off between short- and long-term benefits and costs [1]. It has been suggested that this second kind of sociality is a kind of large-scale coordination problem that depends on bonding processes underpinned by more sophisticated cognitive mechanisms (associated with larger brains [2]). Indeed, the social brain hypothesis [27] implies that the size of group that can coordinate its behaviour is limited by the cognitive capacity (essentially, the neural processing capacity) that organisms can bring to bear on the problem. Although there is considerable neuroanatomical evidence (at the individual as well as the species level) to support the social brain hypothesis in both primates [8] and specifically humans [912], the role that cognition plays in this remains unspecified. The recent resurgence of interest in the relationship between communication complexity and social complexity (the social complexity hypothesis) [13,14] offers a possible mechanism by postulating that (i) the complexity of information processing limits social group size and (ii) these communication competences depend on computationally expensive cognitive capacities. We here use an agent-based model to show that the cognitive demands of coordination impose an upper limit on the size of social group that a species can maintain, but that costly increases in information processing, while disadvantageous when optimal group size is small, can allow the evolution of much larger groups providing there is sufficient benefit from doing so. In our model, the objective function is the size of group that can achieve effective social coordination, and we use a central processing unit's time required to achieve this objective function as an estimate of the cognitive demands of different information-processing strategies. In this, we assume that the size of the brain affects the speed and volume, and hence complexity, of decisions that can be made. We use a novel coordination task for this. A pure