Dunbar’s Number

Dunbar’s number is a proposed cognitive limit on the number of stable social relationships a person can have. These are relationships in which a person not only knows every other person but knows how each person relates to every other person in the group. The number was suggested in 1990 by the British anthropologist Robin Dunbar. He discovered a correlation between the average social group size and the primate brain size. He extrapolated this data from primates to suggest a social number for humans. Can you guess what his proposed number of stable relationships for humans is? These are the people you would not feel embarrassed about spontaneously sitting down to have a meal with. 

Generally, it’s 150. In order for groups or tribes of people to maintain stability and cohesion in numbers larger than this, there must be rules, laws, and other forms of socially enforced norms. Everyone’s number is different, 150 is the average, and most of our numbers lie somewhere between 100 and 250. These are the number of people with whom social contact is maintained, it does not include people from past relationships with whom you’ve stopped communicating. But at the outer limits it can include people with whom you would eagerly become reaquainted with, for example, a high school classmate. The number is a direct reflection of and limited to our neocortical size, our long term memory. 

When compared to different types of historical and contemporary groups, the predicted number is highly accurate. 200 is about the maximum number of academics in a given subject’s sub specialization. 150 was the average size of a Neolithic farming village. It is also the proposed size of most basic modern military companies. Hutterite settlements split up at about 150 people. It is also the unit size of professional armies from Roman antiquity. You can think of this as the number of people in a group that would maintain a high incentive to stay together. 

Today businesses can find high moral by keeping around this number of employees. In business, keeping contact with around 150 different people produces a very high rate of job offer success. Even criminal organizations, dynamic terrorist networks, and cyber crime networks do not tend to extend beyond 150 participants.

Cohesive groups that actually achieve this number tend to be under intense survival pressure and they maintain physically close connections. Think of the number of participants in a successful, close knit gym or athletic training facility. Because of our social dispersal and less intense environmental and economic pressures, we likely keep close relationships at far fewer numbers. As humans, our ability to use language and the internet somewhat replaced our need to socially groom each other, or remain socially and physically close to so many people. 

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