Many of us know the claim that people can maintain no more than 150 friendships. This number is called Dunbar’s number after evolutionary psychologist Robin Dunbar, who first introduced the idea three decades ago. Dunbar argued that the number of neurons in the neurocortex limits the body’s ability to process social information. This, in turn, limits the number of relationships a person can maintain. The results of testing this theory were published by The Conversation.
But although the number has become widely known and is often mentioned in the plans of business managers and software developers, it is not widely recognized in scientific circles. Andreas Wartel, a specialist in evolutionary biology, has investigated the empirical basis of Dunbar’s number and found that it does not hold up when using larger data sets and more modern statistical methods.
The idea that there may be a correlation between social relationships and intelligence was first proposed in 1976 by Cambridge neuropsychologist Nicholas Humphrey. Unfortunately, there is no generally accepted “intelligence test” for animals, so researchers instead turned to measuring brain size as a hypothetical measure of intelligence. Robin Dunbar hypothesized that the neocortex, the top layer of the cerebral hemispheres, is the intellectual part of the brain that processes social information.
Dunbar went on to determine the correlation between the relative size of the neocortex and the size of the primate social group: the larger the neocortex, the larger the social group. Using this correlation and the average size of the human brain, he extrapolated the data to arrive at an estimate of the size of the human group with which humans can interact. It is this estimate that has since been referred to as Dunbar’s number.
The correlation has been confirmed in other studies, although almost all have used the same data set. However, the correlation is completely absent when additional data is added to the statistical models, such as information about other aspects of primate life. Researchers have found that neocortex and brain size in primates can be better described by diet than by sociality.
In addition, many researchers have questioned the value of extrapolating cognitive tendencies from other primates to humans. Although the human brain is anatomically very similar to that of other primates, it functions differently in terms of memory and information processing.
After putting all this data together, the experts replicated Dunbar’s original analysis with a larger data set and more advanced statistical methods. To ensure reliable results, they used three different but overlapping data sets, two different statistical approaches, and analyzed both all primates and a more limited sample that included only monkeys and apes.
The results were clear. The Dunbar number estimates were highly inconsistent, and the 95% confidence intervals, measures of the validity of the estimates, were too large to define any one estimate as the cognitive limit of human group size.
These, however, are not new suggestions for a cognitive limit on human group size, but simply additional illustrations of how poorly this approach works. Overall, Dunbar’s number is a concept with limited theoretical support and no empirical support.