Group size in primates is strongly correlated with brain size, but exactly what makes larger groups more ‘socially complex’ than smaller groups is still poorly understood. Chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla) are among our closest living relatives and are excellent model species to investigate patterns of sociality and social complexity in primates, and to inform models of human social evolution. The aim of this paper is to propose new research frameworks, particularly the use of social network analysis, to examine how social structure differs in small, medium and large groups of chimpanzees and gorillas, to explore what makes larger groups more socially complex than smaller groups. Given a fission-fusion system is likely to have characterised hominins, a comparison of the social complexity involved in fission-fusion and more stable social systems is likely to provide important new insights into human social evolution