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Durham University

Department of Anthropology


Publication details for Professor Russell Hill

Zhou, W.X., Sornette, D., Hill, R.A. & Dunbar, R.I.M. (2005). Discrete hierarchical organization of social group sizes. Proceedings of the Royal Society B: Biological Sciences 272(1561): 439-444.

Author(s) from Durham


The ‘social brain hypothesis’ for the evolution of large brains in primates has led to evidence for the coevolution
of neocortical size and social group sizes, suggesting that there is a cognitive constraint on group size
that depends, in some way, on the volume of neural material available for processing and synthesizing information
on social relationships. More recently, work on both human and non-human primates has suggested
that social groups are often hierarchically structured. We combine data on human grouping patterns in a
comprehensive and systematic study. Using fractal analysis, we identify, with high statistical confidence, a
discrete hierarchy of group sizes with a preferred scaling ratio close to three: rather than a single or a continuous
spectrum of group sizes, humans spontaneously form groups of preferred sizes organized in a geometrical
series approximating 3–5, 9–15, 30–45, etc. Such discrete scale invariance could be related to that
identified in signatures of herding behaviour in financial markets and might reflect a hierarchical processing
of social nearness by human brains.