Members of the IBSEN team (Ignacio Tamarit, José A. Cuesta, Robin I.M. Dunbar, and Angel Sánchez) have recently published an article in the journal PNAS entitled “Cognitive resource allocation determines the organization of personal networks” —open access. The paper proposes a quantitative theory to explain the way we organize our social relationships, which is key to understanding the structure of our society. This research has received a lot of media attention. We provide here a translated version of an original article (in Spanish) from “The Conversation“:

We relate to 150 people

A quarter of a century ago, the British scientist Robin Dunbar proposed that the number of people with whom we usually relate is approximately 150.

Some primatologists had observed that there is a relationship between the number of individuals with whom primates relate socially and the size of their cerebral neocortex, which is considered, from an evolutionary point of view, the most modern part of the brain.

According to these observations, the ability to relate to more or fewer individuals would be limited by the volume of this part of the brain since its volume would condition cognitive capacity. Dunbar estimated the number 150 from the above ratio using data from 38 primate genera. And since then that number, 150, has been called “Dunbar’s number “.

He also proposed that the size of real human groups only reaches 150 individuals when the conditions in which the group operates are very rigorous and its members have a strong incentive to stay together.

Only groups under intense survival pressure, such as subsistence villages, nomadic tribes and military cantonments, would reach 150. In the absence of such circumstances, the group would be smaller, although the ability to establish relationships would still be at that approximate limit.

The circles of friendship

Researchers at the Universidad Carlos III de Madrid and Dunbar himself, from Oxford, have developed a theoretical model of social relations based on the fact that the capacity to relate to different people is limited and that different types of relationships require varying degrees of involvement (Ignacio Tamarit, José A. Cuesta, Robin I.M. Dunbar, and Angel Sánchez, 2018: Cognitive resource allocation determines the organization of personal networks. PNAS).

The theory explains empirical observations according to which human relations usually unfold according to a structure of circles.

It is typical for us to have close relationships with very few people, between three and five; this circle includes the closest relatives and sometimes close friends.

The next circle is made up of ten other people; they’re good friends.

Somewhat further away there is a group of about 30 to 35 people, who are those with whom we deal frequently. Surely it is no coincidence that the bands of hunter-gatherers in which human populations were structured during most of the history of our species had, at most, about 50 individuals; perhaps those first three circles are reminiscent of those bands.

And finally, we have a hundred acquaintances with whom we usually relate.

However, the model also takes into account a different possible social structure, with an inverse configuration to the one we have just seen.

It happens, for example, when the community to which an individual belongs is small (less than 55 people); in that case, almost all their relationships are in the first circles, and the group has a great cohesion. This “inverse” structure is typical of individuals who, because of their personality, tend to relate to very few people. Or also when the individual belongs to special communities, with very few members, such as those formed by certain groups of immigrants.

What seems to be deduced from these studies is that we have a sort of more or less fixed cognitive capital and that if we dedicate that capital to relating to a few people, the relationship with them can be very intense. But if, because of our personality or other circumstances, we have a tendency or need to relate to many people, then we will not be able to dedicate more than a small amount of relational cognitive capital to each one of them. And although we have a large neocortex, its volume is not infinite.


Original article by Juan Ignacio Pérez Iglesias, Professor of Physiology at the University of the Basque Country / Euskal Herriko Unibertsitatea (UPV/EHU)
This is a translated version (from Spanish) of the article originally published in The Conversation.