The complexity-sustainability trade-off and Dunbar’s Number

Further reflection on Vladislav Valentinov’s (2014) “The Complexity- Sustainability Trade-Off in Niklas Luhmann’s Social Systems Theory.” As social systems grow in complexity, they risk their own sustainability. Brain evolution, specifically neocortical volume, sets limits on functional social group size. Robin Dunbar (1992):

It is suggested that the number of neocortical neurons limits the organism’s information-processing capacity and that this then limits the number of relationships that an individual can monitor simultaneously. When a group’s size exceeds this limit, it becomes unstable and begins to fragment. This then places an upper limit on the size of groups which any given species can maintain as cohesive social units through time.

R.I.M. Dunbar, Neocortex size as a constraint on group size in primates, Journal of Human Evolution, volume 22, Issue 6, 1992, Pages 469-493,

Dunbar’s Number is a suggested cognitive limit to the number of people with whom one can maintain stable social relationships—relationships in which an individual knows who each person is and how each person relates to every other person. When hunter-gatherer groups grow beyond 150 members, they fragment.

It can be found in the fields of anthropology, evolutionary psychology, and sociology. It reflects the fact that the maximum number of individuals with whom any person can maintain stable social relationships is determined by the size of their neocortex. Dunbar’s number lies between 100 and 230 but the commonly detected value is 150.

Hernando, Villuendas, D., Vesperinas, C., Abad, M., & Plastino, A. (2009). Unravelling the size distribution of social groups with information theory on complex networks.

Social groups larger than 150 require laws, ethical codes, police forces, etc., to function. These mechanisms are designed to regulate factions and prevent the Hobbesian “war of all against all.”

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