Dunbar’s number: what is it and what does it tell us about human societies?

According to researcher Robin Dunbar, our brain delimits the maximum size of human groups.

Have you ever heard of Dunbar's number? It is the number proposed by psychologist, anthropologist and biologist Robin Dunbar to refer to the number of people with whom we usually interact.

What are its origins and how does it relate to our ancestors, to primates, and to the neocortex of the brain? In this article we will answer all these questions and, in addition, we will explain how the number of Dunbars is related to religious congregations, according to data from a recent study.

What is Dunbar's number?

Dunbar's number is a number that the British psychologist, anthropologist and biologist Robin Dunbar (full name Robin Ian MacDonald Dunbar) made known more than 25 years ago. It consists of the number of people with whom we habitually relate to, which is approximately 150..

According to Dunbar, this number is related to the size of our cerebral neocortex and its processing capacity. Recall that the cerebral neocortex is the area of the brain that allows us to reason and think logically and consciously. In other words, it gathers our higher mental functions, and allows the functioning of the executive functions.

Social brain hypothesis

Dunbar's number is framed within the social brain hypothesis, also developed by Robin Dunbar, according to which there exists a correlation between the size of the brain (specifically, of the cerebral neocortex) and the number of social relationships that we humans can establish (although it also applies to primates, as we will see below). (although it also applies to primates, as we will see later).

It is a number that aroused much curiosity in different fields and sciences, such as sociology and anthropology, but also other more "number" sciences, such as business administration and statistics.

Origin of this concept in the work of Robin Dunbar

What is the origin of Dunbar's number? Many years ago, primatologists (i.e. professionals who study primate behavior) observed the following: primates have a highly social nature, which makes them maintain (and need) social contact with other members of their group.

But not only did they observe this, but they also found that the number of group members with whom the primates maintained social contact was directly related to the volume of their cerebral neocortex. That is, they determined that there is an index of social group size in each primate species, which differs from one to another according to the volume of the neocortex of each species..

A few years later, in 1992, Robin Dunbar used the correlation that had been determined in non-human primates to predict what size the social group would be in humans (i.e., he applied Dunbar's number to humans).

Specifically, Dunbar determined that the Dunbar number in humans was the size of 147.8 (which normally rounds to 150), although Dunbar specified that this was an approximate value.

Findings in human societies

The cerebral neocortex is an area of the brain that developed about 250,000 years ago. Dunbar began to investigate different nomadic societies, tribes and villages, to find the number of Dunbars in each of these societies..

Thus, he investigated the social group sizes of all these societies, and found that the Dunbar number could be classified into three categories: 30 to 50 people, 100 to 200, and 500 to 2,500.

In terms of his findings and observations, he also noted that a group of 150 people required a very high incentive to stick together..

In this regard, one conclusion Dunbar reached is that for a group of this size to remain together and cohesive, its members would need to spend a minimum of 42% of their time socializing with the other members of the group.

Which groups achieved Dunbar's number?

Dunbar also found that only those groups or societies that were under great pressure to survive, or had a very strong need (such as some nomadic tribes, subsistence villages and different military groups) could reach Dunbar's number.

In addition, he found that these people were almost always in physical contact (or at least, close to each other).. In contrast, dispersed groups (members of which were not physically close) had fewer ties, fewer bonds.

The importance of language

Dunbar not only studied the importance of socialization and needs in explaining Dunbar's number, but also the importance and power of language. According to him, language could have emerged as a tool to facilitate socialization.. This, in turn, could improve cooperation, production, survival....

Thus, language constitutes a tool for cohesion in societies, which in turn reduces the need to be in intimate contact with others, both physically and socially.

Relationship with religious communities

A recent article (2020) by Bretherton and Dunbar relates Dunbar's number to religion; specifically, to the literature on church growth. Thus, this study reveals that Dunbar's number could also be applied to the size and growth of religious communities..

The study goes a bit further, and also analyzes other aspects surrounding the famous Dunbar number; specifically, the researchers made the following findings or conclusions:

Salient findings.

On the one hand, they found that larger congregations have less active participation on the part of each of their members. On the other hand, and this has a lot to do with Dunbar's number, congregations that have only one leader tend to have a number of participants around 150.

In addition, these types of congregations (with 150 members) stratify into even smaller functional or social groups.

But what about congregations with more than 150 members? The researchers revealed that they suffer from great internal tensions that lead to internal reorganization.. These same congregations (of more than 150 members), in fact, need structural subdivisions for the active participation of their members to take place.

The article, which is very interesting to read, basically provides a theoretical framework that unifies the observations of the literature on church growth, together with the Social Brain Hypothesis and Dunbar's number.

Referencias bibliográficas:

• Bretherton, R. y Dunbar, R. (2020). Dunbar’s Number goes to Church: The Social Brain Hypothesis as a third strand in the study of church growth. International Association for the Psychology of religion.
• Dunbar, R. (1988). Primate Social Systems. Chapman Hall and Yale University Press.
• Dunbar, R. (1992). Neocortex size as a constraint on group size in primates. Journal of Human Evolution 22 (6): 469-493.
• Dunbar, R. (1993). Co-Evolution of Neocortex Size, Group Size and Language in Humans. Behavioral and Brain Sciences 16: 681-735.