Discrete Hierarchical Organization of Social Group Sizes

TL;DR

This study reveals that humans naturally organize social groups into discrete hierarchical sizes following a geometric series, supporting the social brain hypothesis and suggesting a fractal-like social structure.

Contribution

It provides the first systematic evidence of discrete scale invariance in human social group sizes using fractal analysis.

Findings

Humans form preferred social group sizes in a geometric series with ratios close to 3.

Discrete hierarchical organization of social groups is statistically significant.

The pattern may relate to hierarchical social processing in the brain.

Abstract

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. Extrapolation of these findings to modern humans indicated that the equivalent group size for our species should be approximately 150 (essentially the number of people known personally as individuals). Here, we combine data on human grouping 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 3: 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, 9, 27,... Such discrete scale invariance (DSI) could be related to that identified in signatures of herding behavior in financial markets and might reflect a hierarchical processing of social nearness by human brains.